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Deformation mechanisms during uniaxial compressive loading of two extruded binary Mg − 3Nd and Mg–3Y alloys (wt.%) were studied by advanced acoustic technique and electron microscopy. Particularly, a contribution of rare earth metals on the microstructure and resulting mechanical properties is addressed with respect to the extrusion direction. The extrusion of both investigated alloys was performed with the same processing parameters. Thus, the differences in initial microstructures and textures is caused by alloying elements. Consequently, deformation behavior of investigated alloys significantly depended on both the content of the alloy and mutual orientation of extrusion direction and deformation axis due to texture. The activity of individual deformation mechanisms during loading was elucidated using the acoustic emission (AE) technique including advanced adaptive sequential k-means (ASK) algorithm analysis. The results of the AE analysis were related to microstructure evolution provided by the electron backscatter diffraction (EBSD) technique, including Schmid factor analysis. The obtained results had shown that activity of the basal slip system was predominantly affected by the texture, whereas the activity of non-basal slip systems and twinning was dependent on both texture and type of rare earth metal in the alloy.

The effect of Zn content on the hot deformation behavior of Mg–2Gd–xZn (x = 0, 1, 2 and 3 wt%) alloys was studied by performing shear punch tests (SPT) in the temperature range of 623–723 K and shear strain rate range of 1.0 × 10−2–1.2 × 10−1 s−1. Arrhenius-type constitutive equations were employed to investigate the hot deformation behavior. It was found that addition of 1 wt% Zn yields the highest shear strength due to imposing higher solute drag pressure resulted from the co-segregation of Gd and Zn at grain boundaries and dislocations. Stress exponents of 2.7–5.5 and activation energies of 171–228 kJ mol−1 indicated that the main deformation mechanism is dislocation viscous glide with some contributions of dislocation climb and grain boundary sliding in various alloys. Finally, the optimum processing condition having the highest power dissipation efficiency was determined by developing processing maps. The microstructures of the safe and unsafe regions were associated with complete and partial dynamic recrystallization, respectively.

International audience; In the present article, metallic nanoparticles (NPs) used as nanogauges and electron backscattered diffraction (EBSD) are combined to investigate deformation mechanisms of the UR45 N duplex steel. The kinematic fields bring precious details in the evolution of the local deformation of austenitic and ferritic phases. The crystallographic information at the initial stage and in the plastic domain is obtained by the EBSD technique. Strain evolutions in each phase are analyzed. The first strain marks appear in austenite just before the elastoplastic transition. Slip and twinning activities in austenite constitute an important part of the work hardening of the material. The slips in ferrite are visible in the plastic domain and present complex geometry. The role of phase boundaries, which act as strain barrier is highlighted. The incompatibilities of deformations between the two phases modify the deformation mechanism of austenite in comparison with a pure austenitic stainless steel as 316 L. Local interactions between phases during the deformation are analyzed. The strong strain heterogeneities in the plastic domain are analyzed in the kinematic fields. Local evolutions of the deformation are also confronted to the macroscopic behavior of the material.

In this study the mechanical behavior and deformation mechanism of 7075 aluminum alloy sheets under dynamic strain aging induced by solution treatment were investigated. The uniaxial tensile tests were carried out at various strain rates (8.33 × 10−5 s−1, 1.67 × 10−3 s−1, 3.33 × 10−2 s−1 and 1.33 × 10−1 s−1) for samples treated by different solution processes (solution for 30 min at temperatures of 573 K, 623 K, 673 K and 748 K, and then quenching). The Portevin-Le Chatelier (PLC) effect (i.e., serration types of stress-strain curves, strain rate sensitivity (SRS) and critical strain) were analyzed, causing changes in tensile strength, work hardening rate and elongation. The morphology, sizes and quantities of precipitates under different solution-quenching conditions were observed by scanning electron microscope (SEM) and transmission electron microscope (TEM). The results show that the dissolution of secondary phase occurs during heat treatment, accompanied by coarsening behavior at lower temperatures, which leads to the transition characteristics of mechanical properties of the aluminum alloy in solid solution state. There exists a critical point of solution temperature (623K), namely, when the solution temperature is lower than this critical point, the microstructure after quenching is mainly composed of the matrix and secondary phase; otherwise it is mainly composed of solid solution. Therefore, the samples heated at different solution temperatures have different types of PLC effects. It is worth noting that the strong effect of dynamic strain aging induced at high solution temperature and low strain rate could simultaneously enhance the strength and toughness.

The microstructure and texture evolutions of 2A97 Al Li alloy with initial unrecrystallized structure during superplastic deformation were investigated. A total elongation of 300% was obtained at 390 °C at an initial strain rate of 3 × 10−3 s−1. The initial banded structure gradually transformed into a recrystallized structure, characterized by equiaxed grains, random boundary misorientation distribution and a weak texture at high strains. The true strain-stress curve exhibited three stages: work hardening (stage I), steady-state (stage II), and deformation instability regions (stage III). The corresponding deformation mechanisms varied at different stages. The strain rate sensitivity index (m) remained constant of 0.35 and the texture density increased with strain at stage I, where the dislocation creep and subgrain rotation were the dominant mechanisms. During stage II, the m-value increased to 0.44, and the texture density decreased drastically with strain. The combination of grain boundary sliding (GBS) and dislocation creeep could explain the behavior of the alloy. Grain growth and cavitation resulted in the decrease in m-value at stage III, where GBS was the main deformation mechanism.

Intermetallic phases, such as Mg2Ca, have been shown to significantly improve the creep strength of magnesium alloys. However, the relevant deformation mechanisms of the intermetallics for further alloy development are largely unknown as the application temperature of the intermetallic-metallic composites lies in their brittle low temperature regime. In this study, we investigated the deformation mechanisms of the hexagonal Mg2Ca Laves phase at the same size scale (μm) as in intermetallic-metallic alloys and at room temperature using nanomechanical test methods. We identified active slip planes by a statistical evaluation of slip traces formed around nanoindentations and measured the corresponding critical resolved shear stresses for each slip system by compression of single crystalline micropillars in selected orientations. Deformation occurs on basal, 1st and 2nd order pyramidal as well as 1st and 2nd order prismatic planes and the critical stresses are of the order of 0.44 GPa–0.59 GPa with the lowest value obtained on the 1st order prismatic planes. Finally, we discuss the possible slip systems on a theoretical basis in terms of their local atomic structure.

