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We introduce a new nanoindentation method to continuously measure the hardness while sweeping through orders of magnitudes of strain rates within a single experiment. While nanoindentation already allows the determination of the strain rate sensitivity of materials by means of strain rate jump tests, these are typically limited to few discrete strain rates. With the new method, the strain rate sensitivity can be measured continuously as a function of the strain rate. Applications to fused silica, Zn-22 %Al superplastic alloy, single crystalline aluminum, various nanocrystalline metals and a palladium-based metallic glass are shown. Besides some discrepancy with the reference measurements, the new method seems only affected by the presence of a strong nanoindentation size effect. Provided this indentation size effect is not excessively large and can be corrected for accurately, the method proves robust, with no suggestion that the direction of the strain rate sweep affects the evaluation of the strain rate sensitivity.

The frequently used metallurgical parameters such as activation volume, internal stress, strain rate sensitivity, and dislocation density are identified in ultrafine-grained (UFG) materials through erudite analytical relations. It is difficult to analyze such parameters optically since the material consists of highly distorted and inhomogeneous microstructure due to the large strain imposed during the constrained groove pressing (CGP) process. Hence, transient tests like stress relaxation tests are advantageous over the optical to use. Apart from the identification of parameters in low-carbon steel materials using uniaxial loading and stress relaxation tests, the relationship between the parameters is nobly investigated in as-received and UFG structure samples produced through the CGP process. In the relationship, the exponential-power law constitutive to describe the accurate stress–strain curves, identification of mobile and forest dislocation density during strain hardening, and exponential variation between grain size and activation volume are proposed. The replenishment or exhaustion of mobile and forest dislocation density significantly affects the metallurgical parameters during uniaxial tensile loading followed by stress relaxation testing. In this experimental and analytical work, the CGP process produces a UFG material that has lower activation volume and higher stress drop, mobile dislocation density, forest dislocations density, strain rate sensitivity, and internal stress. [ABSTRACT FROM AUTHOR]

Strong obstacles can greatly impede the motion or transmission of dislocations, which can be reflected by strain rate sensitivity and activation volume. In this study, the strain rate sensitivity and activation volume of overaged Al-Zn-Mg-Cu alloys with a grain size of 3.1 μm fabricated by powder metallurgy were measured by two different methods: a stress relaxation test and strain rate jump test. It was found that the former method gave much higher strain rate sensitivity values. After reviewing the conventional theory of stress relaxation test, it is unreasonable that the activation volume at each cycle during the stress relaxation test is a constant. At a strain rate higher than 10−5 s−1, ∂ ln ε ˙ / ∂ τ *, which is proportional to the activation volume in the conventional theory of the stress relaxation test, increases significantly, and nearly linearly increases with the strain rate in its logarithmic form, while at a strain rate lower than 10−5 s−1, the value of ∂ ln ε ˙ / ∂ τ * is nearly a constant. The grain boundary sliding mechanism was incorporated into the plastic deformation during the stress relaxation test, and the strain rate sensitivity and activation volume obtained by stress relaxation after modification agree well with that obtained by the strain rate jump test. [ABSTRACT FROM AUTHOR]

Background: Understanding the dynamics of deformation processes is of interest for determining the dominant thermally activated processes during plasticity [1] and fracture [2-3]. Strain rate jump (SRJ) tests have been recently introduced to investigate intrinsic deformation mechanisms and have been successfully utilized on some nanomechanical test platforms [4–6]. Objective: The goal is to create standardized SRJ testing and analysis protocols for Hysitron nanoindenters, which cannot be found in literature, besides our previous work [7]. Methods: Presented here is software to create SRJ test load functions for the TI980, which could be adapted to other instruments, and standardized protocols for analyzing the associated data. Results: These protocols are validated using single crystal tungsten as a model material and agree well with literature values from other instruments. Conclusions: New freely available software, validated on single crystal tungsten, creates opportunities for others to investigate intrinsic deformation mechanisms in more complex systems. [ABSTRACT FROM AUTHOR]

A single‐crystal Bi2Se3 topological insulator is fabricated using the Bridgman–Stockbarger method. The crystal structure and atomic lattice parameters are identified by X‐ray diffraction analysis. The nanoindentation size effect on creep displacement, activation volume, and strain rate sensitivity (SRS) with different maximum holding loads between 1000 and 5000 μN is investigated using depth‐sensing nanoindentation. Furthermore, the effect of the loading rate on the steady‐state creep displacement and SRS is analyzed and discussed. Results show Bi2Se3's low resistance to plastic deformation and a significant increase of creep displacement with increasing holding load and holding rate. Additionally, creep strain rate, activation volume, and SRS are also calculated from the secondary stage creep, and results are compared with those of other flexible electronic materials. [ABSTRACT FROM AUTHOR]

Strong obstacles can greatly impede the motion or transmission of dislocations, which can be reflected by strain rate sensitivity and activation volume. In this study, the strain rate sensitivity and activation volume of overaged Al-Zn-Mg-Cu alloys with a grain size of 3.1 μm fabricated by powder metallurgy were measured by two different methods: a stress relaxation test and strain rate jump test. It was found that the former method gave much higher strain rate sensitivity values. After reviewing the conventional theory of stress relaxation test, it is unreasonable that the activation volume at each cycle during the stress relaxation test is a constant. At a strain rate higher than 10−5 s−1, ∂lnε˙/∂τ*, which is proportional to the activation volume in the conventional theory of the stress relaxation test, increases significantly, and nearly linearly increases with the strain rate in its logarithmic form, while at a strain rate lower than 10−5 s−1, the value of ∂lnε˙/∂τ* is nearly a constant. The grain boundary sliding mechanism was incorporated into the plastic deformation during the stress relaxation test, and the strain rate sensitivity and activation volume obtained by stress relaxation after modification agree well with that obtained by the strain rate jump test.

