Search

Commercially pure Ni was additively manufactured using laser powder bed fusion. The printed specimens were then subjected to annealing treatments in a range of 700 °C to 1200 °C to investigate the evolution of the microstructure using electron backscatter diffraction mapping and a five-parameter analysis of the grain boundary character distribution. The post-printing annealing treatment resulted in microstructural changes in respect to grain shape and size, the grain boundary character distribution and the overall texture. However, the extent of these changes strongly depended on the annealing temperature. The grain structure and overall texture changed only modestly when annealed below 900 °C. The grains grew by about 10 pct and this led to a small (≈ 15 pct) increase in the areas of boundaries with low energy (111) planes at the expense of higher energy grain boundary planes with the (001) orientation. Static recrystallization was the dominant microstructure evolution mechanism at annealing temperatures greater than 1000 °C, where new equiaxed grains replaced the initial printed microstructure, enhancing the relative areas of low energy grain boundaries (i.e., $$\sum 3$$ ∑ 3 and $$\sum 9$$ ∑ 9 ) by a factor of more than five. This change increased the population of boundaries with the low energy (111) plane by a factor of five and decreased the strength of the texture by more than 50 pct through twinning. The resultant microstructure is expected to ultimately improve the material properties, where the increase in the relative areas of $$\sum 3$$ ∑ 3 boundaries would enhance the corrosion resistance and fracture toughness of material, and the texture weakening diminishes the anisotropy in mechanical behaviour.

Thermo-mechanical treatments were performed to exploit the recrystallisation and deformation induced ferrite transformation (DIFT) during processing and study their effect on the microstructure and strength-ductility balance of a low-carbon, low alloy CMnSiAl TRIP-aided steel containing a small amount of Mo and Nb. A particular focus of the work was to study the influence of prior transformation of retained austenite to martensite (i.e., absence of the TRIP effect) on the strength-ductility balance of the steel by comparing the tensile properties of samples quenched in water and in liquid nitrogen. It was observed that the formation of high carbon martensite as a result of the prior transformation of retained austenite does not necessarily lead to brittle failure. The work also revealed that the TRIP effect per se may not play a major role and it is the deformation of a composite microstructure comprising hard and soft phases, which allows TRIP aided steels to offer a better combination of mechanical properties (strength, ductility and strain hardening) in comparison with conventional high strength steels (HSS) like high strength interstitial free (HS-IF) steel and high strength low alloy steel (HSLA) and other first generation advanced high strength steels (AHSS) like dual phase (DP) steels. The new insight into the TRIP effect may further be exploited in the design of alloy composition and subsequent thermo-mechanical treatments.

The present work investigates the effect of double-pass deformation and annealing on the evolution of NbC precipitates during hot deformation of a Fe-30Ni-Nb microalloyed model steel. The investigation has been performed using transmission electron microscopy. NbC precipitation mainly took place on the periodic dense dislocation networks constituting the microband (MB) walls that largely maintained their crystallographic alignment up to large strains. The double-pass deformation has markedly reduced the precipitate size compared to the equivalent single-pass straining despite twice as long post-deformation annealing time. This has largely been attributed to relocation of the precipitates at the second pass to solute-richer areas, which shifts the precipitation process back to the nucleation and growth stage, and to particle rotations that disrupt their (semi-)coherency with austenite and thus restrict their growth due to increased misfit stresses. The extended holding after two-pass deformation did not increase the precipitate size due to widespread dislocation annihilation through recovery processes, leading to coarsening and disintegration of the MB wall dislocation networks. This resulted in decreased efficacy of the pipe diffusion and complete loss of access to this type of diffusion for a large fraction of precipitates, which markedly hindered the precipitation growth and coarsening processes.

Uni-axial compression (UAC) tests and further post deformation annealing (PDA) were done for 23Cr-6Ni-3Mo duplex stainless steel (DSS). The initial morphology was equiaxed (EQ) in nature. In the first stage of PDA, austenite showed limited static recrystallization (SRX) followed by static recovery (SRV); however ferrite showed static recovery (SRV). In the second stage of PDA, the austenite showed grain coarsening followed by disintegration of substructures (DIS); and ferrite revealed mostly SRV leading to grain coarsening. The third stage of PDA envisages substructural disintegration of unstable substructure leading to saturation in both austenite and ferrite. The sub-structural characteristics were provided by Electron backscattered diffraction (EBSD) and its post processing were done by using HKL Channel 5 software.