The solid-solution strengthening effect for the single and simultaneous addition of Zr and Si on the compression strength and the creep property in Ti-10Al-2Nb based alloys (at%) with an equiaxed α phase designed by us, was investigated. The compression strengths of these alloys were investigated at temperatures between room temperature and 650 °C. The largest solid-solution strengthening effect was obtained for the alloys with Zr and Si simultaneously added, followed by alloys with the single addition of 2Zr or 0.5Si. For the same amounts of addition elements, the solid-solution strengthening effect of Si was larger than that of Zr. This was attributed to the larger atomic size misfit between Ti and Si than that of between Ti and Zr. A creep test was conducted for Ti-10Al-2Nb-2Zr and Ti-10Al-2Nb-2Zr-0.5Si within a temperature range of 550–650 °C under an applied stress between 137 and 240 MPa. The alloy with Si exhibited a longer creep life for all tested conditions due to a large solid-solution strengthening effect and high compression strength. The deformation mechanisms of the compressive deformation and tensile creep deformation were analyzed using an Arrhenius-type equation. Considering the stress exponent and activation energy, the deformation mechanism of the compressive deformation was identified as low-temperature power-law creep controlled by dislocation core diffusion. While, the deformation mechanism of the creep deformation was identified as high-temperature power-law creep controlled through lattice diffusion.

The application of cellular structure for energy dissipation requires the investigation on the deformation and failure process of the foam under dynamic loading. In this study, the split-Hopkinson pressure bar technique with high speed video camera was used to directly observe the dynamic deformation in quasi-closed-cell aluminum foam fabricated by an in-house direct foaming methodology. The characterization of cellular structure was first performed on the developed aluminum foam, indicating that the cell size follows a log-normal distribution. The measurements revealed that the deformation mode varies with the strain rate. After that an experimentally validated X-ray micro-computed tomography based 3D finite element model of the as-fabricated specimen was established to predict the stress distribution and deformation history under impact loading, and correlated to the experimental observations to explore the deformation mechanisms. It was found that shear band is formed after peak stress. The dynamic deformation and failure process of the quasi-closed-cell aluminum foam result from the coexistence of cell wall bending and buckling, cell collapse and shear traction induced tearing breakage.

Using thermal-mechanical treatments, specimens with different grain sizes were produced for two Cr-Ni-based austenitic stainless steels with different stacking fault energies (analogues of AISI 316L and AISI 321 steels). The effect of strain-rate on the tensile deformation behavior and strength properties was evaluated for these steels. In given grain size interval, (3–73)μm for 316 steel and (0.2–32)μm for 321 steel, the yield strength σ 0.2 varies with grain size D in accordance with Hall-Petch relationship σ 0.2 = σ 0 + k H P D − 1 / 2 . The Hall-Petch coefficient kHP depends on steel composition (stacking fault energy) and possesses higher value for 321 steel as compared to 316 steel. Increase in strain-rate in the interval of 1.0 × 10−4 s−1 to 1.0 × 10−2 s−1 causes growth in stress σ0, but weakly changes coefficient kHP in Hall-Petch relationship: 322–327 MPa × m0.5 for 316 steel and 404–413 MPa × m0.5 for 321 steel. The strain-rate dependence of the constants in Hall-Petch relationship was discussed in terms of deformation mechanism and dislocation arrangement in both steels.

The hot compressive deformation mechanism and processing maps of the equiatomic FCC CoCrFeMnNi high-entropy alloy (HEA) were studied in the temperature range between 1023 and 1323 K and in the strain rate range between 10−3 and 10 s−1. At high strain rates above 1 s-1, strain hardening was dominant even at the very high temperature of 0.84Tm, which may be attributed to the sluggish diffusion coefficient and low stacking fault energy of the CoCrFeMnNi HEA, leading to suppression of dynamic recovery. According to the processing maps, the best condition for hot working was near 10−3 s−1 at 1323 K. Power-law breakdown and unstable flow occurred at low temperatures and high strain rates where the strain hardening was pronounced. The activation energy for plastic flow measured in the power-law creep regime when considering the dependence of elastic modulus on temperature was 312.2 kJ/mol; this value is close to the activation energy for the weighted diffusion coefficient calculated by weighting the contribution of each element in the CoCrFeMnNi HEA (284 kJ/mol). The size and fraction of the dynamically recrystallized grains increased as the strain rate decreased and the temperature increased, as in conventional metals. Both discontinuous dynamic recrystallization and continuous dynamic recrystallization (CDRX) occurred. CDRX became more distinct as the temperature increased. The deformation mechanism and behavior of the CoCrFeMnNi HEA were very similar to those of FCC pure metals in terms of the stress exponent and the effect of the stacking fault energy and diffusivity on the creep rates.

High-strain rate response of low C steel/304 stainless steel (SS) laminates was characterized by hat-shaped specimen using Hopkinson-bar technique at a strain rate of about 7 × 104 s−1. Better dynamic shear properties were observed in the laminates, compared to the plain low C steel plate and the plain 304 SS plate. The laminates were found to postpone the nucleation of adiabatic shear band (ASB) in the hard zone and to delay the propagation of ASB from the hard zone to the soft zone. The conventional maximum stress criterion on ASB nucleation was found not valid any more in the laminates. The hardness difference between the hard zone and the soft zone in the laminates was found to have great influence on the patterns of ASB evolution. Nanotwins were formed in the 304 SS and grain refinement was observed in the martensite low C steel for strain hardening under dynamic shear loading. The mechanical incompatibility across the interfaces was observed to result in strain gradient and geometrically necessary dislocations at the interfaces under dynamic shear loading, contributing to extra strain hardening. The extra hardening was also found to be triggered at the propagation tip of ASB, which helps for achieving better dynamic ductility in the laminates.

A β-solidified nearly lamellar γ-TiAl ( α 2 + γ ) based alloy was investigated to understand the evolution of microstructure during hot isothermal compression test at 1050 °C (below eutectoid). Cylindrical samples were compressed to different true strains up to 60%. The deformed samples were then characterized using scanning electron microscopy (SEM), electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM) to understand the active deformation mechanisms. Dynamic flow softening behavior, i.e., dynamic recrystallization (DRX) type flow was observed. While no new strain induced phases were observed, there were minor changes in the phase fractions as a function of strain. Kinking and bending of lamellae at certain areas in the microstructure of compressed samples were predominant. Preliminary analysis by TEM indicated the presence of twinning in both γ and β phases.