This study identifies the role of solute Cr in the thermal activation process in the plastic deformation of α-(Fe, Cr) ferrite (BCC solid solutions) using micropillar compression tests for micron-sized single-crystals fabricated on various Fe–Cr binary alloys that were solution-treated at 1573 K. Fe–18%Cr single-crystal micropillars with a mean diameter d of ∼2 μm exhibited a relatively higher strain rate sensitivity (m) of 0.12 for the stress required for slip initiation. Larger Fe–18%Cr micropillars (d = 5 μm) exhibited a lower m value of 0.04. The corresponding activation volume of 50 b 3 was equivalent to the millimeter-sized pure Fe and Fe–18%Cr alloy specimens (60–70 b 3 ), indicating the prevalence of a general mechanism involving dislocation motion (kink-pair nucleation of screw dislocations). This mechanism controls the thermal activation process of plastic deformation in the Fe–18%Cr alloy, which has sufficient sources for dislocation multiplication. In the Fe–25%Cr and Fe–40%Cr single crystal micropillars, a less pronounced size-dependent m value (0.01–0.03) was determined, almost independent of specimen size. The corresponding activation volumes of large micropillars (74–77 b 3 ) were similar to those of millimeter-sized specimens, which indicates a mild effect of solute Cr atoms on the thermal activation process of dislocation motion in Fe–Cr binary alloys at ambient temperature. Initial dislocations, rather than Cr content in Fe–Cr binary alloys, led to the low changes in size-dependent activation volume of Fe–Cr alloys with higher Cr content. Planar faults (Cr-enriched regions) were observed in Fe–40%Cr alloys, which may become sufficient sources for dislocation nucleation. However, the influence of Cr-enriched regions on dislocation motion remains unclear. [ABSTRACT FROM AUTHOR]

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Ni–18wt% Co foils made by electrodeposition possesses an average grain size, computed using diffraction contrast in TEM, of about 30 nm. Vickers microindentation and depth-sensing nanoindentation have been adapted to assess the deformation parameters such as hardness, strain rate sensitivity (SRS) and activation volume. These foils with single-phase fcc-structured solid solution exhibit the hardness values of 4.5 ± 0.1 GPa and 6.2 ± 0.2 GPa measured by microindentation and nanoindentation, respectively. The dependence between hardness and applied load in the present solid solution cannot be attributed to the indentation size effect, but to the changes in internal friction and/or reduction in stacking fault energy that may have resulted from the Co additions. An elastic modulus of 193 ± 3 has been realized in these foils. Performing nanoindentation at various loading rates and subsequent analysis has resulted in a SRS of 0.017 and an activation volume of 7.6 b3. These values suggest that in these nanocrystalline Ni–18Co foils made by electrodeposition, interfaces such as grain boundaries, triple junctions and quadruple junctions play a governing role in dictating the deformation kinetics. A model reported in the literature has been successfully modified to reasonably explain the dependence of SRS on grain size for various Ni-based alloys including the one reported in the present study. However, the model fails to follow the established dependence for the materials with grain size below 10 nm as the deformation mechanisms at these extremely finer length scales (below 10 nm) are expected to be totally different from considerations applicable for the present alloy with a grain size of 30 nm. [ABSTRACT FROM AUTHOR]

The influence of Mn and Si on the mechanical properties of solid solution Al-Mn and Al-Mn-Si alloys have been studied at 4 K, 78 K, and 298 K. The solute strengthening potency of the yield strength increases with decreasing temperature. The addition of Si stimulates the precipitation of Mn-containing dispersoids and decreases the solubility of Mn in the Al matrix, reducing the strengthening effect. The alloys exhibit Stage III of declining work hardening rate with the flow stress during the plastic flow at three temperatures. The mean slip length Λ in the work hardening model of Nabarro, Basinski, and Holt (NBH) evaluated from σ Θ curves correlates with the scale of the microstructure. The strain rate sensitivity (SRS) measurements conducted at 78 K show positive SRS of Al and the alloys. At 298 K, one observes a change in SRS due to the change of the mechanism from solute-dislocation to dislocation-dislocation dominated interactions. Mn and Si decrease SRS at 298 K. Al-Mn-Si alloys exhibit negative SRS and jerk flow at a sufficiently low strain rate at 298 K. Analysis of the apparent activation volume, distance, and work has been conducted at 78 K and 298 K to understand the rate-controlling mechanisms. The results indicate that dislocation intersections are rate-limiting processes, but the atomistic mechanism of dislocation cutting varies vastly depending on the temperature and alloy composition. It is suggested that at 78 K, forest obstacles are passed by the formation of jogs and sequential activation of individual partials without making constriction to the perfect defect. At 298 K, the cutting involves constriction, the creation of jogs, and the creation of point defects at each intersection site. The calculated activation work and distance provide signatures of these processes. [ABSTRACT FROM AUTHOR]

• Strain rate jump tests are performed to address the rate controlling deformation mechanisms in additively manufactured SS316 alloy. • Despite having highest dislocation density in the as-printed condition, alloy is found least rate sensitive compared to heat treated and wrought alloy. • Flow transients on strain rate change are understood using dynamic pileup readjustment mechanism. We conducted uniaxial tensile strain rate jump tests to account for the strain rate controlling mechanisms in the additively manufactured SS316L stainless steel. Special emphasis is placed on the role of high dislocation density and chemical segregation on dictating the rate sensitive behavior. Lowest strain rate sensitivity is found, despite the highest dislocation density in the as build state, which signified the role of solute segregation at cell walls in increasing the activation volume and forms the strong basis for rejecting dislocation forest as the strain rate controlling obstacles. Consistent increase in the strain rate sensitivity with annealing is found consistent with the chemical homogenisation of the solutes causing smaller inter-obstacle spacing causing decreasing activation volume. Experimental findings are supported by CALPHAD simulations and mechanical threshold stress (MTS) modelling framework. Significant difference between the instantaneous and steady state rate sensitivity is understood using the dynamic pileup dislocation density readjustment mechanism causing considerable flow transient while adjusting the mobile dislocation density on strain rate change in the presence of solute decorated dislocation cells. Present investigation helps in gaining newer insights into the deformation mechanisms of the additively manufactured alloys featuring cellular microstructures. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • A new nanoindentation method was developed to continuously measure the strain rate sensitivity and activation volume over the strain rate. • Validation was performed on seven different materials with constant strain rate and strain rate jump nanoindentation testing as reference. • A transition of superplastic deformation regimes was found in the superplastic alloy Zn–22 %Al. We introduce a new nanoindentation method to continuously measure the hardness while sweeping through orders of magnitudes of strain rates within a single experiment. While nanoindentation already allows the determination of the strain rate sensitivity of materials by means of strain rate jump tests, these are typically limited to few discrete strain rates. With the new method, the strain rate sensitivity can be measured continuously as a function of the strain rate. Applications to fused silica, Zn-22 %Al superplastic alloy, single crystalline aluminum, various nanocrystalline metals and a palladium-based metallic glass are shown. Besides some discrepancy with the reference measurements, the new method seems only affected by the presence of a strong nanoindentation size effect. Provided this indentation size effect is not excessively large and can be corrected for accurately, the method proves robust, with no suggestion that the direction of the strain rate sweep affects the evaluation of the strain rate sensitivity. [ABSTRACT FROM AUTHOR]

