Strain-hardening Rate Research Articles

Effect of tempering conditions on adiabatic shear banding during dynamic compression and ballistic impact tests of ultra-high-strength armor steel

In this study, roles of strain-hardening rate on susceptibility of adiabatic shear band (ASB) formation and subsequent cracking were investigated in two ultra-high-strength armor steel plates heat-treated differently. The quenched and tempered steel contained ~2% of retained austenite in the tempered martensitic matrix, while the quenched and austempered steel contained ~4% retained austenite in the bainitic matrix partly with the tempered martensite. The actual ballistic impact test results revealed the lower sensitivity of ASB formation in the austempered steel than in the tempered steel, which corresponded well to the higher critical strain for ASB formation in the dynamic compressive test using a laboratory-scale split Hopkinson pressure bar (SHPB). The austempered steel caused the higher internal stress among various constituents, and all the retained austenite transformed into martensite during the deformation, thereby leading to transformation-induced plasticity (TRIP) effect. The higher strain-hardening rate induced by these higher internal stress and TRIP effect increased resistance to ASB formation, which was confirmed by a calculation of ASB susceptibility. Thus, the austempered steel was much less susceptible to the ASB formation during the ultra-high-speed deformation. Consequently, the increased resistance to ASB formation retarded the initiation and propagation of ASBs and cracks.

Effect of silicon addition on the microstructures, mechanical properties and helium irradiation resistance of NiCoCr-based medium-entropy alloys

A series of NiCoCrSix (x = 0, 0.1, 0.2, 0.3) medium-entropy alloys were synthesized to investigate the effect of Si addition on the microstructures, mechanical properties and helium irradiation resistance of the NiCoCr-based alloys. The microstructures and the phase evolution of these alloys were examined using electron microscopy as well as the existing models. The results show that the alloys exhibit a single face-centered-cubic structure when the Si content is in the range of 0–6.25 at.%, whereas phase decomposition occurs as the Si content further increases (∼9 at.%). Comparing with NiCoCr, NiCoCrSi0.2 exhibited improved strength without sacrificing the ductility. The yield strength and ultimate tensile strength increased from 438 to 599 MPa and 921 to 1032 MPa, respectively, whilst the ductility kept as high as ∼50%. Our analysis indicates that the increased strength is mainly ascribed to the grain boundary strengthening. The large ductility is primarily due to the low stacking fault energy and shear modulus of the NiCoCrSi0.2 alloy, which makes mechanical twinning in small grains accessible and provides a steady strain-hardening rate in a wide strain regime. Besides, the enhanced compositional complexity and local lattice distortion induced by the introduction of Si can improve the helium irradiation resistance of NiCoCr, resulting in reduced average bubble size.

Effects of strain rate and strain hardening on a cantilever under impact

An investigation is carried out to assess the response of elastic–plastic and ductile circular section cantilevers subjected to blast wave loading. A mathematical equation is formulated for the effects of strain rate and strain hardening for ductile (rigid–plastic) cantilever subjected to impact. It is assumed that the cantilever deforms when the moment produced by drag force about the clamped end exceeds the maximum resisting moment of the material, resulting in an equation for deformation. For the transient and modal phase, a correction factor is proposed, which improves the accuracy of response prediction. Full-scale explosion tests are carried out at Woomera (Australia) to investigate the response of cantilevers subjected to air-blast loading, where sixty cantilever test specimens were tested. The test specimens used were designed on the basis of different approaches such as elastic, elasto-plastic, quasi-static, impulse technique and plastic approach. Correction factor ‘K’ for the effects of strain rate and strain hardening were developed and used during the design of cantilever specimens. The prediction of a mathematical model for the response of cantilevers was in line with the response of test specimens during field explosion tests due to the modification factor for the effects of strain rate and strain hardening.