Gradient nanograined Ti (GNG Ti) samples were produced by a surface rolling treatment. The mechanical properties were evaluated by microhardness and tensile tests. The results indicated that GNG Ti achieved an optimized combination of strength and ductility relative to the free-standing nano/ultrafine-grained Ti. The deformation and fracture mechanisms were studied using transmission electron microscope (TEM) and scanning electron microscope (SEM). As the grain size reduced from the coarse-grained (CG) matrix to the nano/ultrafine-grained (NG/UFG) layer, the transition from conventional plastic deformation mechanisms, such as dislocation activity and twinning deformation, to grain boundary-mediated deformation mechanisms, such as stress-inducing grain growth, was identified. At the early stage of tensile deformation, the microcracks were first initiated in the CG and deformed grain (DG) regions rather than in the NG/UFG region and the CG/DG interface had a deflection effect on crack propagation. After tensile failure, the fracture morphology showed a mixture of ductile dimples and shear band-like regions.

Materials science and engineering / A 754, 562 - 568 (2019). doi:10.1016/j.msea.2019.03.073, A discontinuous yielding was observed in the as-extruded Mg-7Gd-2Y-1Zn-0.6Zr (wt. %) alloy under uniaxial tension along the extrusion direction, which was eliminated by the peak-aging treatment. The deformation mechanisms of the as-extruded and peak-aged alloys were studied by in-situ synchrotron diffraction measurement and elastic-viscoplastic self-consistent (EVPSC) modeling. The in-situ synchrotron diffraction results suggest that the unpinning of basal dislocations from solute atmospheres causes the stress relaxation in the basal slip favored grains. The EVPSC modeling results indicate that the discontinuous yielding in the as-extruded alloy is associated with the interaction between basal dislocations and solute atmospheres. After peak-aging treatment, the interaction between basal dislocations and solute atmospheres is weakened due to the consumption of a number of solute atoms. Consequently, the discontinuous yielding is eliminated in the peak-aged alloy., Published by Elsevier, Amsterdam

In the current study, microstructural evolution and superplasticity of an extruded Mg–2wt% Gd sheet were studied after the constrained groove pressing (CGP) process. Microstructural observations by scanning electron microscopy and electron backscattered diffraction revealed that after 4 cycles of CGP, a rather homogeneous fine-grained microstructure with an average grain size of 4.3 μm, and a large fraction of high angle grain boundaries was obtained. By performing shear punch tests (SPT) at different temperatures and various shear strain rates, a peak strain rate sensitivity index (m-value) of 0.49 was obtained after 4 cycles of CGP process at 673 K, while peak m-values of 0.31 and 0.36 were obtained for the as-extruded and 2 cycle CGP process conditions, respectively. An m-value of 0.49 and an activation energy of 113 kJ/mol, obtained for the fine-grained material after 4 cycles of CGP, suggest that the dominant deformation mechanism in the superplastic regime is grain boundary sliding (GBS) controlled by grain boundary diffusion.

Ti 5Al 5Mo 5V 3Cr alloy (Ti-5553) is a high strength near β titanium alloy for aircraft structural applications and has found its specific application in landing gear components. In the present paper a systematic approach was taken to generate and choose three distinct microstructures; one with full β, and the other two with two-phase lamellar α+β microstructures with distinctly different aspect ratios of α precipitates. Tensile behavior of these three microstructures was studied. Few of the distinct observations include 1) Even in the single phase β microstructure, yield strength (YS) was reasonably high, 2) For the two-phase lamellar α+β microstructure high YS and no work hardening were observed, 3) Among the lamellar microstructures, small aspect ratio of α showed much higher YS, but less ductility as compared to the larger aspect ratio of α. Detailed TEM based investigations of deformation micro-mechanisms along with study of eventual failure mechanisms were carried out to explain the above mentioned tensile responses of these microstructures. Differences between the microstructures with small vs. large aspect ratios of α were specifically addressed in terms of tensile behavior and underlying deformation micro-mechanism and failure mode.

An analysis of the fatigue behavior of rolled AZ31 magnesium alloy through numerical simulations of polycrystalline aggregates using the crystal plasticity finite element method is presented. A phenomenological pseudo-slip model including twinning, detwinning and subsequent slip in the twinned region was considered in this study. It was calibrated against uniaxial monotonic and cyclic experiments on rolled AZ31 Mg alloy. Multiple periodic two-dimensional synthetic microstructures were generated based on morphological and crystallographic data obtained from electron backscattering diffraction (EBSD) measurements. Fatigue simulations were conducted at different stress amplitudes and mesoscopic non-local fatigue criteria for slip-induced and twin-induced crack initiation were investigated. It was observed that basal slip and twinning-detwinning are the main deformation mechanisms taking place during cyclic loading but that these deformation modes generally occur in different grains. The comparison of the investigated criteria with experimental data also revealed that the twin-induced criterion exhibit better trends in terms of mean behavior, relative dispersion and stress amplitude sensitivity compared with the slip-induced criterion. Finally, the predictive capabilities and limitations of the current modeling approach are discussed.

The mechanical properties from room temperature to cryogenic temperature were investigated for a novel high-Mn cryogenic steel with 0.5C, 25Mn and 4Cr (in wt. %) based on understanding the relationship between stacking fault energy (SFE), microstructural evolution and deformation mechanism. The superior cryogenic toughness around ∼201 J was achieved and no ductile-brittle transformation temperature was found due to the highly thermal stable austenite. Only ∼2.5% e-martensite was detected in impact sample fractured at −196 °C and consequently the ductile-dimpled fracture is dominant from room temperature to cryogenic temperature. The yield strength of 350 MPa, tensile strength of 810 MPa at room temperature increased to 820 MPa and 1380 MPa at −196 °C. It is controlled by the dominant twin deformation hardening mechanism with the SFE of 24.1 mJ/m2 at room temperature and combination of twin deformation hardening mechanism and martensitic transformation hardening mechanism at −196 °C. Due to the lower SEF at cryogenic temperature, and encountered by strain and stress, martensite transformation should be enhanced.

The mechanical behavior of a metastable β titanium alloy, Ti-2Al-9.2Mo-2Fe (wt%), was studied through high compression strain rate (~3000 s−1) at ambient temperature by Spilt Hopkinson Pressure Bar (SHPB) tests after solution treatment at 850 ℃ for 0.5 h and 950 ℃ for 1 h. The mechanism of deformation and microstructure evolution was investigated by electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). Multiple deformation mechanisms, including primary and secondary {332} twinning, stress-induced martensite, and stress-induced ω-phase and dislocation slips, were identified in specimens subjected to solution treatment at 850 ℃ for 0.5 h. In contrast, only primary and secondary {332} twinning and dislocation slips were detected in specimens subjected to solution treatment at 950 ℃ for 1 h. The athermal ω precipitation produced after solution treatment and elevated temperature generated during dynamic loading played a crucial role in the different deformation mechanisms observed in the two different specimens. Moreover, the deformation mechanisms active during plastic deformation in the different strain regimes affected the development of different textures in the Ti-2Al-9.2Mo-2Fe alloys from each regime.

