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The primary objective of this contribution is to numerically and graphically evaluate engineering stress–strain curves, transform them into true stress–strain curves, and de-scribe the key points of material processed by cold rolling with strains of εRoll = 0%, 10%, 30%, and 50%. The initial and final conditions for uniform plastic deformations have been described. The initial point of uniform deformation lies above the onset yield strength value (σT,S > RP0,2). The necking point, as the final point of uniform deformation, was determined as the intersection point of the curves of the true stress–strain and strain hardening rate. The strain hardening coefficient and the recovery rate, as a function of cold rolling deformations, were derived. Convex polyhedra were derived which describe the dependencies of the development of maximal strain hardening rate values (θMax) and initial strain hardening rates (θ0) as a function of cold rolling deformations and the diameter of grain. The decisive point at which the curves showed a local maximum was a cold rolling deformation εRoll = 30%. The saturation stress required to initiate dynamic recovery of the microstructure is significantly higher than the stress necessary for necking (σT,Sat > σT,Neck). The saturation strain required to initiate dynamic recovery of the microstructure is significantly higher than the strain needed for necking formation (εT,Sat > εT,Neck).

Laser powder bed-fused Ti6Al4V alloy has numerous applications in biomedical and aerospace industries due to its high strength-to-weight ratio. The brittle α′-martensite laths confer both the highest yield and ultimate tensile strengths; however, they result in low elongation. Several post-process heat treatments must be considered to improve both the ductility behavior and the work-hardening of as-built Ti6Al4V alloy, especially for aerospace applications. The present paper aims to evaluate the work-hardening behavior and the ductility of laser powder bed-fused Ti6Al4V alloy heat-treated below (704 and 740 °C) and above (1050 °C) the β-transus temperature. Microstructural analysis was carried out using an optical microscope, while the work-hardening investigations were based on the fundamentals of mechanical metallurgy. The work-hardening rate of annealed Ti6Al4V samples is higher than that observed in the solution-heat-treated alloy. The recrystallized microstructure indeed shows higher work-hardening capacity and lower dynamic recovery. The Considère criterion demonstrates that all analyzed samples reached necking instability conditions, and uniform elongations (>7.8%) increased with heat-treatment temperatures.

Additive manufacturing is increasingly used to produce metallic biomaterials, and post-processing is gaining increasing attention for improving the properties of as-built components. This study investigates the effect of work hardening followed by recrystallisation annealing on the tensile and nanoindentation behaviour of laser powder bed-fused (LPBF) 316L stainless steel, with the aim of optimising its mechanical properties. As-built and thermally stabilised (at 900 °C) specimens were prestrained in a uniaxially tensile manner at room temperature (0.12 plastic strain, ~75% of maximum work hardening) and subsequently annealed (at 900 °C or 1050 °C for 1 h). The microstructure and mechanical properties were then characterised by optical microscopy, SEM, EBSD, XRD, nanoindentation, and tensile testing. It was found that prestraining increased yield tensile strength (YTS) 1.2–1.7 times (to 690–699 MPa) and ultimate tensile strength (UTS) ~1.2 times (to 762–770 MPa), but decreased ductility 1.5 times. Annealing led to recovery and partial static recrystallisation, decreasing YTS (to 403–427 MPa), restoring ductility, and increasing the strain hardening rate; UTS and indentation hardness were less affected. Notably, the post-LPBF thermal stabilisation hindered recrystallisation and increased its onset temperature. Mechanical property changes under prestraining and annealing are discussed with respect to microstructure and crystalline features (microstrain, crystal size, dislocation density). All specimens exhibited ductile fractures with fine/ultra-fine dimples consistent with the as-built cellular structure. The combined treatment enhanced tensile strength whilst preserving sufficient ductility, achieving a strength–ductility product of 40.3 GPa·%. This offers a promising approach for tailoring LPBF 316L for engineering applications.

At room temperature, the mechanical properties and microstructural evolution of magnesium-calcium alloys (Mg-Ca) were investigated. Magnesium with calcium 0.3, 0.9, 1.5, 2.1, and 2.7 wt.% alloys were cast in an SF6 inert atmosphere. When the amount of calcium in the alloy changes from 0.3 to 2.7 wt.%, the grain size of the alloy was refined from 716 to 47 micrometres. Hardness (23-47 HV) was found to increase with an increase in Ca addition up to 1.5 wt.%. The work-hardening behaviour of Mg-Ca alloys deformed in tension was analysed and using Considere’s criteria, the absence of necking phenomena was found for all five alloys. Analysis of wear tests showed that the average COF values for Mg-0.3Ca (0.554) and Mg-2.7Ca (0.644) were higher than it was for other compositions with a similar average COF (∼0.3) value. Microstructural properties, such as grain size, volume fractions of Mg2Ca precipitates, and change in grain morphology (equiaxed into dendrites), were correlated to mechanical performance and wear rate variation to determine the optimal alloy composition.

The mechanical characteristics of hydrate-bearing sediments (HBS) are not only dominated by the cementing and filling effects of hydrate, which makes them show the characteristics of overconsolidated soil, but also easily become overconsolidated sediments under the influence of the surrounding environment. In this paper, marine clay collected from hydrate-bearing areas in the South China Sea and quartz sand were used to remold hydrate-bearing clayey–silty sediments (HBCSS). A series of consolidated-drained triaxial tests of HBCSS were carried out to obtain the mechanical characteristics of overconsolidated fine-grained HBS under the combined influence of overconsolidation ratio (OCR) and confining pressure. The results show that the improvement in OCR can significantly improve the strength and stiffness of HBCSS and easily lead to strain softening and dilatation. Increasing confining pressure can weaken the strain softening caused by OCR, while particle breakage is easily caused by double compaction of overconsolidation and confining pressure. In addition, a new parameter strain hardening rate was proposed, which can effectively describe the hardening and softening of stress–strain relationship. The effect of overconsolidation on the mechanical properties of HBCSS is essentially compaction, and the shearing mechanism lies in the continuous evolution of particle breakage and compressive hardening.