Microstructure, Tensile Properties and Fracture Behavior of Squeeze-Cast Mg Alloy AZ91 with Thick Cross Section

As the most popular magnesium alloy, AZ91 is desired in the automotive-related manufacturing industry because of its high specific strength and good corrosion resistance. AZ91-based automotive components are made by conventional high-pressure die casting (C-HPDC) and are unable to provide adequate mechanical properties to components with heavy cross sections because of the high level of gas and shrinkage porosity in C-HPDC parts. To expand the usage of AZ91, a replacement for the C-HPDC to minimize the porosity content is needed. In this research, Mg alloy AZ91 with a cross-sectional thickness of 10 mm was squeeze cast (SC) under an applied pressure of 90 MPa. A counterpart was made by C-HPDC for comparison with SC AZ91. The porosity of the SC and C-HPDC AZ91 specimens was evaluated by optical microscopy and density measurements. The SC AZ91 was almost pore free, whereas the counterpart had a porosity of 3.57%. Microstructural analyses by scanning electron microscopy showed that the SC and C-HPDC AZ91 contained phases of primary α-Mg, β-Mg17Al12 eutectic and Al-Mn intermetallic. Tensile testing indicated that the ultimate tensile strength and elongation (ef) of the SC alloy were 171.7 MPa and 3.0%, which resulted in improvements of 33 and 77% compared with those of the C-HPDC counterpart. The strain-hardening rate and tensile toughness of the SC AZ91 during deformation were higher than those of the HPDC specimen. SEM fractography revealed characteristics of plastic deformation on the flat fractured surface of the SC AZ91, whereas brittle fracture prevailed on the HPDC counterpart.

The role of Mn on twinning behavior and tensile properties of coarse- and fine-grained Fe–Mn–C twinning-induced plasticity steels

The twinning behavior, strain hardening and tensile properties of coarse- and fine-grained Fe-13/18/22Mn-1.0C twinning-induced plasticity (TWIP) steels were investigated by microstructural observations and mechanical tests. These steels have stacking fault energy ranging from ~27 to ~37 mJ/m2 with increasing Mn content, and thus deformation twinning is the dominant mechanism. Tensile test results show that, with increasing Mn content, the yield and tensile strengths decrease slightly for both coarse- and fine-grained steels while the ductility increases significantly, especially for coarse-grained steels. Microstructural observations indicate that, at lower strains, an increase in Mn content suppresses the formation of twins, resulting in a lower twinning rate; however, at intermediate and higher strains, even higher twinning rate is observed in higher-Mn steels. In addition, increasing Mn content enables deformation twins to form in a wider range of strain. Such a varying twinning behavior with Mn content is even more apparent for coarse-grained steels. Furthermore, the sizes of twins vary with Mn content and also with grain size: both the thickness and spacing of twins or twin bundlers increase with increasing Mn content but decrease with refining grain sizes. Moreover, increasing Mn content decreases strain-hardening rate, but allows persistent strain hardening to higher strains. These strain-hardening behaviors and tensile properties observed in the twinning-dominated Fe–Mn–C steels with different Mn contents are correlated well their microstructural evolutions.

Effect of constitutive material model on the finite element simulation of shear localization onset

One of the most challenging problems in the field of machining is to determine the onset of shear localization. The consequences of the emergence of shear localized chips are fluctuations in the machining forces, tool wear, deterioration of the surface quality and out-of-tolerance machined components. Several constitutive material models are developed for the simulation of shear localization during machining, especially for Ti6Al4V. However, the accuracy and capability of the proposed models for the prediction of shear localization onset have not been investigated yet. In this study, the effect of different constitutive material models in the prediction of shear localization onset has been investigated. Different material models are studied including the Johnson-Cook (J-C) material model with Cockcroft-Latham damage model, J-C material model with a J-C damage model, models based on modified J-C material models (MJ-C) with strain softening terms, and material model with power-law type strain hardening and strain rate sensitivity, with polynomial thermal softening and polynomial temperature-dependent damage. The results of the finite element models are verified using orthogonal cutting experiments in terms of chip morphology and machining forces. Metallography techniques are used along with SEM observations to elucidate the distinction between continuous and shear localized chips. The results of this study indicate that three models are capable of predicting shear localization onset. However, when compared to the experiments, where a critical cutting speed of 2.8 m/min is obtained for shear localization onset, the results revealed that the model proposed by Sima and Ozel (2016) which is a model based on MJ-C model with temperature-dependent overarching modifier and temperature-dependent material model parameters is more accurate for the prediction of shear localization onset during machining Ti6Al4V. This model is shown to reveal a good prediction for the machining forces as well.