Low-cycle fatigue (LCF) behavior of wrought superalloy GH4742 was studied at room-temperature (RT) and 700 °C. Scanning electron microscope (SEM) and Transmission electron microscope (TEM) were used to study the development of cracks and deformation mechanism. An obvious decrease in fatigue life occurred as the temperature was increased from RT to 700 °C and as the total strain amplitude was raised from 0.5% to 1.0%. The Halford-Marrow relationship based on plastic strain energy density was used to predict the fatigue life, which is in good accordance with experimental data. At RT, for all investigated samples, initial cycle hardening is followed by gradual cycle softening especially at higher strain amplitude ( ∆ e t ≥ 0.8 % ). Detailed TEM observation revealed that γ′ phase was sheared by dislocations. The shearing mechanism, reducing the ordering degree and diameter of γ′ phase, should be responsible for fatigue softening. Differently, at 700 °C, the tested samples exhibited continuous cycle hardening. A sharp drop in the number of stacking faults occurred. Lots of dislocation loops existed around tertiary γ′ phase and paired dislocations moved in secondary γ′ phase, indicating that Orowan and shearing mechanism have occurred. Fatigue cracks usually initiated from the surface. SEM investigation showed that the failure mode changed from transgranular to mixed transgranular and intergranular modes with increasing temperature.

As one of the most mature titanium alloys, Ti6Al4V is widely used in many critical aerospace applications. However, the flow behavior of this alloy cannot be easily predicted using general computational models, mainly due to the existence of various microstructural morphology in titanium alloys and relatively complex deformation mechanisms during hot deformation. Principally, the variation in initial grain morphology and grain size/plate thickness significantly influences microstructural evolution during hot working, thus leading to dissimilar work hardening and related softening behaviors. In the current study, a microstructural based Estrin Mecking (EM) +Avrami model was developed and used to model the deformation behavior of Ti6Al4V alloy with different initial grain morphologies (i.e. equiaxed vs martensitic) during simulative hot compression testing in the α + β phase region. Herein, the effect of initial grain size was considered as a function of Hall-Petch strengthening, where experimental validation revealed very good accuracy on predicting the work hardening behavior, peak stress, peak strain and flow softening with varying grain morphologies. In addition, the current model was extended to predict the material flow behavior of Ti6Al4V alloy during bulk metallic deformation (i.e. forging) in 3D as an FEM based simulation tool.

In this work, the microstructure, phase transformation, and tensile properties were investigated for Ti55-xZr20Hf15Al10Nbx (x = 1, 2, 3…, 9, at%) medium entropy alloys (MEAs). It has been found that the phase composition of the as-cast MEAs changes from hexagonal α′ to orthorhombic α″, then to metastable β, finally to stable β phase at x = 2, 4 and 9, respectively. The phase transformation of Nb4–Nb8 MEAs evolved in the sequence of β + α″→ω + α″ + β→α + β′→β during heating process. The phase constitution and microstructure of these MEAs strongly influence their tensile properties. The formation of metastable α″ and β phase in certain compositional and temperature range results in “double yielding” phenomenon and remarkable work-hardening effect. Extraordinary static toughness represented by the product of strength and elongation has achieved for MEA with 8% Nb. The deformation mechanisms for these MEAs changes with the phase composition. For the alloy with most α″ phase, the deformation is dominated by de-twinning of martensite variants; for the alloys with most β phase, stress induced martensitic transformation from metastable β to α″ phase and the refinement of the newborn martensite by the dislocation have been verified. These integrated effects are considered to endow the MEAs excellent deformation ductility.

The effects of heat treatment conditions on the microstructure and the deformation behavior of Haynes 282 superalloy in tension at 750 °C were investigated. The standard 2-step aging heat treatment (1010 °C/2 h + 788 °C/8 h) was compared to alternative, more economical, 1-step aging treatment (800 °C/4 h). Moreover, three different cooling rates from the solution temperature of 1135 °C were studied to represent the effective cooling rates that large-scale components may experience. Regardless of the heat treatment conditions, as much as about 20% of fine spherical intragranular γ' particles were successfully precipitated with an average size between 12 nm and 39 nm. The average γ' particles size increased as the cooling rate from the solution temperature decreased. All four heat-treated alloys exhibited good mechanical properties at the high temperature of 750 °C with a yield strength in particular well over 620 MPa. As it could be expected, the yield strength increased and the ductility decreased as the average γ' particles size decreased. The alloys exhibited a mixed mode of deformation characterized by shearing and bypassing. However, the dominant deformation mechanism depended on the γ' characteristics resulting from different heat treatment conditions: only 1-step aged specimens with the largest γ' particles of 39 nm size, which was furnace-cooled from solution temperature, exhibited a plastic behavior typical of a dislocations shearing mechanism while the other alloys showed primarily an Orowan dislocations bowing and looping mechanism behavior. The major operative deformation mechanism could be well predicted by strength increment calculations based on the precipitation strengthening model. Our results suggest that wrought Haynes 282 produced by a more economical 1-step aging treatment may be a reliable candidate for high temperature applications under advanced ultra-super-critical (A-USC) conditions.

Laser direct melting deposited (LDMD) Ti-6Al-4V alloys are mainly used in aerospace fields. It is very important to study their deformation behavior as a function of the application conditions (i.e. temperature and stress). This work compared the dynamic microstructure evolution and mechanical properties of LDMD Ti-6Al-4V alloys at 23 ℃ and 650 ℃. A method based on in-situ scanning electron microscopy (SEM) tensile testing was used to identify the initiation of cracks and propagation mode of slip bands. The deformation mechanisms at low and elevated temperatures were discussed. The results showed that the strength decreased and the elongation increased with increasing temperature. These results were verified by the propagation modes of slip bands and opening of the active deformation systems.

The plastic deformation processes and fracture mechanisms of as-consolidated powder metallurgy (PM) and as-cast ingot metallurgy (IM) metastable beta titanium alloys (Ti-5Al-5V-5Mo-3Cr) were thoroughly investigated by room temperature tensile tests with in-situ scanning electron microscopy (SEM) observations and further microstructure characterizations including transmission electron microscopy (TEM) and atomic force microscopy (AFM). The results suggested that IM alloy has a better compatible deformation capability than PM alloy characterized by varying slip deformation mechanisms and the constantly changing slip modes. The microcracks in PM alloy were observed to be originated at the interfaces of α″/β and grain boundary α/β. Whereas, the microcracks in IM alloy were observed on the grain boundaries and the intracrystalline α precipitation areas, where the slip steps were generated. Particularly, the common defects in PM (residual pores and microvoids) and IM (casting cavity) alloys did not exhibit significant negative effects, contrary to previous awareness. Finally, the coalescence of the grain boundary α and the widened α″/β microcracks led to the sudden brittle transgranular-intergranular mixed fracture of PM alloy. For IM alloy, the serious cracking initiated the V-shape notch made its way through the microcracks at the α/β slip steps, resulting in the gradual transgranular fracture with better ductility.