The tensile properties of additively manufactured (AM) metals and alloys are among the most important variables that impact the potential applications of these materials. Here we examine and report on the tensile properties of AM 316L stainless steels fabricated by the laser powder-bed-fusion (L-PBF) technique, via twelve sets of optimized laser processing parameters that produce materials with density >98.8 ± 0.10%. A heterogeneous microstructure is observed in all L-PBF samples, including microscopic features such as dislocations, cellular walls, elemental segregations, local misorientations, impurities, precipitates, and a large fraction of low-angle grain boundaries (2-10°, ∼40–60%). The derived average grain size defined by high-angle grain boundaries (>10°) is ∼30–50 μm. Tensile testing reveals a yield strength ranging from 552 to 635 MPa and a tensile-elongation-to-failure (TEF) of 0.09–0.42 for directly-printed samples, whereas these values are 592–690 MPa and 0.29–0.50 for samples machined from the as-built rectangular thin plates. In all samples, we observe a variation of tensile yield strength within ∼15% but not the TEF, suggesting marginal microstructural changes despite a wide range of laser processing parameters. The large scatter of TEF in directly-printed samples originates from the sensitivity of thin gauge geometry (∼2 mm2 cross-section area) to the built-in flaws. We measured a substantially higher strain rate sensitivity (m ∼0.02–0.03) of L-PBF 316L compared to the coarse-grained counterparts (∼0.006), together with a small activation volume of ∼20–30 b 3 (where b is the Burgers vector of 316L). These deformation kinetics parameters suggest that the tensile plasticity of L-PBF 316L is controlled by a much finer microstructural length scale than the measured grain size, consistent with the high strength and juxtaposed nano- to macro-structures seen in these materials. Strategies to optimize the tensile properties of AM materials are discussed. • The yield strength of L-PBF 316L from a single build machine exhibits a variation within ∼15% but not the tensile elongation-to-failure (TEF). • The large variation of TEF in L-PBF 316L is due to its sensitivity to sample geometry and built-in flaws. • An m ∼0.02–0.03 is measured in L-PBF 316L compared to coarse-grained counterpart (∼0.0061), together with a small V∼20–30 b 3. • Nanoinclusions in L-PBF 316L SS were found located in the dimple center and may impact its fracture strength. [ABSTRACT FROM AUTHOR]

The rapid change in strain rate sensitivity (SRS) of metals at a critical true strain rate (ε ̇ Tc) is commonly attributed to change in rate-controlling deformation mechanism from thermally-activated process to viscous drag. In this study, metastable AISI 321 stainless steel with grain sizes of 0.24 μm (ultrafine [G1]), 3 μm (fine [G2]) and 37 μm (coarse [G3]) was deformed at high strain-rate. A change in SRS with increasing ε ̇ T is observed only in G2 and G3 and it occurs at ε ̇ Tc of ~5900 and 6800s−1, respectively; suggesting that grain size have an effect on the switch of rate-controlling deformation mechanisms. While the SRS of G1 is 0.101 and unchanged, those of G2 and G3 change from 0.094 to 0.326 and 0.091 to 0.634, respectively, once ε ̇ Tc is reached. Similarly, the estimated activation volume in G1 specimen is ~1.57 b 3 , while those of G2 and G3 specimens change from ~2.95 b 3 to ~0.65 b 3 and from ~4.10 b 3 to ~0.45 b 3 , respectively. Analysis of the deformed specimens using XRD, EBSD and TEM techniques revealed that the complexities in the activated deformation mechanisms in G2 and G3 specimens led to a change in ε ̇ Tc. Constant SRS in G1 specimen indicated no change in the operational deformation mechanisms. Unlabelled Image • Strain rate sensitivity (m) and activation volume (ϑ ∗) of AISI 321 steel were studied at high strain-rate regime. • m and ϑ ∗ were determined for UFG (0.24 μm), FG (3 μm) and CG (37 μm) samples. • m and ϑ ∗ remains unchanged for UFG at all investigated strain rates. • A change in both m and ϑ ∗ occurred at critical true strain rates (ε ̇ Tc) of ~5900 and 6800 s−1 for FG and CG specimens, respectively. • The change in ε ̇ Tc with increasing grain size is attributed to complexities in the operating deformation mechanisms in AISI 321 steel. [ABSTRACT FROM AUTHOR]

Abstract The creep behavior of nanocrystalline Cu with an average grain size of 25 nm was investigated by nanoindentation test at room temperature. Using the creep strain rate versus creep stress data obtained at different loading rates, the activation volume and strain rate sensitivity were determined obtained by cooperating the continuous stiffness measurement (CSM) technique. The results showed that the activation volume first increases and then decreases, and the strain rate sensitivity first decreases and then increases with increasing the creep stress. The experimental activation volume and strain rate sensitivity versus the creep stress data exhibit very good agreements with the theoretical values calculated by the previous models, respectively. The analysis based on the data of the activation volume and strain rate sensitivity revealed that at lower stress, the grain boundary activities dominate and lead to the lower creep strain rates; at higher stress, the dislocation activities dominate and lead to the higher creep strain rates. The analysis based on the data of the nanoindentation test also revealed that the use of the CSM technique can lead to the continuous creep strain rate versus creep stress data, which allows us to uncover the creep mechanisms over a wide range of the creep stress from the initial to steady stage. [ABSTRACT FROM AUTHOR]