The characteristics of the intervariant boundary network that resulted from the $$\beta \to \alpha^{\prime}$$ martensitic phase transformation in a Ti–6Al–4V alloy were studied using the crystallographic theories of displacive transformations, five-parameter grain boundary analysis and triple junction analysis. The microstructure of Ti–6Al–4V martensite consisted of fine laths containing dislocations and fine twins. The misorientation angle distribution revealed four distinct peaks consistent with the intervariant boundaries expected from the Burgers orientation relationship. The phenomenological theory of martensite predicted four-variant clustering to have the lowest transformation strain among different variant clustering combinations. This configuration was consistent with the observed Ti–6Al–4V martensitic microstructure, where four-variant clusters consisted of two pairs of distinct V-shape variants. The $$63.26^\circ /[\overline{10}\, 5\, 5\, \overline{3}]_{{\alpha^{\prime}}}$$ and $$60^\circ /[1\, 1\, \overline{2}\, 0]_{{\alpha^{\prime}}}$$ intervariant boundaries accounted for ~ 38% and 33% of the total population, respectively. The five-parameter boundary analysis showed that the former had a twist character, being terminated on the $$(\overline{3}\, 2\, 1\, 0)_{{\alpha^{\prime}}}$$ plane, and the latter revealed a symmetric tilt $$(1\, 0\, \overline{1}\, 1)_{{\alpha^{\prime}}}$$ boundary plane. The $$63.26^\circ /[\overline{10}\, 5\, 5\, \overline{3}]_{{\alpha^{\prime}}}$$ and $$60^\circ /[1\, 1\, \overline{2}\, 0]_{{\alpha^{\prime}}}$$ had the highest connectivity at triple junctions among other intervariant boundaries. Interestingly, the boundary network in Ti–6Al–4V martensite was significantly different from the commercially pure Ti martensite, where only $$60^\circ /[1\, 1\, \overline{2}\, 0]_{{\alpha^{\prime}}}$$ intervariant boundaries largely were found at triple junctions due to the formation of three-variant clustering to minimize the transformation strain. This difference is thought to result from a change in the martensitic transformation mechanism (slip vs twinning) caused by the alloy composition.

The influence of the parent austenite deformation state on the intervariant boundary network (i.e., population, plane orientation, and connectivity) of a lath martensitic microstructure was investigated using conventional EBSD mapping and five-parameter boundary analysis approach along with quantification of boundary connectivity using homology metrics. The lath martensite largely revealed a bimodal misorientation angle distribution, closely matched with the Kurdjumov–Sachs (K–S) orientation relationship. The application of deformation significantly changed the distribution, gradually reducing the intensity of the 60° misorientation angle peak. This was largely ascribed to substructure development within the parent austenite upon deformation, which stimulates particular variant/s having a habit plane (011)α′ closely parallel to the primary (111)γ and/or secondary (1$$\overline{1 }$$ 1 ¯ 1)γ slip plane. The interaction of these variants eventually promoted specific intervariant boundaries (e.g., 60°/[111], 10.5°/[011], and 49.5°/[110]) at the expense of 60°/[011]. The application of deformation in the parent austenite did not change the intervariant boundary plane character distribution, which mostly exhibited an anisotropic character terminated on {110} planes because of the displacive nature of the martensitic transformation. However, the extent of anisotropy progressively decreased with increasing strain in the austenite prior to transformation. The grain boundary network connectivity was markedly altered due to the local variant selection induced by the deformation. Deformation in the austenite regime generally decreased the connectivity of boundaries having a {110} plane orientation. The intervariant boundaries with the {110} twist character also displayed a similar trend, though the connectivity of {110} tilt boundaries progressively enhanced with increasing strain. The former was closely matched with a decrease in the population of 60°/[110] intervariant boundaries with the strain. The current findings suggest that the intervariant boundary network of lath martensite can be manipulated through changes in the parent austenite deformation state which, in turn, can be used to enhance key final product properties such as toughness. Graphical abstract

In the present study, microstructural analysis of post-deformation annealed sample has been carried out for 23Cr–6Ni–3Mo duplex stainless steel. It was observed that the restoration mechanism and substructural characteristics were different for austenite and ferrite during post-deformation annealing. In the initial stage of post-deformation annealing, the restoration mechanism of austenite envisaged meta-dynamic recrystallization (MDRX), while the dominating mechanism of softer ferrite showed static recovery (SRV) with a minimal of static recrystallization (SRX). In the intermediate stage, the austenite showed SRX followed by coarsening of meta-dynamically recrystallized grains, and ferrite revealed mostly SRV. The last stage of post-deformation annealing showed the disintegration of substructures in both austenite and ferrite. The microstructure after 20 min of post-deformation annealing displayed coarse subgrains. Finer austenite subgrains were observed as compared to ferrite subgrains after the post-deformation annealing of 20 min. High-resolution EBSD was used to study substructural characteristics of microstructure, and post-processing of microstructural results was made by HKL Channel 5 software. Misorientation angle distribution, subgrain size as well as restoration mechanism corresponding to both austenite and ferrite were determined over intensive sample area.

The ferrite-to-austenite phase transformation during water quenching of a duplex stainless steel was studied, where a duplex stainless steel was heated to 1370 °C (delta ferrite region) and quenched to room temperature. The microstructure consisted of coarse ferrite grains and fine needle-like austenite particles. The phase transformation mechanism appeared to be “diffusion-limited” displacive where shear was dominant, but also accompanied by prior or simultaneous diffusional elemental redistribution. A small fraction of the interfaces followed Kurdjumov–Sachs and Nishiyama–Wassermann orientation relationships (ORs) where austenite/ferrite interfaces terminated on {111}A//{110}F planes. The high undercooling associated with the fast cooling rates resulted in a considerable deviation from rational ORs. This was mostly due to the formation of intragranular austenite on Cr2N particles, which most likely caused a random OR with respect to the ferrite matrix. A detailed transmission electron microscopy (TEM) analysis revealed that the planar interphase boundaries characterised by the rational ORs typically contained one dominant set of parallel, regularly spaced dislocations. TEM analysis also showed the occurrence of small protrusions appearing on the edge/face of some austenite particles. Some of these did not leave a sub-boundary behind and formed a finger-like austenite morphology resulting from the instability mechanism. In some other cases, the protruded austenite possessed a low-angle grain boundary with the substrate austenite grain, which was the result of sympathetic nucleation of austenite on a pre-existing austenite particle.