The effect of solid solution and gamma prime on the deformation modes in Ni-based superalloys

Understanding the deformation, strengthening and failure mechanisms in polycrystalline nickel-base superalloys is necessary to develop next generation alloys for application in highly demanding environments. Here, the aim is to examine the various ways in which solution- and γ’ precipitation-strengthening affect the deformation behaviour of three Ni-based superalloys through deformation mapping and investigation at multiple length-scales. This is achieved using high-resolution digital image correlation to quantify local strain, electron backscattered diffraction for lattice rotations and electron channelling contrast imaging to investigate dislocation-mediated mechanisms of deformation. This approach bridges the gap between nano-scale microscopy of dislocation-γ’ interactions and macro-scale measurements of mechanical properties, such as yield stress, flow stress and the strain-hardening rate. Deformation in solution-strengthened alloys progresses by a slip band refinement mechanism, which results in low levels of dislocation pileup at grain boundaries and so better grain deformation compatibility with neighbours. Deformation in the γ’-strengthened alloys evolves through a glide plane softening mechanism and the resulting high strain localisation impinges on grain boundaries, creating diffuse strain regions at the boundary. In the coarse-γ’ variant there is more Orowan looping and cross slip around larger precipitates, more slip planes are active and more grain-scale cross slip takes place, resulting in greater local friction stresses and therefore greater macroscopic flow stresses and strain-hardening rates. We provide evidence of greater interaction between intersecting non co-planar slip bands in the γ’-strengthened alloys, which contributes to the strain-hardening by progressively decreasing the slip distance. This mechanism is not observed in the solution-strengthened alloy.

A phenomenological model utilizing Voce’s strain-hardening model describing high strain rate behavior of alloy 800H during hot working

In this study, a simple mathematical model is introduced to predict the stress–strain curves under the occurrence of minor dynamic recrystallization. In this regard, first, with the assumption of nonlinear variation of strain-hardening rate versus strain, a mathematical equation was derived to obtain the strain-hardening rate. Then, using the Voce’s equation, a new mathematical model was developed to predict the hot deformation behavior of metallic alloys at high strain rates. The most important feature of the proposed model is its simplicity so that the only constant of the stress–strain equation can easily be interpolated at different temperatures and strain rates. Accordingly, it was found that the defined material constant varies linearly and exponentially with strain rate and temperature, respectively. The high correlation factors (average correlation factor, r2 = 0.85) obtained from regression analysis showed that the model can provide a precise estimation of flow stress curves for the defined deformation conditions.

Length-scale dependent deformation, strengthening, and ductility of fcc/fcc Ni/Al nanolaminates using micropillar compression testing

The deformation and strengthening behaviors of face-centered cubic/face-centered cubic (fcc/fcc) nickel (Ni)/aluminum (Al) nanolaminates were systematically investigated in uniaxial compression tests using micropillars with a varied range of individual layer thickness h from 5 nm to 100 nm. The deformation behavior of micropillar samples of Ni/Al nanolaminates was strongly dependent on the individual layer thickness of Ni and Al, where at larger h the deformation of micropillars took place first by plastic deformation of the Al layers and proceeded to dislocation pileup–based Hall–Petch (H-P) strengthening, which further nucleated in the Ni layers. The deformation behavior of the Ni/Al nanolaminates transitioned from dislocation-dominated homogeneous co-deformation with multiple shearing in Ni and Al layers and grain boundary–mediated massive fractures in the Ni layers at larger h to stress-concentrated broken interfaces and valleys with plastic barreling due to layer shearing and smaller fractures in the Ni and Al layers at smaller h. With decreasing h from 100 nm down to 5 nm, the Ni/Al micropillars exhibited a gradual transition from lower to higher strain hardening, and the strain-hardening rate reached a peak at h = 20 nm; while it started declining with further decreasing of h. The yield strength (σ0.2%) and flow strength (σ8%) of the Ni/Al micropillars reached the peak of ~1.93 GPa and ~2.35 GPa, respectively, with decreasing h to 10 nm. The confined layer slip (CLS) strengthening mechanism operated well at h = 10 nm, whereas H-P strengthening model was applicable for h = 20–100 nm, and surprisingly the strengthening at h = 5 nm was independent of h. Interestingly, significant strain softening was observed at h = 5 nm due to the severe plastic deformation at the broken interfaces, interfacial sliding of layers, and defects related to the presence of intermixed Ni and Al layers at the interface, and valleys between the islands of stacked constituent layers.