The hot compression tests of Al-5.6Zn-2Mg aluminum alloy were conducted on a universal testing machine at temperature of 300–500 °C and strain rate of 0.1–0.0001 s −1 . The work hardening rate curves for the σ c and ε c for the onset of dynamic recrystallization were identified. The correlation among the key features σ c ( ε c ), σ p ( ε p ) and σ SS , and the Z coefficient are determined. Four constitutive models include the Arrhenius-type model, modified Johnson Cook (MJC), modified Zerilli-Armstrong (MZA), and an artificial neural network (ANN) developed. The results showed that the ANN and Arrhenius-type models had the lowest AARE values of 0.486% and 3.36%, while the MZA and MJC models had higher AARE values of 8.84% and 3.93%, respectively. The Arrhenius-type model was found to be the most appropriate prediction model due to its ability to handle the nonlinear relationship among factors, but the MJC model could be a simpler alternative in cases where material properties are unknown or experimental data are limited. The MZA model was found to be unsuitable for estimating flow stress in hot compression. In addition, the highest predictive performance is seen in the best-trained ANN model, with an AARE of 0.486% and an R value of 0.99.

For the evaluation of cavitation erosion resistance of metallic materials, it has always been a time-consuming technical problem. This paper innovatively proposed a parameter named cavitation hardening rate to characterize the plastic deformation and work hardening behavior of metallic materials during the cavitation process and then found a positive correlation between cavitation hardening rate and strain hardening index. Finally, the relationship between cavitation volume loss and strain hardening index was obtained, and a simple and rapid evaluation method for cavitation erosion resistance was established.

This study investigated the influence of the initial grain size on the plastic deformation and tunnel defects that occurred from friction stir welding of CoCrFeMnNi high entropy alloys (HEAs). The rolled and cast HEAs had a grain size of 2.8 and 308 μm, respectively. After friction stir welding, the cast HEA weld had a grain size of 1.8 μm, which was coarser than that of the rolled HEA weld (1.4 μm). Therefore, the dynamic recrystallization ratios were 60.7 and 99.6% for the rolled and cast HEAs, respectively. The cast HEA weld with a large grain size contained a higher density of high-angle boundaries and twins than the rolled HEA weld with the small grain size. The cast HEA had a larger resistance to plastic deformation owing to the larger fraction of ∑3 twin boundaries than the rolled HEA during friction stirring. This was associated with the high strain hardening rate during tensile testing and to the significant amount of W dissolved from the stirring tool in the cast HEA weld. Thus, the cast HEA weld had a higher tunnel defects ratio than the rolled HEA weld. The total unbonded ratios of the rolled and cast HEA welds were 0.2 and 7.2%, respectively, indicating that the rolled HEA had better weldability than the cast HEA.Graphic Abstract

The mechanical behavior of VCoNi medium-entropy alloys with five different grain sizes at three different temperatures was investigated. The VCoNi alloys with different grain sizes exhibit a traditional strength–ductility trade-off at 77 K, 194 K and 293 K. Both the yield strength and the uniform elongation of the VCoNi alloys with similar grain size increase with decreasing the deformation temperature from 293 to 77 K. Obvious strain hardening rate recovery characterized by an evident up-turn behavior at stage II is observed in VCoNi alloys with the grain size above 11.1 μm. It is found that the extent of the strain hardening rate recovery increases with increasing grain size or decreasing deformation temperature. This may mainly result from the faster increase in the dislocation multiplication rate caused by the decrease in the dislocation mean free path, the decrease in the absorption of dislocations by grain boundaries and the dynamic recovery from the cross-slip with increasing grain size, as well as the suppressed dynamic recovery at cryogenic temperatures. The critical grain sizes for the occurrence of the recovery of strain hardening rate are determined to be around 9.5 μm, 8.3 μm and 3 μm for alloys deformed at 293 K, 194 K and 77 K, respectively. The basic mechanism for the strain hardening behavior of the VCoNi alloys associated with grain size and deformation temperature is analyzed.

In the present investigation, stress–strain curves and strain hardening rates on samples rolled at ambient temperature with thickness reductions of 0%, 10%, 30%, and 50% were studied. On the processed samples, static tensile tests at ambient temperature were performed. Transformation of the engineering stress–strain curves to true stress–strain curves and their numerical processing by first derivation (θ = dσ/dε) was carried out. Dependencies θ = f(εT) characterizing the strain hardening rates were derived. From the curves and the true stress–strain and strain hardening rates, the three stages describing different rates of strain hardening were identified. A rapid increase in true stress and a rapid decrease in the strain hardening rate in Stage I were observed. Quasi-linear dependencies with an increase in true stress but with a slow, gradual decline in the strain hardening rate in Stage II were obtained. Slowly increasing true strains, accompanied by a decrease in strain hardening rates and their transition to softening, led to the formation of plastic instability and necking in Stage III. The endpoints of the strain hardening rate depending on the cold rolling deformations lie in the following intervals: θStage I ∈ <1904;3032> MPa, θStage II ∈ <906;−873> MPa, θStage III ∈ <−144;−11,979> MPa. While in Stage I and Stage II, the plastic deformation mechanism is predominantly dislocation slip, in Stage III, the plastic deformation mechanism is twinning accompanied by dislocation slip.