The five-parameter analysis approach was used to measure the grain boundary character distribution of randomly textured AZ31 Mg alloy produced through casting followed by annealing at 450°C...

The tribological performance of Ti–29Nb–14Ta–4.5Zr alloy against Ti–6Al–4V ELI was investigated for the mild condition, emphasized on the superficial changes and changes underneath the wear track. The results of X-ray photoelectron spectroscopy indicated that TiO2, Nb2O, Ta2O and Zr2O oxides aggregated to form a protective tribo-layer, which effectively prevented further superficial damages. Development of fine-grain layer and the occurrence of phase transformation of β → α’’/ω were characterized as the main wear-induced evolution of the subsurface layer. The wear-induced transformation increases the hardenability of the subsurface layer, effectively support the worn surface and prevent premature cracking. These phenomenal evolutions were led to a simultaneous increase in the strength and ductility and subsequently wear loss reduction in comparison to the Ti64 ELI.

The effect of hot rolling and austenite revert transformation (ART) on the microstructure development and mechanical properties for a series of newly developed manganese steels with small carbon and/or boron content was investigated in this study. The cast steels were hot rolled, ART annealed and subjected to tensile testings after each step. The carbon-bearing alloy revealed a significant combination of ultimate tensile strength (UTS) and elongation (El. pct) with 1.1 GPa and 42 pct values, respectively. The boron-bearing alloy exhibited a UTS of 760 MPa having 30 pct elongation. Moreover, the carbon and boron alloyed steel had a fully martensitic structure with a UTS of 1.4 GPa and a negligible elongation. X-ray diffraction, SEM and electron backscatter diffraction analyses were used to rationalize the mechanical behaviour of the developed alloys. The higher ductility values for the boron alloyed and carbon alloyed steels were attributed to the presence of the austenite phase in conjunction with α′- and e-martensite and the transformation-induced martensite during the tensile loadings. The ART annealing at 290 °C, 450 °C and 510 °C increased the austenite fraction in the final microstructure of the boron alloyed and carbon alloyed steels but had an insignificant influence on the boron and carbon alloyed steels. It was concluded that the α′ → γ transformation occurring at higher ART annealing temperature is the dominant factor for the improved mechanical properties in the developed alloys.

A high-strain scratch tester was employed to investigate the abrasive behavior of four different ferrous microstructures with similar bulk hardness levels. The microstructures revealed unique groove characteristics and significant deformation in their sub-surface layers. Bainite and pearlite displayed superior abrasive wear performance in comparison with martensite and tempered martensite microstructures. The characteristics of microstructure constituents were responsible for the distinct groove characteristics and material removal mechanisms. The effect of normal load was influential in the transition of the material removal mechanism from ploughing to cutting. The sub-surface layer of microstructures revealed significant deformation (i.e., nanostructure formation), which was largely determined by the initial microstructure characteristics. The occurrence of work-hardening phenomenon in the sub-surface layer demonstrated a positive impact during abrasion.

The use of Ti6Al4V alloy in critical aerospace applications among other titanium alloys is higher due to the high strength, toughness and corrosion resistance characteristics. Achieved combination of strength and toughness is directly related to the bimodal (equiaxed alpha plus transformed beta) microstructure obtained by hot working of this alloy in a narrow strain rate and temperature domain. Present study distinctly describes the evolution of microstructure during hot deformation as an effect of process conditions and initial microstructure characteristics of Ti6Al4V alloy. The initial microstructure and processing conditions significantly influenced the evolution of β phase and dynamic recrystallization behaviour. An increase in β phase fraction was observed due to the increased adiabatic temperature at higher strain rates during α+β working. The increase in volume fraction of stable β phase as result of vanadium diffusion during hot working revealed the existence of strain induced phase transformation. Continuous dynamic recrystallization (CDRX) led to the fragmentation of initial α grains, and the effect of initial grain orientation on the CDRX mechanism was studied. The initial grains with the basal plane parallel to the compression direction revealed large fragmentation due to the high resistance to deformation. A CDRX mechanism map was developed based on the effect of processing conditions and initial grain characteristics.

The microstructural evolution and mechanical performance of α + β brass alloy (Cu-40%Zn) processed through the accumulative back extrusion (ABE) were investigated in this study. The detailed microstructural evolution during the ABE process exhibited a significant difference in deformation mechanisms operating within each phase, consequently altering their grain refinement mechanism. The rotation of the mechanical twinning along the shear bands contributed to the strain accommodation in the α-phase, while the subgrain formation and rotation within the shear bands resulted into a significant α-phase grain refinement at higher ABE passes. On the other hand, the β-phase, which was strain hardened at the early stages of straining, at higher ABE passes, was fragmented into refined subgrains by the highly strained α/β interphase boundaries. The formation of the newly recrystallized grains with ~0.24 μm (α-phase) and ~1.21 μm (β-phases) sizes, mechanical twin boundaries, shear bands and strain incompatibility zones contributed to a significant increase in the ultimate strength from ~350 MPa for the as-received sample to ~800 MPa in the three-pass ABE processed sample.