Hot workability and deformation mechanism of β-solidifying γ-TiAl alloy with a nominal composition of Ti-44Al-5Nb-1Mo-2V-0.2B (in at%) were systematically studied under uniaxial hot compression. The strain rate sensitivity exponent (m) and processing maps at strains of 0.2 and 0.6 were developed. Based on an analysis of the processing maps and the corresponding microstructure, 1200 °C/0.1 s−1 was defined as the optimum deformation condition. At low temperatures and high strain rates, pileup of dislocations at lamellar colonies was found to be responsible for deformation, which provided the driving force for sub-grain boundary formation. As deformation progressed, twinning-induced dynamic recrystallization (DRX) and near complete breakdown of lamellae through the rearrangement of sub-boundaries were the main deformation mechanism. Moreover, nano-lamellar structure and stacking faults (SF) were generated. The hot isostatically pressed (as-HIPed) alloy had relatively good workability, and crack-free sheets were obtained from the as-HIPed alloy with different reductions. A multiple phase, homogeneous as-rolled microstructure without coarse residual lamellar colonies (RL) or shear bands was obtained when the total rolling reduction was 78.8%. The ultimate tensile stress (UTS) and ductility at 800 °C ranged from 523 to 553 MPa and from 21% to 32%, respectively, for different deformation reductions.

In this work, the effect of equal channel angular pressing (ECAP) on the microstructure and mechanical properties of zinc and zinc alloys with Ag, Cu, and Mn additions (0.5 at%) was investigated. Four passes of ECAP Route BC was performed at room temperature for each material. Properties of investigated materials after ECAP were compared to their coarse grained counterparts obtained via indirect hot extrusion at 300 °C. Highest strengthening effect was observed for alloy containing the Mn addition. Grain refinement in materials after ECAP was obtained, mean grain diameter is equal to 20 µm in the case of pure zinc, and less than 3.2 µm for alloys. Strain rate dependent plasticity increase was observed for all fine grained materials, with maximum elongation of 510% measured for Zn-Cu alloy after ECAP. Grain refinement did not result in increased yield and ultimate tensile strength of alloys after ECAP. In all investigated materials tensile properties after ECAP were 20 ~ 60% lower than in hot extruded samples. Based on the tensile properties, microstructure and texture analysis, the changes in the main deformation mechanisms were considered. It was presented that the crystallographic texture and grain size are the main factors affecting twinning, slip and non-slip deformation mechanisms resulting in large differences in observed mechanical properties.

Mechanical twinning plays a significant role in the plastic deformation of Twinning Induced Plasticity steel (TWIP). The effect of grain size on the twinning nucleation stress, yield stress, twin morphology and deformation mechanisms of Fe-22 wt% Mn steel have been studied using molecular dynamics simulations. The nanostructure materials with mean grain sizes of 11.1, 14.3, 17.7 and 32 nm are constructed using the Voronoi tessellation method, and the plastic deformation behavior is studied by molecular dynamics simulation. A meta-atom potential is applied to represent the atomic interactions. The result of tensile test showed that the yield strength and twin nucleation stress increase by grain size decreasing. Structural analysis of the samples deformed to 10% and 20% total strain disclosed thicker primary twins in the coarse-grained material in comparison with the fine-grained one. Moreover, a high volume fraction of twins was formed in the coarse-grained sample. Dislocation analysis exhibited that the dislocation structure influenced by the initial grain size. The density of nucleation sites for twins and subsequently the developing twin substructure is affected by the dislocation substructure. It is concluded that with decreasing grain size, deformation of nanocrystalline TWIP steel by mechanical twinning can become difficult.

The impact of grain boundary structures on mechanical properties and its deformation behavior was investigated on a wrought processed Mg-0.3 at%Mn binary alloy. The Mg-Mn alloy, which had an average grain size of 1.2 µm and a high density of low-angle grain boundaries, was fabricated via caliber rolling. This alloy exhibited yield strength of more than 300 MPa and had low strain rate dependence in both tension and compression tests at room temperature. The major deformation mechanism was dislocation slip for tension and deformation twinning for compression, respectively, as is well established for conventional wrought processed magnesium alloys. However, an extruded Mg-Mn alloy having an average grain size of 1.8 µm is reported to exhibit a good elongation-to-failure and large strain rate dependence. While the same cast billet was used, the result obtained from caliber rolled alloy is inconsistent with that from the extruded alloy. This is due to the difference in grain boundary structures, i.e., the existence of low-angle grain boundaries in the caliber rolled alloy. Low-angle grain boundaries, which do not have free volume, are difficult for segregation of alloying elements. The mechanical properties and deformation mechanism can be clearly controlled by grain boundary structures through wrought process.

Cyclic strain controlled tension-compression tests were performed on samples made of the Mg-base alloy AZ31, which were taken from warm rolled sheets in rolling direction. Due to the strong preferred orientation of the grains, a pronounced strength asymmetry was observed with a higher yield strength in tension compared to compression. This can be attributed to the formation of twins as the dominant deformation mechanism, which occurs only under compression and not under tensile loading. Twinning during compression is associated with the formation of macroscopic twin bands which pass through the sample. Thus a strong localization of compressive strains exists. This leads to a complex inhomogeneous strain distribution during cyclic strain controlled deformation. The local strain distribution was analyzed using digital image correlation. Compressive strain is almost exclusively realized by the strongly localized formation and growing of a macroscopic twin band while tensile deformation is achieved by the dissolution of twin bands as well as by homogeneous deformation based on dislocation glide. In following loading cycles, the areas where during the preceding loading cycle twins had been formed and dissolved by detwinning are again areas of preferred formation of new macroscopic twin bands. Local strain distributions also depend on whether the cyclic tests were started in tension or compression and on the related strain hardened volumes.