Abstract In the present work, titanium alloys doped with different amounts of impurity solutes were studied as regards their strain rate sensitivity, work-hardening and fracture manner. The experimental results revealed a transition of both the strain hardening behavior and fracture manner with increasing the impurity content. Namely, the low impurity content Ti exhibited features of intergranular crack, whereas the high impurity content Ti showed transgranular cracking instead. In addition, the physical activation volumes were found to reduce with the impurity content, which is attributed to both dislocation pinning and solute drag effects. A constitutive model was proposed to understand the mechanical behavior of the materials. The highlights of the model are of separating deforming grains into plastic and elastic groups and of giving them different properties. To describe the volume of grains changing from elastic to plastic deformation with strain, a controlling function was applied based on an empirical analysis. It turns out that the proposed model predicted stress-strain curves in well agreement with the experimental results. [ABSTRACT FROM AUTHOR]

In a recent experimental study of indentation creep, the strain rate sensitivity (SRS) and activation volume v* have been noticed to be dependent on the indentation depth or loading force for face-centered cubic materials. Although several possible interpretations have been proposed, the fundamental mechanism is still not well addressed. In this work, a scaling law is proposed for the indentation depth or loading force-dependent SRS. Moreover, v* is indicated to scale with hardness H by the relation ∂ln(v*/b3)/∂lnH=−2 with the Burgers vector b. We show that this size effect of SRS and activation volume can mainly be ascribed to the evolution of geometrically necessary dislocations during the creep process. By comparing the theoretical results with different sets of reported experimental data, the proposed law is verified and a good agreement is achieved.

The strength of nanocrystalline (NC) metal has been found to be sensitive to strain rate. Here, by molecular dynamics simulation, we explore the strain rate effects on apparent Young’s modulus, flow stress and grain growth of NC gold with small size. The simulation results indicate that the apparent Young’s modulus of NC gold decreases with the decrease of strain rate, especially for strain rates above 1 ns−1. The rearrangement of atoms near grain boundaries is a response to the decrease of apparent Young’s modulus. Indeed, the flow stress is also sensitive to the strain rate and decreases following the strain rate’s decrease. This can be found from the change of strain rate sensitivity and activation volume with the strain rate. Temperature has little effect on the activation volume of NC gold with small grain size, but has an obvious effect on that of relatively large grain size (such as 18 nm) under low strain rate (0.01 ns−1). Finally, grain growth in the deformation process is found to be sensitive to strain rate and the critical size for grain growth increases following the decrease of strain rate.

Highlights • Activation volume in the presence of semi-coherent β'' precipitates is significantly lower compared to other microstructural features. • Effective stress is almost two times higher in the presence of semi-coherent β'' precipitates compared to other microstructural features. • Exhaustion rate of mobile dislocation density is negligible in the over-aged sample compared to solutionized and peak-aged sample. • Relaxation strain is higher in the presence of semi-coherent β'' precipitates compared to other microstructural features. Abstract In the present investigation, stress relaxation tests were carried out on an aluminium magnesium silicon alloy to determine rate controlling mechanism in different temper conditions. Single and multiple relaxation tests were carried out to determine apparent and physical activation volume. It was found that in the presence of solutes and incoherent precipitates, dislocation-dislocation interaction is the rate controlling mechanism, whereas dislocation-precipitate interaction is the rate controlling mechanism in the presence of semi-coherent precipitates. Moreover, the nature of deformation was found to be more viscous (higher strain rate sensitivity) in the presence of semi-coherent precipitates compared to other microstructural features (solutes, incoherent precipitates). Special emphasis is placed upon understanding exhaustion rate of mobile dislocation density during relaxation period. It was found that the exhaustion rate of mobile dislocation density is negligible in the presence of incoherent precipitates, whereas a significant reduction in mobile dislocation density was observed in the presence of solutes and semi-coherent precipitates. Furthermore, thermal and athermal components of the flow stress were determined to develop a comprehensive understanding regarding deformation behavior in different temper conditions. Graphical abstract Image, graphical abstract [ABSTRACT FROM AUTHOR]

Eutectic high entropy composites (EHECs) are novel class of material with excellent combination of strength and ductility, thus having a large potential for industrial applications. However, the mechanisms operating behind the trade-off between strength and ductility has not been investigated in detail. In this work, the influence of severe straining imposed by high-pressure torsion (HPT) was evaluated for a series of CoCrFeNiNb x alloys with varying Nb content ( x molar ratio), hypoeutectic ( x  = 0.25), eutectic ( x  = 0.65) and hypereutectic ( x  = 0.80) compositions. Strain rate sensitivity ( m ) and activation volume ( V * ) calculations were calculated from constant strain rate (CSR) nanoindentation experiments, revealing that dislocation interaction with lamellae interfaces become the rate-limiting step for the strength-ductility trade-off in these EHECs. [ABSTRACT FROM AUTHOR]

The strain rate sensitivity (m) of Ti- x Al- y Mo- z V and Ti- x Al- y Mo- z Cr bcc-Ti alloys has been investigated as a function of composition by a high throughput approach based on diffusion multiples and mechanical property screening by micropillar compression. It is shown that the influence of a solute species depends on the alloy composition. In particular, increasing levels of Mo result in a decrease of m in Ti- x Al- y Mo- z V alloys, while they do not have a significant effect in the Ti- x Al- y Mo- z Cr system. The measured m values are related to the active deformation systems and to the flow stress and activation volume of the investigated alloys. [ABSTRACT FROM AUTHOR]

Energetic electron beams (e-beams) are used in the welding, 3D printing and imaging of various materials. It is commonly accepted that the e-beam irradiation inside transmission electron microscope has a negligible effect on the intrinsic mechanical properties of small-volume metallic materials during in situ investigation. However, as small-volume Al is usually covered by a thin layer of native oxide, we show here that high-energy (200 keV) e-beam irradiation can dramatically alter the deformation behavior. E-beam irradiation can drastically rejuvenate the amorphous structure of the oxide layer, producing a defective interface/surface, and promoting the transition of the deformation mode from internal dislocation multiplication to interface-dominated dislocation nucleation mediated plasticity, as indicated by the enhanced strain rate sensitivity and reduced activation volume. Moreover, the amorphous oxide layer undergoes viscous flow under the e-beam irradiation, such that nanoscale single-crystal Al confined by a 29% volume fraction of native oxide shell exhibits more than 60% uniform elongation. [ABSTRACT FROM AUTHOR]