In the current study, the effect of deformation in the non-recrystallization region on the characteristics of interphase precipitation was extensively studied in a Ti-Mo steel. The present observation revealed a novel mechanism affecting the interphase precipitation characteristics during an austenite-to-ferrite transformation, as a result of the thermomechanical processing. The hot deformation primarily led to the fragmentation of an austenite grain interior through the formation of elongated dislocation walls, locally deviating the orientation within a grain. Relatively coarse strain-induced precipitates were initially formed on the dislocation walls upon cooling. During the isothermal austenite to ferrite transformation, the interphase precipitation took place on the advancing austenite-ferrite interface front. This was similar to the strain-free condition, though the alignment of planar rows was abruptly altered while encountering the microband walls due to local orientation change at either side of a given microband. In addition, the deformation reduced the planar spacing of interphase precipitates up to a given strain (i.e., ∼0.3), beyond which it became nearly constant. This was due to the saturation of dislocation density within austenite above a certain strain and/or the consumption of alloying elements through the strain-induced precipitation formed on the microbands prior to the austenite to ferrite transformation.

Hydrogen depth profiles and microhardness of the electrochemically hydrogen-charged nanostructured bainitic steels (produced at two different transformation temperatures, i.e. 200 °C (NBS200) and 350 °C (NBS350)) were obtained using elastic recoil detection analysis (ERDA) technique and Vickers microhardness testing, respectively, and compared to that of mild steel. The ERDA results showed that the subsurface hydrogen concentration was higher in NBS200, followed by NBS350 and mild steel. However, the microhardness data of the hydrogen-charged steels revealed material softening in NBS200 and NBS350, whereas the mild steel exhibited material hardening effect. The microhardness along the cross-sectional depth of the steels showed that the softening effect in NBS200 was closer to the hydrogen-charged surface compared to that of NBS350. The plausible mechanisms for the softening effect in the NBS200 and NBS350, and hardening effect in mild steel have been discussed in this paper.

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.

Two types of austenite morphologies, equiaxed and Widmanstatten, were produced through different phase transformation routes to evaluate the critical factors affecting the intergranular corrosion susceptibility in a 2205 duplex stainless steel. These distinct austenite morphologies behaved quite differently in secondary phase precipitation on exposure to sensitization temperature. Although the Widmanstatten microstructure was found to have a larger degree of coherent ferrite/austenite interface area compared with the equiaxed one, it showed a higher degree of sensitization. It was clarified that, in addition to the ferrite/austenite interface coherency, the extent of an interface area and presence of un-stable ferrite also play prominent roles in intergranular corrosion susceptibility.

In this study, the microstructure dependence of impact toughness was studied for a 2205 duplex stainless steel in the temperature range of −196 to 25 °C. Three markedly different austenite morphologies (i.e., rolled (R), equiaxed (E) and Widmanstatten (W)) were produced through different thermomechanical routes. It was found that while the room temperature impact toughness of all microstructures were quite similar, the microstructure dependence of impact toughness significantly increased with decreasing testing temperature. At cryogenic temperatures, microstructure R showed significantly higher toughness compared to microstructures E and W. Considering a 40 J criterion, the ductile to brittle transition temperature was estimated to be ∼ −80 °C for microstructures W and E, while microstructure R showed impact toughness values higher than 40 J even at −196 °C. The lamellar character of microstructure R and the termination of ferrite phase on the (100) plane orientation in this microstructure were found to have a positive effect on the toughness. The occurrence of deformation twinning within ferrite at low temperatures, facilitated by significantly coarser grain sizes in microstructures E and W compared to R, appeared to be the main reason behind the observed deterioration of the impact toughness of the former microstructures at these temperatures.

By time-of-flight (TOF) neutron diffraction experiments, the influence of segregation-induced microstructure bands of austenite (γ) and martensite (α′ ) phases on the partitioning of stress and strain between these phases was investigated. Initially, tensile specimens of a Co-added stainless steel were heat treated by quenching and partitioning (Q&P) processing. Tensile specimens were subsequently loaded at 350 °C parallel to the length of the bands within the apparent elastic limit of the phase mixture. Lattice parameters in both axial and transverse directions were simultaneously measured for both phases. The observation of a lattice expansion for the γ phase in the transverse direction indicated a constraint on the free transverse straining of γ arising from the banded microstructure. The lateral contraction of α′ imposed an interphase tensile microstress in the transverse direction of the γ phase. The multiaxial stress state developed in the γ phase resulted in a large deviation from the level of plastic strain expected for uniaxial loading of single phase γ. Since segregation-induced banded microstructures commonly occur in many engineering alloys, the analysis of stress and strain partitioning with the present Q&P steel can be used to interpret the observations made for further engineering alloys with two-phase microstructures.