Achievement of High Ductility and Ultra-high Strength of V-Nb Microalloyed Spring Steel by Austempered Multiphase Microstructure

A V-Nb microalloyed spring steel was investigated through a series of austempering heat treatments. Optimal properties were obtained after austempering at 240 °C for 2 hours because of the good coordination of multiphase martensitic/bainite, filmy retained austenite and fine carbides. The tensile strength was 2239 MPa and the total elongation was 16.1 pct. With an increase in temperature, the matrix changed from martensite to bainite. The volume fraction of retained austenite decreased from 13.7 pct at 240 °C to 9.6 pct at 280 °C and then increased to a maximum of 19.2 pct at 360 °C. The tensile strength remained above 1500 MPa and the impact energy increased to 30 J. MC carbides formed in martensite lathes and ɛ- or θ-type carbides with acicular nanoscale were precipitated in bainite lathes at a low temperature. MC- and M3C-type precipitated in bainite lathes at high temperature. Samples that were subjected to a low isothermal temperature had a high strain-hardening rate before necking, whereas samples at a high isothermal temperature could exert a TRIP effect for a long time. After stretching to fracture, a hierarchical nanotwinned structure and many shear cells formed, which enhanced the austempered sample ductility and toughness at 360 °C.

Nitrogen effect on hydrogen diffusivity and hydrogen embrittlement behavior in austenitic stainless steels

Hydrogen embrittlement behavior of three austenitic stainless steels with 0.1–0.3 wt% N was investigated by electro-chemical charging of hydrogen. The degradation of tensile properties was alleviated by nitrogen. Hydrogen permeation testing revealed that the increase of nitrogen content decreased the hydrogen diffusivity, and thereby, reduced the depth of hydrogen-affected zones. Intergranular cracks were formed in an early stage of deformation by hydrogen charging and reduced the strain-hardening rate. The cracks propagated further into the grains along martensite/austenite interfaces especially in steels with less nitrogen.

The effect of iron on the microstructure and mechanical properties of a cast Cu–12Sn-1.5Ni (wt. %) alloy

Effect of Fe content on microstructure and mechanical properties in a gravity-cast Cu–12Sn-1.5Ni (wt. %) alloy has been investigated. Addition of 0.053, 0.63 and 1.44 wt % Fe caused grain refinement, vanishment of continuous interdendritic δ phase, and formation of nano-sized iron precipitates within the dendritic matrix. There was a simultaneous increase in the yield and tensile strength, uniform and total elongation and hardness of Cu–12Sn-1.5Ni-(0.05–1.5)Fe alloys relative to their Fe-free counterpart. Fracture surface analysis revealed an intergranular fracture along the interdendritic δ phase in Cu–12Sn-1.5Ni alloy. But in Cu–12Sn-1.5Ni-(0.05–1.5)Fe alloys, after initiation at interdendritic area, crack propagated into the dendrite matrix in a dimple rupture mode. The evolution mechanism of mechanical properties upon alloying content was discussed in terms of the microstructure characterization, analysis of strain-hardening rate, fractographic examination, as well as the strengthening model. This work sheds light on the alloying effect of minor Fe below 1.0 wt % on the microstructure-mechanical optimization in tin bronze alloys.

Enhancement of strength and ductility by Cu-rich precipitation in Cu-bearing 304L austenitic stainless steel

This work investigated the effects of copper (Cu) addition and Cu-rich precipitates on the deformation mechanism and strain hardening behavior of 304L austenitic stainless steel. The results indicated that high addition of Cu could suppress the formation of martensite and improve the amount of deformation twins after tensile deformation, which was accordance with the calculated stacking fault energies. After aging treatment, the finely dispersed Cu-rich precipitates were involved in the deformation process and retarded the decreasing tendency of strain-hardening rate. The synergistic effects of transformation-induced plasticity (TRIP), twinning-induced plasticity (TWIP) and Cu-rich precipitation-induced plasticity were well homogenized and improved the excellent strength-ductility balance in 304L austenitic stainless steel.