The crystal orientation effect on mechanical heterogeneity of LiTaO3 single crystals is well known, whilst the time-dependent plastic behavior, i.e., creep is still short of understanding. Relying on nanoindentation technology, we systematically studied room-temperature creep flows at various holding depths (100 nm to 1100 nm) in three typical orientations namely the X-112°, Y-36° and Y-42° planes. Creep resistance was much stronger in the X-112° plane than the others. In the meanwhile, creep features were similar in the Y-36° and Y-42° planes. The orientation effect on creep deformation was consistent with that on hardness. The nanoindentation length scale played an important role in creep deformation that creep strains were gradually decreased with the holding depth in all the planes. Based on strain rate sensitivity and yield stress, the activation volumes of dislocation nucleation were computed at various nanoindentation depths. The activation volumes ranged from 5 Å3 to 23 Å3 for the Y-36° and Y-42° planes, indicating that a point-like defect could be the source of plastic initiation. In the X-112° plane, the activation volume was between 6 Å3 and 83 Å3. Cooperative migration of several atoms could also be the mechanism of dislocation activation at deep nanoindentation.

The flow stress and work hardening behaviour during uniaxial compression of dilute binary Mg-Sc alloys have been studied at 4.2 K, 78 K and 298 K. The influences of atomic misfit, chemical misfit, solute electronic properties and stacking fault energy (SFE) on strength and ductility have been examined. The results suggest that the small atomic misfit and the reduction of SFE due to Sc additions are decisive factors in determining the mechanical behaviour of the alloys. Mg-Sc alloys deform by basal, prismatic 〈 a 〉 , pyramidal 〈 c + a 〉 slip and extension twinning. Sc exerts limited strengthening on these modes due to the small atomic size and weak solute-dislocation and solute-twin boundary interactions. Constitutive modelling of the flow stress permits for the evaluation of the critical resolved shear stress (CRSS) of slip and twinning systems and determination of their activity during deformation. Analysis of activation volume and activation distance suggests that dislocation-dislocation interactions determine the flow stress. The strain rate sensitivity parameter is unaffected by alloy composition, but it decreases sharply with decreasing temperature. Mg-Sc alloys show some improvement in strength and ductility in comparison to Mg, however not as much as demonstrated by common Mg-RE binary systems. The enhancement in ductility is attributed in part to the activity of twinning and non-basal slip. [ABSTRACT FROM AUTHOR]

A single phase fcc based nanocrystalline solid solution in equiatomic AlCoCrCuFeNi high-entropy alloy (HEA) has been synthesized using ball milling. The milled powders were of “plate-like” morphology and possessed a precise lattice parameter of 3.641 Å. Compaction of ball milled powders into bulk components using spark plasma sintering (SPS) at 1023 K led to the precipitation of ordered bcc (B2). Detailed structural and microstructural investigations on the sintered alloy indicate the presence of bimodal grain size distribution with average grain sizes of 112 ± 46 nm and 1550 ± 500 nm, solid solutions (fcc and B2 phases), dislocations and twin boundaries. A high hardness value of 6.5 ± 0.1 GPa was measured for the sample sintered at 1023 K/15 min using Vickers microindentation. Comprehensive analysis on probable strengthening mechanisms suggests that frictional stress, Taylor hardening, Hall-Petch strengthening, solid solution strengthening and twin boundary strengthening mechanisms are responsible. The Taylor hardening arising from intersection of dislocations and grain boundary (Hall-Petch) strengthening arising from grain boundary-dislocation interactions together account for 85% of the observed flow stress. The Tabor's ratio, (H/σ flow ) attained a value of 2.7 which is in close agreement with that for conventional polycrystalline materials. Nanoindentation at a peak force of 8000 μN yielded a high hardness value of 8.13 ± 0.15 GPa and an elastic modulus of 172 ± 10 GPa. A low strain rate sensitivity of 0.0084 and an activation volume of 13 b 3 (b is 0.23 nm) were measured, suggesting that grain boundaries, twin boundaries and interphase boundaries (fcc/B2) are influential in governing the deformation kinetics. [ABSTRACT FROM AUTHOR]

Formation of chunks of various sizes ranging between 2 and 6 mm was achieved using high-energy ball milling in Al–1at.%Pb–1at.%W alloy system at room temperature during milling itself, aiding inin situconsolidation. X-ray diffraction and transmission electron microscopy (TEM) studies indicate the formation of multi-phase structure with nanocrystalline structural features. From TEM data, an average grain size of 23 nm was obtained for Al matrix and the second-phase particles were around 5 nm. A high strain rate sensitivity (SRS) of 0.071 ± 0.004 and an activation volume of 4.71b3were measured using nanoindentation. Modulus mapping studies were carried out using Berkovich tip in dynamic mechanical analysis mode coupled within situscanning probe microscopy imaging. The salient feature of this investigation is highlighting the role of different phases, their crystal structures and the resultant interfaces on the overall SRS and activation volume of a multi-phase nc material. [ABSTRACT FROM PUBLISHER]

The strain rate sensitivity (m) and the activation volume (V*) of arc melted ingots (AMIs), suction cast rods (SCRs) of lamellar CoCrFeNiNb x (0.45 ≤ x ≤ 0.65) and CoCrFeNiTa y (0.2 ≤ y ≤ 0.5) eutectic high entropy alloys (EHEAs) comprising of FCC γ-Ni and Fe 2 Nb/Co 2 Ta-type Laves phases have been studied at strain rates of 8 × 10-5- 8 × 10-3/s. Furthermore, specimens were cold worked up to 55% strain to investigate the effect of prior deformation on the rate sensitivity in bulk specimen using jump test under compression as well as the localized m in the nanoeutectic matrix and micrometer-size pro-eutectic phases using nanoindentation. The m value of the bulk EHEAs with varying lamellae thickness (λ w) of 70–202 nm lies in between 0.0070 and 0.0082, and the V* value lies in between 24.8 b 3 and 43.7 b 3 . Whereas, the localized m value of the eutectic matrix increases from 0.0066 to 0.0075 upon cold working of x = 0.5 SCR. Cold working increases the dislocation density up to 1016/m2, forms cells, sub-boundaries and distorts the lamellae interface, thus reducing the dislocation mean free path (Ʌ) and V*. The local lattice distortion, lamellae thickness, and the dislocation pile-up at the lamellae interface are correlated with the rate sensitivity and related mechanisms in the solute concentrated EHEAs. [ABSTRACT FROM AUTHOR]