The energies of 388 grain boundaries with a range of misorientations and grain boundary plane orientations have been calculated using the meta-atom embedded atom method potential recently developed to simulate an austenitic twinning-induced plasticity (TWIP) steel. A comparison between the simulated grain boundary energies and the measured grain boundary population in an austenitic TWIP steel revealed that at fixed misorientations, there is a strong inverse correlation between the energy and the population. In addition, the Bulatov–Reed–Kumar five-parameter grain boundary energy function for face-centered cubic metals was used to produce a larger, more nearly continuous set of grain boundary energies. Interestingly, these interpolated grain boundary energies were consistent with the simulated energies and also inversely correlated with the measured grain boundary populations in an austenitic TWIP steel.

The microstructures and tensile properties of a conventional Hadfield steel subjected to explosion and cold rolling deformation modes were investigated in the current study. The deformation mode significantly changed the microstructure and tensile properties of the Hadfield steel. The grains in cold rolling condition were largely elongated along the rolling direction; however most grains in the exploded sample retained their equiaxed morphologies. The strain concentration was localized at the grain boundaries, and the change in the population of ∑3 annealing twin and ∑9 boundaries was smaller in the exploded sample than in the rolled condition. Moreover, the deformation twins were formed uniformly along grain boundaries in the Hadfield steel under the explosion condition. However, they primarily appeared as twin bundles in the cold rolled state. The change in the microstructure characteristics resulted in higher yield strength but lower tensile strength and elongation in the exploded sample compared with the cold rolled condition.

In the present study, wedge-shaped samples were used to determine the effect of nominal equivalent strain (between 0 and 1.2) and carbon content (0.06--0.35%C) on ferrite grain refinement through dynamic strain-induced transformation (DSIT) in plain carbon steels using single-pass rolling. The microstructural evolution of the transformation of austenite to ferrite has been evaluated through the thickness of the strip. The results showed a number of important microstructural features as a function of strain which could be classified into three regions; no DSIT region, DSIT region and the ultrafine ferrite (UFF) grain region. Also, the extent of these regions was strongly influenced by the carbon content. The UFF microstructure consisted of ultrafine, equiaxed ferrite grains (

The hydrogen permeation behavior of twining-induced plasticity (TWIP) steel was studied using a Devanathan-Stachurski hydrogen permeation cell. The TWIP steel exhibited three orders of magnitude lower hydrogen permeation rate as compared to the mild steel at room temperature. The hydrogen permeation rate of the TWIP steel was 1.71 × 10−18 mol cm−1 s−1 at 25 °C, but it increased with the increase in the electrolyte temperature: 5.55 × 10−17 mol cm−1 s−1 at 30 °C, 6.56 × 10−17 mol cm−1 s−1 at 40 °C and 8.84 × 10−17 mol cm−1 s−1 at 50 °C. Interestingly, the effective hydrogen diffusivity of TWIP steel was significantly higher as compared to that of mild steel, at all the four test temperatures. Activation energy calculations suggest that the hydrogen permeation was primarily through the grain boundaries in TWIP steel, and therefore exhibited higher effective hydrogen diffusivity in comparison with mild steel.

The intervariant boundary characteristics of a commercially pure Ti microstructure formed by the β → α martensitic phase transformation were described according to the crystallography of the displacive transformation and the boundary plane orientation. The martensitic transformation created a microstructure whose grain boundary misorientation angle distribution had four distinct peaks that were consistent with the misorientations between the variants produced by the Burgers orientation relationship. Interestingly, about 60% of population corresponded to 60°/ [ 11 2 ¯ 0 ] intervariant boundaries. Three-variant clusters with a triangular morphology were observed frequently. This configuration is consistent with the phenomenological theory of martensite, which predicts that these clusters, separated by 60°/ [ 11 2 ¯ 0 ] boundaries, have a lower transformation strain than other possible variant cluster arrangements. Other intervariant boundaries resulted from the impingement of different combinations of distinct three-variant clusters. The five-parameter boundary analysis revealed a strong anisotropy in the plane orientation distribution, showing that boundaries have a tendency to terminate on prismatic { h k i 0 } and pyramidal { 10 1 ¯ 1 } planes, when misorientation was ignored. The dominant 60°/ [ 11 2 ¯ 0 ] intervariant boundaries had symmetric tilt ( 1 ¯ 101 ) boundary planes, which are a low energy configuration.

A gradient nanocrystalline layer with a thickness in a range of millimeter magnitude was successfully produced on the surface of Hadfield steel by a novel severe plastic deformation technology, high speed pounding. The surface hardness was measured, and the microstructure evolution during nanocrystallization process was characterized by X-ray diffraction and transmission electron microscopy. Results showed that the hardness increment and nanocrystallization in Hadfield steel were obtained at different stages under high speed pounding. The first stage was strain hardening, where surface hardness of Hadfield steel increased gradually during high speed pounding until a steady-state value was obtained. The hardening degree and rate of Hadfield steel were determined by deformation stress and strain rate, respectively. The second stage was microstructure nanocrystallization, at which twin boundaries interacted with dislocations to form general high angle grain boundaries. In this stage, the surface hardness of Hadfield steel remained basically the same. Moreover, a physical model was established to explain the strain hardening and surface nanocrystallization behaviors in accordance with the microstructure evolution at different stages in Hadfield steel.