It is known that mechanical properties of hot working die steel deteriorate during service due to dynamic recovery, recrystallization and plastic deformation of tempered martensite. In order to clarify the associated microstructure evolution and fracture mechanism, a systematic investigation was carried out on a widely used hot working die steel. Samples were tensile deformed at different temperatures ranging from 25 ℃ to 640 ℃ and examined using SEM and TEM. Results obtained have shown that mechanical properties as well as fracture mode change with increasing testing temperature. These phenomena have been elaborated together with microstructure evolution during high temperature deformation. Deformation mechanisms and roles of different carbides in crack formation and propagation have been discussed.

The microstructure and properties of cast and hot-rolled Al5Ti5Co35Ni35Fe20 High-Entropy Alloy were studied. Presented alloy after initial homogenization for 20 h at 1100 °C is characterised by γ/γ’structure. After solid solution annealing at 1200 °C γ’ precipitates were dissolved that allowed further hot plastic deformation. Heat treatment performed after hot-rolling were used to increase the strength by γ’ precipitation strengthening. Tensile testing was performed at 20 °C, −120 °C and −196 °C. Deformation mechanisms observation was conducted by means of SEM-EBSD and TEM. To evaluate the mechanical properties after solid solution treatment the σy0.2 and σu at 20 °C were measured approx. 775 MPa and 1045 MPa, respectively. The decrease in temperature to −196 °C increase mentioned above parameters of about 20%. Precipitation strengthening enhances the σy0.2 and σu up to 900 MPa and 1200 MPa at room temperature and to 1020 MPa and 1460 MPa at −196 °C, respectively. Investigated alloy exhibits a high elongation to failure over 18% regardless of heat treatment and deformation temperature. Despite the face-centers cubic crystal structure and high Co, Fe and Ni content the twinning were not observed. Deformation behaviour at all testing temperature was the same, the main deformation mechanism was dislocation slip.

Tensile properties, work-hardening, microstructure and fracture mechanisms were studied in two vanadium-containing high-nitrogen austenitic steels, Fe-19Cr-21Mn-1.3V-0.22C-0.81N and Fe-17Cr-10Mn-7Ni-1.0V-0.09C-0.65N, in low-temperature deformation regime. Vanadium-alloying provides several concurrent effects – decreases a solid solution hardening in the steels (softening effect) due to formation of nitrides(carbonitrides), causes precipitate hardening by vanadium-based particles (hardening effect) and suppresses grain growth during solid solution treatment (hardening effect). The yield stresses at room temperature deformation and 77 K-deformation are analyzed taking into account the contributions from these softening/hardening mechanisms. As a result of complex hardening and softening effects, both steels possess high tensile properties, show striking temperature dependences of the yield strength, an ultimate tensile strength and an elongation (the ultimate tensile strength and elongation at 77 K reaches 2600 MPa and 14% for Fe-19Cr-21Mn-1.3V-0.22C-0.81N steel) in low-temperature deformation regime. Striking temperature dependence of the yield strength for particle-containing steels is in accordance with deformation behavior of high-nitrogen particle-free steels with high solid-solution strengthening. In spite of similarities in temperature dependence of strength properties and deformation mechanisms with precipitate-free high-nitrogen steels, V-containing steels do not undergo ductile-to-brittle transition during deformation at cryogenic temperatures and they fracture in ductile manner even during tensile deformation at 77 K. Deformation microstructures at different strain levels and test temperatures are studied in order to reveal the dominating structural parameters responsible for fracture micromechanisms. Despite the activation of mechanical twinning and planar dislocation slip as dominating deformation mechanisms during tension at 77 K, precipitate-hardened high-nitrogen steels show rather high elongation values of 14–15% and ductile fracture with dimple micromechanism. Vanadium-based precipitates of ≈ 300 nm in diameter do not cause strong precipitate hardening, but strongly influence dislocation arrangement and suppress low-temperature brittle fracture in high-nitrogen austenitic steels.

In the present study, a 304 stainless steel was deformed at different temperatures of room temperature (cold rolling) and 200 °C (warm rolling) for elucidating the distinct deformation mechanisms, and then, the influence of the deformed microstructures on the subsequent microstructural evolution and texture development during annealing treatments was studied. The results indicated that deformation-induced martensite transformation was the dominant deformation mechanism during cold rolling, and a mixed microstructure consisting of ~60% volume fraction of α'-martensite and deformed austenite was obtained. Deformation twinning of austenite dominated the deformation during warm rolling. The underlying reason was the increasing SFE with deformation temperature, which inhibited the martensitic transformation but promoted the formation of deformation twins. On annealing, the recrystallization of retained austenite in cold-rolled samples was retarded because of martensitic reversion compared to the warm-rolled samples. Completely recrystallized structure with average grain size of ~0.7 µm was obtained in both cold and warm-rolled samples after annealing at 800 °C. Texture evolution indicated that martensitic reversion enhanced the intensity of {110} texture in cold-rolled and annealed samples. However, the final recrystallization texture components were consisted of {110} and {113} regardless of rolling process. Furthermore, structure-property relationship was established, and combination of high strength and excellent plasticity can be obtained in both cold and warm-rolled sheets by tuning the subsequent annealing parameters.

Low cycle fatigue (LCF) behavior of CP-Ti along transverse direction under symmetrical strain and stress-controlled modes was compared systematically. For the strain-controlled tests, tension-compression asymmetry (TCA) has little effect on LCF behavior due to the negligible compressive mean stress. For the stress-controlled tests, TCA is detrimental to LCF resistance. The microstructure evolution during cycling deformation was characterized by EBSD and TEM to identify different deformation mechanisms. It was found that strain cycling deformation was dominated by dislocation slip and exhibited the transition from planar slip to wavy slip with the increase of strain amplitude. For the stress-controlled tests, tensile twinning together with dislocation slip accommodated the increased ratcheting deformation. The strain-life curve under stress-controlled tests is lower than that at strain-controlled tests due to the heterogeneous plastic deformation caused by ratcheting deformation. In order to obtain a unified fatigue life prediction model for different control modes, a novel effective strain range related model was proposed by introducing the TCA factor into the original Manson–Coffin relation. Furthermore, the application of this effective strain range related model was extended to the condition of asymmetrical stress cycling by correcting fatigue ductility coefficient and exponent.

AZ31 Mg and Mg-1Gd alloys were subjected to electroplastic rolling (ER) and electroplastic-asymmetric rolling (EASR). Due to the additional shear stress in EASR, different deformation behaviors, including twinning, dislocations and DRX, were witnessed in EASR. For AZ31, significant grain refinement was achieved in the EASR sample. The refined grains with weakened texture contributed to the largely enhanced tensile ductility of the EASR sample. While for Mg-1Gd, the formation of extension twins was booming and propagated throughout the EASR sample, which was considered to be attributed to the high participation of non-basal slip during deformation. Extension twins and their related recrystallization strongly influenced the texture evolution and mechanical properties of the EASR sample, so that both the tensile ductility and strength of the EASR sample were improved.