The variation in apparent and true activation volumes of 304L SS with true plastic strain has been evaluated at three temperatures (973 K, 1073 K and 1123 K) over the strain rates of 3 × 10−3 s−1, 3 × 10−4 s−1 and 3 × 10−5 s−1. The instantaneous strain rate sensitivity parameter was derived from strain-rate jump tests across different strain levels to estimate apparent activation volume. The values of apparent activation volume were higher at 973 K than at other temperatures. In contrast to the apparent activation volume, the estimated values of true activation volume using internal-stress-based kinetic rate law systematically increase with the increasing temperature from 973 K to 1123 K. The higher flow stress with a larger strain hardening rate at 973 K leads to a lower true activation volume in the steel compared to other temperatures. The model outcomes revealed that the instantaneous strain rate jump-up or jump-down leads to the change in the internal and effective stresses. The observed variations in geometrically necessary dislocation density (GND) and intra-grain reference orientation deviation (GROD) in the substructure substantiated the predicted variation in the internal stress level upon strain rate jump up/down. The true activation volume was estimated for a fixed value of mobile dislocation density (ρ m = 5 × 1011 m−2) and for varying mobile dislocation density. Irrespective of ρ m values, the estimated true activation volume values for 304L SS were in the order of 102 b3 to 103 b3, thereby implying that the intersection of dislocations coupled with the climb of jogged dislocations is the dominant mechanism during tensile deformation at high temperatures. Based on the analysis using the kinetic rate law with a fixed value of mobile dislocation density, it has been realized that the multiplication of effective stress and true activation volume remains constant across the plastic strain levels for the given combination of strain rate and temperature. • In the range 973–1123 K, higher apparent activation volume ΔV app was noticed at 973 K. • Estimated true activation volume (ΔV true) ranged from 370 b3–1200 b3. • In contrast to ΔV app , ΔV true increases systematically with increasing temperature. • Effective stress (σ e) increases with strain rate jump-up and decreases with jump-down. • For a fixed mobile dislocation density, the product of σ e and ΔV true remains constant. [ABSTRACT FROM AUTHOR]

Two different thermo-mechanical treatments (TMT) were performed on an alumina-forming austenitic (AFA) stainless steel, DAFA 29, which was recently developed at Oak Ridge National Laboratory for application in advanced ultra supercritical power plants. The resulting fine-grained TMT materials showed yield stresses more than double the yield stress of the as-received material at room temperature and more than 50% higher at 600 ºC, but lower yield stresses (but greater elongations) at 700 ºC and above. Strain rate jump tests performed at both 600 ºC and 700 ºC showed that the TMT materials have greater strain rate sensitivity and lower activation volumes at both temperatures. The strain rate has a power law relationship with the flow stress with a stress exponent of ~5. The as-received AFA alloy showed serrated stress–strain curves at 600 ºC suggesting the occurrence of dynamic strain aging. [ABSTRACT FROM AUTHOR]

Using the magnetron sputtering technique, two typical high-entropy alloy (HEA) films namely CoCrFeNiCu (Al-0) with a face-centered cubic (fcc) structure and CoCrFeNiCuAl 2.5 (Al-2.5) with a body-centered cubic (bcc) structure were prepared by alloy targets. The as-deposited HEA films have a columnar-growth mode and nanocrystalline grains. The creep behaviors of both HEA films were systematically investigated by nanoindentation with a Berkovich indenter. The bcc Al-2.5 exhibited a stronger creep resistance than the fcc Al-0. In addition, with the increase of holding load and/or loading rate, the creep deformation was significantly enhanced in the fcc Al-0. Interestingly, it was almost history-independent in the bcc Al-2.5. The creep characteristics of HEA films could be related to the distinct lattice structures, which apparently affect the kinetics of plastic deformation. The strain rate sensitivity (SRS) and activation volume of the dislocation nucleation were carefully estimated for both HEA films. In view of the large differences of activation volumes between Al-0 and Al-2.5, we present discussions to explain the observed creep characteristics in HEA films. [ABSTRACT FROM AUTHOR]

The aim of the present study was to identify the effect of solute Ni element on the plastic deformation and its thermal activation process of 18Cr ferritic stainless steels. Single-crystal micropillars with different diameters of approximately 2 and 5 μm were fabricated on the surface of Fe–18%Cr–Ni alloy sheets containing different contents of Ni (0–2 mass%). Compression tests were performed on the micropillars at different initial strain rates to investigate the strain rate sensitivity (m) of stress for the slip initiation and the specimen-size dependence of the m value. In micropillars with a small diameter of 2 μm, the Fe–18%Cr binary alloy specimen exhibited a relatively higher strain-rate sensitivity of initial slip stress (m = 0.12), whereas m was reduced to 0.01 by adding 2% Ni. The same trend was observed in micropillars with a larger size of approximately 5 μm. The experimental results were utilized to evaluate the activation volume of plastic deformation. The activation volumes of the Fe–18%Cr alloy and alloy containing 1% Ni followed the specimen-size dependence trend of activation volume in bcc metals; however, the alloy containing 2% Ni exhibited larger activation volumes, suggesting a different thermal activation process presumably due to the significant interaction that occurred between dislocations and solute Ni atoms in bcc solid-solutions. [ABSTRACT FROM AUTHOR]

Two TiNi alloys, Ti 49.4 Ni 50.6 and Ti 50 Ni 50 , are subjected to equal-channel angular pressing (ECAP) resulting in the formation of a homogeneous ultra-fine grained microstructure. Tensile tests and strain rate jump tests are carried out in the temperature range of 25–400 °C to measure mechanical properties and strain rate sensitivity of both alloys before and after ECAP processing. Effect of grain size on mechanical behavior, strain rate sensitivity and mechanisms operating during plastic deformation of both alloys in the given temperature range is discussed. [ABSTRACT FROM AUTHOR]