This paper developed high-temperature deformation constitutive models for a Ti6Al4V alloy using an empirical-based Arrhenius equation and an enhanced version of the authors’ physical-based EM + Avrami equations. The initial microstructure was a partially equiaxed α + β grain structure. A wide range of experimental data was obtained from hot compression of the Ti6Al4 V alloy at deformation temperatures ranging from 720 to 970 °C, and at strain rates varying from 0.01 to 10 s−1. The friction- and adiabatic-corrected flow curves were used to identify the parameter values of the constitutive models. Both models provided good overall accuracy of the flow stress. The generalized modified Arrhenius model was better at predicting the flow stress at lower strain rates. However, the model was inaccurate in predicting the peak strain. In contrast, the enhanced physical-based EM + Avrami model revealed very good accuracy at intermediate and high strain rates, but it was also better at predicting the peak strain. Blind sample tests revealed that the EM + Avrami maintained good predictions on new (unseen) data. Thus, the enhanced EM + Avrami model may be preferred over the Arrhenius model to predict the flow behavior of Ti6Al4V alloy during industrial forgings, when the initial microstructure is partially equiaxed.

In the current study, the dependence of mechanical twinning on grain orientation and grain boundary characteristics was investigated using quasi in-situ tensile testing. The grains of three main orientations (i.e., 〈111〉, 〈110〉, and 〈100〉 parallel to the tensile axis (TA)) and certain characteristics of grain boundaries (i.e., the misorientation angle and the inclination angle between the grain boundary plane normal and the TA) were examined. Among the different orientations, 〈111〉 and 〈100〉 were the most and the least favored orientations for the formation of mechanical twins, respectively. The 〈110〉 orientation was intermediate for twinning. The annealing twin boundaries appeared to be the most favorable grain boundaries for the nucleation of mechanical twinning. No dependence was found for the inclination angle of annealing twin boundaries, but the orientation of grains on either side of the annealing twin boundary exhibited a pronounced effect on the propensity for mechanical twinning. Annealing twin boundaries adjacent to high Taylor factor grains exhibited a pronounced tendency for twinning regardless of their inclination angle. In general, grain orientation has a significant influence on twinning on a specific grain boundary.

The complexity of interphase precipitation in a Fe-0.19C-1.54Mn-0.4Si-0.06Al-0.13Mo-0.06Ti (at%) high strength low-alloy (HSLA) steel at an early stage of the austenite-to-ferrite transformation was studied by analyzing the solute distribution across ferrite-austenite interfaces with Kurdjumov-Sachs (K-S) and non-K-S orientation relationships (OR). Structural characterization i.e. ferrite/austenite OR and habit plane characteristics was performed by electron backscatter diffraction (EBSD) and the clustering back-calculation approach, while solute distributions i.e. the solute concentration spikes in the interface regions were studied by atom probe tomography (APT) on the specimens specifically prepared across/near the ferrite/austenite interfaces. It was shown for the first time that interphase precipitation is promoted at both types of interface: (i) a K-S OR and habit plane deviated from ideal (110)α//(111)γ and (ii) a non K-S OR. The key aspect of interphase precipitation is the distribution of solute atoms across the interface, which is pronounced Mn, Ti, Mo and C concentration spikes at the interphase boundary. In contrast, interphase precipitates were not formed at the coherent interface with a K-S OR and habit plane of (110)α//(111)γ. This was correlated with the interfacial condition, where the compositional ratio of substitutional solute and solvent elements remains almost constant across the interface, i.e. Mn and C spikes. Interface compositions in this study did not match with local equilibria (negligible partitioning local equilibrium and paraequilibrium) limits. In addition, it appeared that the interfaces with Mn, Ti, Mo and C concentration spikes form ledges leading to randomly redistributed interphase precipitates.

The fully martensitic microstructures of a manganese bearing precipitation hardenable (PH) stainless steel and a commercial 17-4 PH steel were compared to investigate the influence of chemical composition on the grain boundary network of the lath martensitic microstructure. The martensitic transformation in both steels led to the bimodal misorientation angle distribution having multiple maxima at misorientations in the range of 10–15° and 50–60°. This closely matched with the misorientations between variants associated with the Kurdjumov-Sachs orientation relationship (K-S OR). However, the Mn addition appeared to relatively reduce the area of boundaries with misorientations near 60°. This was a result of the reduction in the area of 60°/[011] intervariant boundaries in the Mn bearing steel. The phenomenological theory of martensite revealed that the Mn addition altered the 3-variant clustering (i.e., V 1 V 3 V 5 ) in 17-4PH steel to the 4-varaint clustering (i.e., V 1 V 2 V 3 V 5 ) arrangement to minimize the strain associated with the displacive martensitic transformation, promoting the population of 60°/[111] intervariant boundaries at the expense of 60°/[011] intervariant boundary. The changes in local variant selection affected the connectivity of the grain boundary network, suggesting that grain boundary network characteristics (i.e., population and connectivity) of the lath martensite microstructure can be manipulated by altering the chemical composition of the steel. The five-parameter grain boundary analysis for both martensitic steels, however, revealed similar grain boundary plane distributions for all boundaries associated with the K-S OR, being terminated on {011} planes due to the constraint that results from the displacive phase transformation.