Deformation behavior of 304L stainless steel with heterogeneous lamella structure mainly contains three stages: yield drop stage (~0.2–2%), involving accumulation of stacking faults/geometrically necessary dislocations and initiation of strain-induced martensitic transformation (SIMT); high-speed work-hardening stage (~2–15%), related to dislocation slipping and SIMT; steady work-hardening stage (~15–32%), dominated by SIMT.

In order to understand the effects of cyclic asymmetry and control mode on low cycle fatigue (LCF) behavior of CP-Ti along rolling direction (RD), symmetrical strain and stress-controlled fatigue tests were conducted in this paper. LCF behavior of CP-Ti along RD under different control modes was systematically compared from the aspects of cyclic response, microstructure evolution and fatigue life with the technique of electron backscatter diffraction (EBSD) and scanning electron microscope (SEM). It was found that cyclic softening resulted by dislocation slip dominates strain-controlled fatigue. On the contrary, cyclic hardening dominates stress-controlled fatigue at high stress amplitude, leading to a higher cyclic stress-strain curve. Owing to asymmetrical hysteresis loop, compressive ratcheting strain is accumulated in stress-controlled LCF test despite of the fully reversed stress cycling. EBSD results show there is a transition of deformation mechanism from dislocation slip to dislocation slip and twinning dominated cyclic deformation with the increase of compressive ratcheting strain. Meanwhile, GOS analysis and SEM observation indicate that the appearance of massive twins results in the heterogeneous plastic deformation and micro-cracks are favorable to be initiated in the twin abundant area. Besides, compressive ratcheting strain has significant effects on fatigue crack growth as the fracture surface presents the serious abrasion of cleavage facets and fatigue striations. Finally, strain-life curves under different control modes are identical, indicating that fatigue life along RD is insensitive to compressive ratcheting strain under the interaction of these factors.

We elucidate here the strain rate dependence on the deformation behavior and accompanying deformation mechanism in Fe-30Mn-0.3C TWIP steel via nanoscale deformation experiments and post-mortem microscopy of the deformed region. The nanoindentaion hardness increased with increased strain rate from 0.01 s-1 to 1 s-1, and exhibited a positive strain rate sensitivity of 0.095 with an activation volume of 18b3. At a low strain rate, dislocations dominated the deformation behavior with a high density of 2.7 × 1016 m-2. With increased strain rate, the dislocations decreased and the stacking faults and nanotwins gradually increased. However, nanotwins with secondary twins were the dominant deformation process at high strain rate of 1 s-1. The deformation behavior was significantly impacted by the interplay between strain rate, stacking fault energy and deformation mechanisms. A critical theoretical analysis suggested that the strain rate influenced the critical shear stress for twinning and dislocation slip, resulting in the change in deformation mechanism from dislocation slip to twinning.

In the present study, V10Cr15Mn5Fe35Co10Ni25 (at%) high-entropy alloy (HEA) of a single phase face-centered cubic structure with various grain sizes was fabricated. The influences of grain size on the work-hardening behavior and deformation mechanisms were investigated. The fine-grained and coarse-grained samples showed different work hardening behaviors during room temperature tensile deformation. Microstructural analysis revealed the presence of a high-density tangled dislocation structure without any mechanical twinning in the fine-grained sample, while mechanical twinning was observed to be the additional deformation mechanism in the coarse-grained sample.

The dynamic tensile behavior of Fe-15Mn-3Al-0.23C TRIP/TWIP steel was studied at intermediate strain rates in the range of 7–700 s−1. The results showed that transformation induced plasticity (TRIP) effect, twinning induced plasticity (TWIP) effect, dislocation glide and adiabatic temperature rise effect coexisted during dynamic deformation. With the increase of strain rate from 7 to 70 s−1, the number of deformation twins increased and some intersecting twins appeared. However, when the strain rate approached 700 s−1, the amount of deformation twins were decreased and intersecting twins disappeared. During deformation at strain rates from 7 to 700 s−1, the strain rate had no significant effect on TRIP effect, and deformation twinning was the main deformation mechanism. The ultimate tensile strength (UTS) showed a positive strain rate sensitivity with increased strain rate, and the experimental steel exhibited remarkable mechanical properties with the product of ultimate tensile strength and total elongation (PSE) of 61.5 GPa% at the strain rate of 70 s−1.

Gradient nano-grained (GNG) materials have shown synergetic high strength and ductility due to their unique gradient microstructure, where the grain size changes from tens of nanometers in the surface region to tens of micrometers in the core. This work develops a deformation mechanisms based constitutive model to describe the deformation behavior of GNG materials and to offer a guidance for the design of their microstructures. The established constructive model employs a modified Hall-Petch relation to describe the initial yielding. The different strain hardening behaviors of nano-grained and ultra-fine grained/coarse-grained regions in GNG materials are correlated with the different mechanisms of dislocation microstructure evolution. In addition, the mechanically driven grain growth observed in experiment is also taken into account to capture the effect of grain size evolution on strength and ductility. It is found that grain growth helps to reduce the heterogeneity of the internal stress and plastic strain fields, and thus delays failure of GNG materials. The model successfully predicts the tensile response of a GNG copper bar and captures the distinctive, inverse linear relation between strength and ductility typical of GNG materials. The model can serve as a potential tool for microstructure design of GNG materials by manipulation and optimization of the grain microstructure.

Hot compression tests were performed on Inconel 718 to study high temperature flow stress-strain response and activation mechanism in the hot deformation. To model the flow stress response, we employed two physically based models namely Schӧcks-Seeger-Wolf and Kocks model in addition to extensively used Sellers model. Flow curves from all three models were compared and it was found that predictions of Sellers model closely resemble the experimental results. However, activation energy calculated using Sellers model differs significantly from the activation energy of micro mechanism as reported in the literature. Schӧcks-Seeger-Wolf model predictions are in close agreement with experiments at low strain rate and the activation energy corresponds to diffusion activation energy. Though Kocks model shows much deviation from the experimental results, qualitative comments on the activation energy profile can be made using this model. We utilized hardening models to derive a constitutive model for DRX and simultaneously establish an understanding of rate controlling deformation mechanism in Inconel 718.