We report the effect of testing conditions on the nanomechanical behavior of the surface and inner core of as-cast plates of Zr 41 · 2 Ti 13 · 8 Cu 12 · 5 Ni 10 Be 22.5 , Zr 52 · 5 Cu 18 Ni 14 · 5 Al 10 Ti 5 , Zr 58 · 5 Cu 15 · 6 Ni 12 · 8 Al 10 · 3 Nb 2.8 and Zr 55 Cu 30 Ni 5 Al 10 bulk metallic glasses (BMGs). A series of instrumented nanoindentation experiments have been performed under control loading rate (CLR) mode at various maximum applied load (P max) in between 100 and 500 mN, and under control strain rate (CSR) mode at strain rates in between 0.001 and 1.0/s at P max = 500 mN on length-width (LW) surface and thickness-width (TW) inner core of 2 mm thick as-cast plates. The solid-liquid like regions in the glassy structure affects the deformation mechanism of BMGs as the liquid like regions triggers the activation of the shear transformation zones (STZs), which promotes formation of shear bands at various testing conditions. The nano-micromechanical behavior has been evaluated in terms of indentation size effect (ISE), strain rate sensitivity (m) and activation volume (V*) under CLR and CSR modes for LW and TW planes. The effect of testing conditions on the rate sensitivity and deformation mechanism have been discussed in terms of size and dynamics of STZs by considering the presence of solid-liquid like regions in the glassy structure. It has been noticed that hardness and rate sensitivity values have decreased and STZ size has increased with increasing P max for both the planes of all the studied BMGs. [ABSTRACT FROM AUTHOR]

High-energy ball milling of conventional coarse-grained aluminium and nanocrystalline W in an Al-10 at.%W composition results in the formation of a two-phase mixture of Al and W with nanocrystalline features. Subsequent compaction of these powders using spark plasma sintering (SPS) at 748 K resulted in the formation of an Al12W phase in the nanocrystalline aluminium matrix. It is suggested that the mere attainment of nanocrystallinity was not enough to trigger a reaction between Al and W to form Al12W but that sufficient thermal activation was also required, as supplied during SPS. The second-phase particles (~175 nm in size) are uniformly distributed in the nanocrystalline Al matrix having a grain size of ~40 nm. The nanocomposite possessed a high hardness of 5.42 ± 0.33 GPa and an elastic modulus of 145 ± 5?GPa, both measured using depth-sensing nanoindentation. At room temperature, this novel nanocomposite exhibited a strain rate sensitivity (SRS) of 0.024 ± 0.001 and an activation volume in the range of 3.78–3.88 b3. Interfacial regions, viz. grain boundaries and triple junctions in the matrix and the reinforcement, matrix/particle boundaries, etc. could be influential factors in deciding the SRS and the activation volume. A scanning probe microscope image of the nanoindent shows a plastic flow region around the periphery of the indent. [ABSTRACT FROM AUTHOR]

The strain rate sensitivity, activation volume and exhaustion of mobile dislocation density in nanocrystalline (NC) Cu-11.1at%Al alloy with low stacking fault energy were studied by strain rate jump tests and repeated stress relaxations at room temperature. It was found that in comparison with NC Cu with similar grain size, the strain rate sensitivity of NC Cu-Al alloys is much small, which might due to the low SFE and the tendency of chemical short range order formation during plastic deformation. The small physical activation volume (~20b³) was determined, which could be ascribed to the dislocation processes mediated by grain boundaries, twin boundaries. Meanwhile, a lower exhaustion rate of mobile dislocation density was found, possibly due to the low stacking fault energy enhancing dislocation storage capability, the high densities of nanotwins preserving mobile dislocations and a lower dislocation velocity in concentrated alloys. [ABSTRACT FROM AUTHOR]

The long-term stress relaxation tests with a relaxation time of about 7 h are performed on the bulk dense nanocrystalline (with a mean grain size of 27 nm) pre-deformed at strain rate from 4.17 s-1- 4.17 × 10-6 s-1, where a phenomenon that the initial relaxation behavior of nc depends on itself deformation history. That is, the nc s pre-deformed at higher strain rate (not less than 4.17 × 10-3 s-1) exhibit a three-staged relaxation process from the initial near linear rapidly stress delayed (LRSD) stage and the subsequent lumber nonlinear stress delayed (LNSD) one and the final near linear slowly stress delayed (LSSD) one while only the later two stages are observed for the nc s pre-deformed at low strain rate. The three-stage relaxation behavior is attributed to the transition from the initial dislocation-dominated plasticity to the mixture of dislocation motion and diffusion-based GB activity and finally to the entire diffusion-based GB activity including GB sliding or grain rotation in the rate-controlling deformation mechanism, which was illuminated by the attained three-staged strain rate sensitivity and activation volume and the exhaustion of mobile density of deformed nc in the first two stages of relaxation. Such rate-controlling deformation mechanism well interpreted the macroscopic tensile mechanical behavior of nc and simultaneously an optimizing strategy in improving the ductility of nc is also mentioned. [ABSTRACT FROM AUTHOR]

Abstract: We investigate the tensile properties, strain rate sensitivity, and activation volume of nanostructured Ti prepared by equal channel angular pressing plus a series of thermal treatment. Simultaneous enhancement of strength and tensile ductility is achieved. The strain rate sensitivity is found to decrease with decreasing grain sizes (∼150–450nm). A new Hall–petch relationship is presented and discussed. The analyses on the internal friction peaks at around −70°C suggest that twins could play the primary role for the strain hardening of the annealed nanostructured Ti, consistent with the miniscule activation volumes. [Copyright &y& Elsevier]

Abstract: In this study, the constitutive equation and DRX(Dynamic recrystallization) model of Nuclear Pressure Vessel Material 20MnNiMo steel were established to study the work hardening and dynamic softening behavior based on the flow behavior, which was investigated by hot compression experiment at temperature of 950°C, 1050°C, 1150°C and 1250°C with strain rate of 0.01s−1, 0.1s−1 and 10s−1 on a thermo-mechanical simulator THE RMECMASTOR-Z. The critical conditions for the occurence of dynamic recrystallization were determined based on the strain hardening rate curves of 20MnNiMo steel. Then the model of volume fraction of DRX was established to analyze the DRX behavior based on flow curves. At last, the strain rate sensitivity and activation volume V * of 20MnNiMo steel were calculated to discuss the mechanisms of work hardening and dynamic softening during the hot forming process. The results show that the volume fraction of DRX is lower with the higher value of Z (Zener–Hollomon parameter), which indicated that the DRX fraction curves can accurately predicte the DRX behavior of 20MnNiMo steel. The storage and annihilation of dislocation at off-equilibrium saturation situation is the main reason that the strain has significant effects on SRS(Strain rate sensitivity) at the low strain rate of 0.01s−1 and 0.1s−1. While, the effects of temperature on the SRS are caused by the uniformity of microstructure distribution. And the cross-slip caused by dislocation piled up which beyond the grain boundaries or obstacles is related to the low activation volume under the high Z deformation conditions. Otherwise, the coarsening of DRX grains is the main reason for the high activation volume at low Z under the same strain conditions. [Copyright &y& Elsevier]