The present study investigated the microstructure evolution and mechanical behavior in a low carbon CMnSiAl transformation-induced plasticity (TRIP) steel, which was subjected to a partial austenitization at 1183 K (910 °C) followed by one-step quenching and partitioning (Q&P) treatment at different isothermal holding temperatures of [533 K to 593 K (260 °C to 320 °C)]. This thermal treatment led to the formation of a multi-phase microstructure consisting of ferrite, tempered martensite, bainitic ferrite, fresh martensite, and retained austenite, offering a superior work-hardening behavior compared with the dual-phase microstructure (i.e., ferrite and martensite) formed after partial austenitization followed by water quenching. The carbon enrichment in retained austenite was related to not only the carbon partitioning during the isothermal holding process, but also the carbon enrichment during the partial austenitization and rapid cooling processes, which has broadened our knowledge of carbon partitioning mechanism in conventional Q&P process.

The crystallography of interfaces in a duplex stainless steel having an equiaxed microstructure produced through the ferrite to austenite diffusive phase transformation has been studied. The five-parameter interface character distribution revealed a high anisotropy in habit planes for the austenite–ferrite and austenite–austenite interfaces for different lattice misorientations. The austenite and ferrite habit planes largely terminated on (1 1 1) and (1 1 0) planes, respectively, for the austenite–ferrite interfaces associated with Kurdjumov–Sachs (K–S) and Nishiyama–Wasserman (N–W) orientation relationships. This was mostly attributed to the crystallographic preference associated with the phase transformation. For the austenite–ferrite interfaces with orientation relationships which are neither K–S nor N–W, both austenite and ferrite habit planes had (1 1 1) orientations. Σ3 twin boundaries comprised the majority of austenite–austenite interfaces, mostly showing a pure twist character and termina...

The work-hardening mechanisms of two novel advanced high-strength steels (Fe67.4−xCr15.5Ni14.1Si3.0Bx [x = 0 (0B), 2 (2B)] wt%) were investigated by means of field emission gun scanning electron microscopy coupled with angle-selective backscattered detection, transmission electron microscopy, and electron backscattered diffraction. The 0B and 2B specimens combined low yield stresses and high ultimate tensile strengths with good total elongation percentages, with results of 219 MPa, 568 MPa, and 83% and 357 MPa, 703 MPa, and 42%, respectively. The 0B and 2B alloys were characterized by a decreasing work hardening rate, followed by a constant and finally a steep decreasing change tendency. Detailed angle-selective backscattered and electron backscattered diffraction microscopy observations on interrupted tensile test specimens revealed that the work hardening rate in these alloys was facilitated by planar (extended stacking faults) and wavy (dislocation cell and wavy microbands) characteristics and mechanical nano-twins. The total flow stresses of the 0B and 2B specimens were calculated from the dislocation density and twin spacing. This indicated that the work hardening contribution of the microband mechanism can be estimated via a dislocation hardening formula. The rule of mixture was also used to evaluate the effect of a boron addition on the total flow stress of the 2B specimen; this illustrated that, in addition to the strengthening contribution of the second hard phase to the yield stress, the rule of mixture must also be considered. The calculated values of the contribution of the mechanical nano-twins and dislocations on the work-hardening for 0B and 2B specimens were about 62% and 18.6% and 52% and 31.8%, respectively.

The effect of the ferrite to austenite phase transformation route on the microstructure and interface plane character distributions was studied in a duplex stainless steel. Two markedly different austenite morphologies (i.e., equiaxed and Widmanstatten) were produced through diffusional (slow cooling) and semi-shear (air-cooling) transformations, respectively. Both austenite morphologies had textures similar to the as-received condition, which was attributed to a “texture memory” effect. The air-cooled microstructure displayed a significantly higher content of Kurdjumov-Sachs (K-S) and Nishiyama-Wassermann (N-W) interfaces (39%) compared with the slow-cooled one (16%), due to the change in the austenite nucleation and growth mechanism during the phase transformation. A five-parameter analysis of different interfaces revealed that for K-S/N-W orientation relationships, ferrite and austenite terminated on (110) and (111) planes, respectively, regardless of the transformation route. The population of these planes, however, increased as the transformation rate increased. A higher fraction of Σ3 boundaries was observed in the equiaxed austenite morphology compared with its Widmanstatten counterpart, which was mainly attributed to the different kinetics and the growth mode of austenite plates during the phase transformation. Σ9 boundaries were mostly formed where two Σ3 boundaries met and were largely of tilt character because of geometric constraints. The intervariant boundary plane distributions of both austenite microstructures displayed more frequent {111} orientations than other planes for a majority of the boundaries. This trend was markedly stronger for Widmanstatten austenite.

A maraging stainless steel was studied by in-situ XRD to monitor the austenite reversion process. The partitioning of Ni aided austenite reversion in this steel was identified from its negative effect on the lattice parameter of the reverse austenite. This chemical enrichment partially retained the reverse austenite against re-transformation to fresh martensite upon cooling to room temperature. The precipitation hardening effect of Cu and martensite tempering controlled the mechanical properties during low temperature aging (i.e., 550–600 °C), despite a significant austenite reversion at 600 °C. On the other hand, the reverse austenite formation by the austenite reversion process during heating and its subsequent transformation to fresh martensite upon cooling dominated the microstructure for higher aging temperatures (i.e., 650–700 °C), and resulted in a serrated prior austenite grain boundary structure. The change in strengthening mechanism from precipitation hardening to martensite strengthening caused a notable change in the tensile flow behavior (especially the yield ratio) with an increase in the aging temperature. As a result, the mechanical strength for the 700 °C aging condition approached the as-quenched value. This suggests that the aging heat treatment at higher temperature resulted in the re-appearance of martensite strengthening due to fresh martensite formation assisted by the austenite reversion process.