Stress relaxation tests were performed on Ti-6Al-4V alloy with different grain sizes at 660 ℃. Based on classical Maxwell exponential decay function, a revised model following the time-hardening rule and corresponding method were proposed to describe the stress relaxation behavior. The stress exponent was calculated using a classical function based on Arrhenius and Norton equations. The microstructure as well as stress exponent analysis show that main deformation mechanism of as-received Ti-6Al-4V sheet is grain rotation combining with grain boundary sliding for accommodation. For the coarser-grained sheet, dislocation movement including dislocation sliding and climb or even dynamic recovery and recrystallization become predominated ones. The combining effect of grain rotation/grain boundary sliding and dislocation movement are the major mechanism for medium-grained sheet. Nevertheless, the relationship between stress relaxation limit and grain size indicates almost linear.

By means of measuring the yield and ultimate tensile strength at room temperature, microstructure observation and contrast analysis of dislocation configuration, the deformation features of a single crystal Ni-based superalloy during tensile are investigated. The results show that the ultimate tensile strength (σb) and yield strength (σ0.2) of the single crystal nickel-based superalloy at room temperature are measured to be 1023 MPa and 958 MPa, respectively. During tensile at RT, the super-dislocations in γ′ phase may decompose on the {111} plane to form only configuration of (1/2) partials plus APB. no configuration of (1/3) partials plus SISF in the alloy is attributed to the dislocation decomposing on {111} plane to form the APB needing a smaller energy than to form the SISF. And the reason of decomposed width with about 20–30 nm keeping in the alloy is attributed to the lower stacking fault energy of alloy and a few deformed heat released during tensile at RT. The super-dislocation in γ′ phase may cross-slip form the {111} to {100} plane to form the K-W locks during tensile at RT, no decomposed feature in K-W locks is attributed to the dislocation decomposing on {100} plane needing a bigger energy. The super-dislocation of shearing into γ′ phase may be also decomposed on the {111} plane to form the Giamei dislocation locks consisting of (1/3) partial loops plus SISF, which is attributed to the combined role of applying a big normal stress and a few deformed heat during tensile.

CoCrFeMnNi high-entropy alloys (HEAs) with partially recrystallized (PR) structure were fabricated by cold rolling and annealing. The microstructures were characterized and the tensile properties were tested at 77 K and 293 K, respectively. In contrast to the early necking at 293 K, an ultrahigh yield strength of 1692 MPa and a considerable uniform elongation of 10.3% were obtained at 77 K. The notable uniform elongation at 77 K can be attributed to the enhanced strain-hardening capability via introducing multiple deformation mechanisms in the recrystallized grains. This work provides a strategy to design high-strength high-ductility HEAs for applications at cryogenic environments.

In this paper, fatigue behavior and microstructure response of laser shock peening (LSP) brass during high cycle fatigue (HCF) test are systematic investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). The results show that broadening, shifting and intensity decreasing of diffraction peaks can be attributed to grain refinement and increase of micro-strain after HCF test. The new grain boundaries in initial grains imply the increase of low angle grain boundaries and the decrease of grain size. The mode of fatigue crack growth is determined by the grain orientation and the grain boundary characteristics. Due to different grain sizes in LSP layers and sub-surface layer, the deformation mechanisms of these layers lead to different microstructural changes. In addition, fracture morphologies indicate that the fatigue crack initiation (FCI) of LSP brass is transferred from surface to sub-surface relative to untreated brass. Based on the investigation above, the fatigue fracture mechanism accompanied by grain evolution is proposed in detail.

Double size effects are uncovered in metastable β Ti-10V-2Fe-3Al alloy single-crystal micropillars subjected to compress along β in the Part I of this paper. In this Part II, a series of samples at different deformation stages were systematically analyzed by transmission electron microscopy (TEM) to understand the deformation mechanism for double size effects. Two distinct phase transformations were observed to take place at different stages of plastic deformation. In the initial stage, dislocation interaction with ω precipitates and deformation-induced ω variants transformation from one to another are predominant plastic deformation mechanisms when the total strain is less than 10%. As a result, high density of ω precipitates contributes to the weak size effect and the continuous stable strain-hardening behavior in the metastable β Ti-10V-2Fe-3Al alloy. In comparison, the martensitic transformation from β to α″ is induced when the critical strain reaches to 10%. The strain-induced martensitic transformation becomes primary deformation modes, which result in higher strain-hardening exponent and stronger size effect, in the second deformation stage. These results provide a new perspective of designing micro-electromechanical materials with an excellent combination of the enhanced yield strength and stable plasticity.

The high-temperature yield drop phenomenon in Al-Mg alloys with high contents of Mg (5–13 wt%) was examined in a wide range of strain rates under compression. Yield drop was highly pronounced when the added amount of Mg was beyond 10 wt%. For example, at 1 s−1 at 698 K, the yield drop ratio between the upper yield stress and the lower yield stress was as high as 1.43 in the Al-13Mg alloy, while the yield drop ratios for the Al-5Mg, Al-7Mg and Al-10Mg alloys were 1, 1.1 and 1.1, respectively. Analysis of the deformation mechanism associated with the yield drop phenomenon suggests that dynamic deformation model based on a sudden increase in the dislocation density during initial straining is more suitable in explaining the observed yield drop behavior than the static theory based on the critical stress for the breakaway of dislocations from the solute atmosphere.

The effects of applied stress (150–250 MPa) on the creep deformation behavior and creep damage (stable and unstable deformation) were studied systematically for a novel tempered martensite ferritic steel G115 at 650 °C. A creep model involving three deformation mechanisms (grain boundary sliding, dislocation glide, and dislocation climb) was applied to fully understand the creep deformation behavior of G115 steel over a wide range of applications. Subsequently, the deformation-mechanism-based creep model was validated for G115 steel over a wide range of stresses (120–250 MPa) and temperatures (625–675 °C). Further, in the stable deformation regions, the martensite laths have no significant change under high-stress conditions, whereas the martensite laths obviously coarsen and micro-cavities formed under low-stress conditions. Four types of precipitates can be characterized after creep. M23C6 carbide and MX carbontride are pre-existing in the initial microstructure. Cu-rich phase was precipitated at 650 °C after very short-term creep (6.87 h), whereas Fe2W Laves phase formed after long-term creep (4404.78 h). The Cu-rich phase particles are cut by a dislocation shearing mechanism and become too small to be stable, leading to the dissolution of the Cu-rich phases after long-term creep. The fine Cu-rich phase particles can significantly retard the recovery of martensite laths in the early stage of creep or during short-term creep. In the unstable deformation (necking) regions, martensite cracking and martensite fracture are the main micro-damage features during creep at high stresses, whereas the growth of micro-cavities and micro-cracks are the main features at low stresses. Ductile fracture is the dominant fracture mode of G115 steel at 650 °C above 150 MPa.