Abstract: Low temperature (77K, 243K and 255K) mechanical tests were carried out at strain rates between 7.10−4 s−1 and 5.10−2 s−1 on pure ultrafine grained (UFG) copper (grain size 100nm) to mimick room temperature high strain rates. Variation of the activation volume measured as a function of the stress is consistent with the analytical model proposed in [C. Duhamel, Y. Bréchet, Y. Champion Int. J. Plast. 26 (2010) 747–757] to explain the rheology of UFG metals. This model is based on dislocations interactions at grain boundaries and grain boundaries sliding. The experiments confirm that the strain rate sensitivity (which should be a criterion for plasticity in absence of macroscopic work hardening) increases with the strain rate (or stress) at high strain rate, as also observed for nanotwinned copper. From the model, a strain rate sensitivity criterion is derived, function of a characteristic UFG strength parameter and grain boundary properties. The analytical description of deformation of UFG including microstructural properties should help for a quantitative control of the macroscopic properties of UFG. [Copyright &y& Elsevier]

Abstract: The present study attempts to investigate room temperature and high temperature flow behaviour of a tungsten heavy alloy (W-7.10Ni–1.65Fe–0.5Co–0.25Mo alloy). The alloy was deformed under compression at room temperature and elevated temperatures (400–700°C) at different strain rates (1–0.0001s−1) to observe plasticity under compression loading at these temperatures and strain rates. The alloy showed higher plasticity and positive strain rate sensitivity at room temperature. Samples hardness after 70% deformation at room temperature increased from 3.20±0.14 to 5.08±0.03GPa. Barreling was observed in room temperature compression tested samples. Microstructure of the alloy after heavy compressive deformation at room temperature showed that severe deformation of W grains took place along a direction at 45° to the direction of applied stress. The alloy showed varying (positive and negative) strain rate sensitivity at elevated temperatures. Samples hardness after 70% deformation at elevated temperatures increased from 3.20±0.14 to 4.60±0.23GPa. At 600 and 700°C, the specimens failed by shear along the direction which is at an angle 45° to the direction of applied stress. Microstructural evidences indicate that the failure at these elevated temperatures seems to be triggered by the excessive void generation in the matrix and their coalescence under the influence of the applied stress. Room temperature deformation mechanism of tungsten heavy alloy was also studied by carrying out tensile testing at room temperature in strain rates ranging from 0.1 to 0.0001s−1. The values of strain rate sensitivity, and apparent activation volume with respect to different level of strain suggest that the deformation mechanism of the alloy is similar to that of bcc metals such as tungsten and molybdenum, i.e. Peierls mechanism controls the dislocation motion. [Copyright &y& Elsevier]

Abstract: Quasi-static and dynamic compression tests on ultra-fine grained (UFG) copper (Cu 99.9) were performed at temperatures ranging from 77 to 573K and strain rates ranging from 10−3 to 5×103 1/s using an electronic universal testing machine and the split Hopkinson pressure bar system, respectively. We focused on the strain rate sensitivity and its dependence on experimental temperatures. The results show that UFG-Cu has enhanced strain rate sensitivity that is apparently temperature dependent. The activation volume of UFG-Cu is estimated to be on the order of ∼10b 3 at current experimental temperature and strain rate range. The plastic deformation mechanism is suggested to be dislocation–grain boundary interactions. [Copyright &y& Elsevier]

Deformation behavior of Gleeble-simulated inter-critical heat affected zone (ICHAZ) of modified 9Cr-1Mo steel has been studied using instrumented microhardness testing. The strain rate sensitivity exponent and activation volume of ICHAZs produced from steels with different prior austenite grain sizes (PAGS) have been estimated. In the simulated ICHAZ, the strain rate sensitivity exponent is lower for larger PAGS (100-120 µm) than for smaller PAGS (20-30 µm), while the activation volume is higher for larger PAGS. The slope of the loading line at any given point was higher in the simulated ICHAZ of the steel with larger PAGS, indicating that the ICHAZ in the steel with larger PAGS is more resistant to deformation than in the steel with smaller PAGS. [ABSTRACT FROM AUTHOR]

Abstract: An aged Al–5Zn–1.6Mg alloy with fine η′ precipitates was grain refined to ∼100nm grain size by means of confined channel die pressing. Microstructure observations and mechanical tests were carried out to characterize the materials before and after various degrees of severe plastic deformation. Deformation processing enhanced the strength of the alloy, but limited its work hardenability. An analysis of deformation mechanisms revealed that plasticity proceeded by dislocation slip through ultrafine-grained cellular and subgrain arrangements. η′ precipitates strengthened the alloy by dispersion hardening, but retarded an increase in the strain rate sensitivity during grain refinement. The influence of η′ precipitates is discussed with respect to their effect on dislocation configurations and deformation mechanisms during processing of the alloy. [ABSTRACT FROM AUTHOR]

A bulk and dense nc Ni-24.7% Co with an average grain size of 15nm was fabricated by a direct current electrodeposition. This Ni-24.7%Co exhibits very high tensile strength of 1813MPa to 2232MPa with relatively good tensile ductility of 6.0%~9.6% under tensile test over a wide strain rate range of 0.417s-1 ~ 1.35×10-5s-1. The combination of high strength and good ductility should be attributed to the increased strain hardening ability induced by the addition of alloying Co element. The interaction of dislocation and grain boundaries is the rate-controlling deformation mechanism controlled in the Ni-24.7%Co based on its high strain rate sensitivity of 0.029 and small activity volume of ~14b3. [ABSTRACT FROM AUTHOR]