Strain-rate sensitivity of Hall-Petch strengthening in Au(Cu) electrodeposits

A triboindentation test method is used to reveal a strain-rate sensitivity in the Hall-Petch hardening coefficient that’s representative of grain size effects. For electrodeposited gold-copper alloys, the strain-rate dependent hardening coefficient kH (ε̇) equals kHo⋅ε̇m where kHo equals 1.28 ± 0.02 GPa⋅nm0.5 and the strain-rate sensitivity exponent m equals 0.267 ± 0.014, for those samples with a 5-34 nm grain size as scratch tested in cross-section over a 0.4–150 s−1 strain-rate range ε̇.

Influence of shock pre-strain on the dynamic mechanical behavior and adiabatic shearing failure of the Ti–6Al–4V alloy with lamellar microstructure

The effects of shock (high-strain-rate) and quasi-static pre-strain on the dynamic mechanical properties and adiabatic shear band (ASB) behavior of Ti–6Al–4V (Ti64) alloy with lamellar microstructure were systematically investigated. The strength improves but the ductility degrades in the all pre-strained Ti64. The strengthening effect is more significantly in the materials pre-strained and reloaded at higher strain rates, due to the more efficient accumulation of defects and the viscous drag effect. The work hardening rate of the shock pre-strained Ti64 shows a complex trend with pre-strain, due to the competition between thermal softening and strain-rate hardening. The failure of the shock pre-strained alloys during dynamic reloading is caused by the cracks that nucleate, grow and propagate along the ASBs. The nucleation and interaction of ASBs in Ti64 are promoted by defects multiplication during the shock wave pulse, resulting in the formation of some special ASB structures such as rings and plugs. Some ASB nodes also generate in the shock pre-strained Ti64 through the more serious localized deformation in the ASB ring formed by the bifurcation, deflection and convergence again of ASBs.

Predicting deformation behavior of α-uranium during tension, compression, load reversal, rolling, and sheet forming using elasto-plastic, multi-level crystal plasticity coupled with finite elements

An elasto-plastic self-consistent (EPSC) polycrystal plasticity formulation is adapted to model deformation of wrought α-uranium (α-U) accommodated by a combination of elasticity, dislocation glide, and deformation twinning. The EPSC model incorporates a strain-path, strain rate, and temperature sensitive dislocation density-based hardening law for the evolution of resistance to slip, twinning, and de-twinning and a slip system-level kinematic back-stress law to influence the driving force for activation. The model is used to interpret the complex deformation behavior of α-U as a function of strain-path and temperature. Samples of α-U with different initial orientation distributions are experimentally evaluated in simple compression, tension, and load reversal at room temperature and in compression and rolling at 573 K under a quasi-static deformation rate. Evolution of texture and twinning is characterized using electron backscattered diffraction and in-situ and ex-situ neutron diffraction during deformation. It is observed that the behavior of the material is highly anisotropic owing to its low-symmetry orthorhombic crystal structure and different activation stresses for crystallographic deformation modes. The model is calibrated and validated under these deformation conditions and predicts the stress-strain responses, amount of twinning, texture evolution, and lattice strains with one set of parameters for the hardening and back-stress evolution laws. Subsequently, the developed model is used as a constitutive law in the implicit finite element (FE) framework to simulate drawing of a hemispherical part from a rolled sheet of α-U. Here, the FE-EPSC model is a two-level homogenization scheme with EPSC relating the grain-level to the polycrystalline aggregate-level response, while the FE framework scales the polycrystalline to the part-level response. The simulation results and insights from the calculations, such as location dependent texture evolution is in good agreement with experiments.