The control of microstructure, texture and deformation inhomogeneity is critical for the hot working of titanium alloys and to optimize their mechanical properties. Herein, a new approach to control the grain refinement, texture and deformation anisotropy of commercial purity titanium (CP–Ti) was established through hot working of a martensitic starting microstructure. The deformation characteristics of the martensitic CP-Ti were investigated through uniaxial hot compression testing at 800 °C and compared with the same alloy with an equiaxed starting microstructure. The associated microstructure, texture development and grain refinement mechanisms were elucidated using electron backscatter diffraction (EBSD) and flow behavior analysis. The equiaxed microstructure displayed a typical flow behaviour of metals undergoing continuous dynamic recrystallization (CDRX). This was accompanied by moderate grain refinement to ∼8.2 μm and macroscopically non-uniform expansion of specimen. In comparison, the martensitic microstructure revealed a stress-strain curve of continuous softening beyond the peak strain. An enhanced grain refinement to ∼5 μm was obtained also via CDRX in the form of kinking of laths and progressive lattice rotation without noticeable macroscopic deformation anisotropy. This distinct deformation behaviour was mainly attributed to the unique microstructure and texture formed through the martensitic transformation, which altered the activation of slip systems, grain refinement kinetics and texture development during hot deformation.

Polycrystalline α iron has been used in various applications, yet its microstructure design via grain boundary engineering (GBE) is not well established. One limiting factor is that while there are many different grain boundaries in the five-dimensional space of grain boundary types, relatively few of the energies have been determined. In this study, a piece-wise continuous grain boundary energy function for α iron is constructed to fill the entire five-dimensional space of grain boundary types using scaffolding subsets with lower dimensionality. Because the energies interpolated from the grain boundary energy function are consistent with the 408 boundaries that have been calculated using atomistic simulations, the energy function is then employed to generate a larger set of grain boundary energies. Comparisons between the interpolated energies and the measured grain boundary population indicate that they are inversely correlated for the high-energy anisotropy misorientations (those for which the difference between the maximum and minimum grain boundary energies is greater than 0.4 J/m2). The results suggest that GBE in the α iron should not only be considered the twinning-related grain boundaries (Σ3, Σ9, Σ27a, and Σ27b) as in the case of fcc metals, but also other high-energy anisotropy misorientations.

In the current study, the microstructure evolution and mechanical behaviour in a commercial low silicon boron steel were investigated after one-step quenching and partitioning (Q&P) treatment under different isothermal temperatures in the range 260–320 °C. This processing approach resulted in a complex microstructure consisting of a tempered martensite matrix with bainitic ferrite, fresh martensite, and retained austenite, offering a superior combination of mechanical properties compared to the directly water-quenched condition, where a fully martensitic structure was exhibited. The Q&P processed steel isothermally held at a high temperature of 320 °C showed lower work-hardening, which was mainly associated with the lower fraction of initially formed martensite and the corresponding tempering responses, compared to the steels held at lower temperatures. The phase transformation behaviour throughout the heat treatment was studied by dilatometry, regarding the martensitic transformation upon initial cooling, carbon partitioning combined with the martensite tempering and austenite decomposition during isothermal holding, as well as the fresh martensite formation on the final cooling. Additionally, the microstructure characteristics were examined using X-ray diffraction, scanning electron microscope, and electron backscatter diffraction techniques.

The ferrite major dynamic softening mechanisms, particularly in duplex microstructures, have long been a matter of debate among steel scientists. It has been shown in the present work that a marked increase in the strain rate at a high temperature leads to a transition in the ferrite softening mechanism in low density steels. In contrast to the current widespread view, ferrite was found to soften through continuous dynamic recrystallization at a low strain rate and via discontinuous dynamic recrystallization at a high strain rate. The latter mechanism is largely associated with the interphase mantle regions, rather than the scarce original ferrite-ferrite boundaries. In these regions, the new grains were observed to develop through the growth of highly-misoriented subgrains. Keywords: Hot deformation, Restoration, Strain rate, Low density steel

The microstructure evolution of a novel advanced high-strength steel (AM2B® = austenite + (Fe,Cr)2B) subjected to room-temperature interrupted uniaxial tensile test was investigated in relation to the mechanical behavior. This steel showed both high ductility and strength, with yield stress of 357 MPa, ultimate tensile strength of 702 MPa, and total elongation as high as 42%. Electron back-scattered diffraction analysis showed that the high formability (29.4 GPa%) of the current steel was mostly due to energy dissipation mechanisms such as (1) formation and rearrangement of geometrically necessary dislocations (GNDs), (2) activation of mechanical nanotwins, (3) grain rotation, and (4) void nucleation and growth. It was found that GND rearrangement mostly occurred in the true strain range of 0.13–0.25, and void nucleation and growth were the most influential mechanisms in the true strain range of 0.25–0.37. Moreover, it was found that the constraint effect of M2B phases may accelerate softening (grain rotation) and mechanical nanotwinning.