Effect of Aluminum and Vanadium on Retained Austenite Stability and Work Hardening in Silicon Free TRIP Steel

Despite that the conventional CSiMn TRIP steel has a promissing mechanical attributes, it has a limitations on the galvanizability of such grades of steel due to Silicon. Thus, aluminum as a strong candidate for substituting silicon has been introduced in this study accompanied by vanadium as a microalloying element. Microstructure of the studied steel was observed by using OM, and SEM. X-ray diffraction analysis, and tent-etching technique carried out on the studied steel to identify the fractions of the retained Austenite after thermal mechanical process, as well as its morphology. In addition, the mechanical properties in term of strength, ductility, strain hardening, and the rate of strain hardening were studied to define the influenced parameters throughout this alteration. The results refer to the possibility of complete replacement of silicon in TRIP steel with aluminum at the presence of vanadium as a micro alloying element.

Analysis and Characterization on Dynamic Recrystallization in Casting AZ31 Mg Alloys Under Plane Strain Compression.

Studies on twinning, twin-induced dynamic recrystallization (TDRX), and their temperature and strain rate dependences are of considerable significance to the ultimate strength and plastic formability of the coarse-grained Mg alloys during severe plastic deformation. Plane strain compression tests were conducted on the parallelepiped samples of casting AZ31 Mg alloys. The twinning and recrystallization behaviors close to and away from the crack boundaries were characterized using electron backscatter diffraction. The results show: (1) with increasing strain rate for tests, the extension twin proliferates significantly. Due to the local stress concentration, the TDRX is more active in the area close to the crack tip and exhibits the positive strain-rate sensitivity as twinning; (2) the TDRX is not only stress-favored but also closely links to the temperature. However, the TDRX is not utterly proportional to the temperature. Compared to 400 °C, 300 °C is more beneficial to the TDRX, achieving the higher strength and plastic deformability. The main reason is that the higher strain-hardening rate and flow stress at the higher strain rate and lower temperature motivates the transformation from twinning to the fine twin-walled grains more efficiently, and the stress-favored TDRX is crucial to refine grains and continue plastic deformation for the casting Mg alloys with coarse grains.

High strain rate compression behaviour of single phase CoCuFeMnNi high entropy alloy

Deformation behaviour of FCC CoCuFeMnNi single phase high entropy alloy (HEA) was studied under quasistatic and dynamic loading conditions in compression at strain rate of 0.001 s−1 and 3000 s−1 using screw driven universal testing machine and split Hopkinson pressure bar respectively. A detailed microstructural characterization was carried out using electron back scatter diffraction (EBSD) while bulk texture measurement was carried out using X-ray diffraction. The high strain rate deformed sample shows high strain rate hardening as well as higher strain hardening that is attributed to operation of deformation twinning as observed from EBSD. The operation of deformation twinning and multiple slip systems leads to grain fragmentation and weakening of characteristic <101> compression texture in sample subjected to high strain rate. Constitutive behaviour of the CoCuFeMnNi HEA explained using the Johnson-Cook model further justifies better strain rate and strain hardening response of the alloy compared to individual elements.

Microstructure Evolution and Compressive Properties of Multilayered Al/Ni Energetic Structural Materials under Different Strain Rates

Multilayered Al/Ni energetic structural materials integrating exothermic properties and mechanical properties were prepared by the method of electrodeposition and hot pressing in this research. Then, the uniaxial quasi-static compression and split Hopkinson pressure bar experiments were conducted at strain rates from 10−4 to 6.5 × 103 s−1 at room temperature. The effects of compression strain rate on the microstructure evolution and the compressive properties of multilayered Al/Ni energetic structural materials were systematically investigated. With increasing quasi-static compression strain rate, the compression strength increased slightly for two kinds of Al/Ni multilayers prepared under different hot pressing time. With the hot pressing process extending to 4 h, the dynamic compression strength of multilayered Al/Ni composites monotonically increased from 494.7 to 564.2 MPa with increasing strain rate. It was shown that Al/Ni energetic structural materials exhibited evident strain hardening and strain rate strengthening. However, when the compression strain rate reached 6500 s−1, the Al/Ni composite prepared with the hot pressing time of 1 h showed prominent thermal softening. Notwithstanding, it was found that the compression strength of Al/Ni composite prepared at 4 h was evidently higher than that at 1 h, since the second phase reinforcement counteracted the thermal softening. In addition, the critical failure strain presented obviously increasing tendency with the increased compression strain rates.