Regions Of Flow Stress Research Articles

Investigation on mechanical behaviors of Cu-Ni binary alloy nanopillars: a molecular dynamics study.

Cu-Ni binary alloy has become the attention of scientific world for its potentials in nanodevices. It is indispensable to investigate on the mechanical properties of this material due to lack of previous work done regarding this binary alloy. Molecular dynamics (MD) studies were performed on nanopillar (NP) structures comprised of Cu-Ni binary alloy having an FCC unit cell with Cu atoms selectively replaced by Ni atoms. This selective replacement resulted in a better stress behavior than the randomly replaced alloy structure when both tension and compression load were applied. The effect of crystal orientation, NP dimensions, temperature, and strain rate on the stress-strain curve of Cu-Ni binary alloy NPs was thoroughly investigated under tensile loading. This investigation reveals significant influence of crystal orientation on ultimate strength and flow stress region. Among four different crystal orientations, <111> orientation shows maximum strength behavior under tensile loading, while <110> shows highest strength under compressive load. However, in both cases, i.e. tension and compression, the poorest stress behavior was observed for <001> orientation. Under tensile load, <111>-oriented binary alloy fails due to the formation of Shockley partials followed by formation of complex dislocation network. On the other hand, <110>-oriented binary alloy fails due to the formation of Lomer-Cottrell (LC) lock from the Shockley partials. Total dislocation length is calculated, and its effect on the stress-strain behavior of the Cu-Ni binary alloy is discussed. Highest Young's modulus and yield stress are observed on <111>-oriented binary alloy among other orientations, and these values for <111>-oriented NP was found to decrease with the increment of temperature. If the temperature is increased, yield stress and Young's modulus decrease. The effect of cross section width was also investigated in this study, and it was found that yield stress decreases with the increment of cross section width due to the effect of surface atom fraction. Increasing the strain rate causes the initiation of amorphous structure, resulting in superplastic behavior of the <111>-oriented Cu-Ni binary alloy NP.

Ultra-Fine Grain Development in an AZ31 Magnesium Alloy during Multi-Directional Forging under Decreasing Temperature Conditions

Grain refinement of a magnesium alloy, AZ31, was studied in multi-directional forging (MDF) with decreasing temperature from 623 to 423 K. The MDF was carried out up to cumulative strains of around 5 with changing the loading direction during decreasing temperature from pass to pass. The structural changes are characterized by the development of many mutually crossing kink bands accompanied by MDF at low strains, followed by full development of very fine grains at high strains. The dynamic changes in grain size evolved can be expressed by two different power law functions of flow stress for the regions of flow stress above or below around 100 MPa. The MDF under decreasing temperature condition can accelerate the uniform development of much finer grains and the improvement in plastic workability, leading to the minimal grain size of 0.36 μm at a final processing temperature of 423 K. The mechanism of grain refinement is discussed in detail.

ＡＺ３１マグネシウム合金の降温中多軸鍛造による微細粒組織の生成

Grain refinement in a magnesium alloy AZ31 was studied in multi-directional forging (MDF) with decreasing temperature from 623 K to 423 K and at a strain rate of 3×10−3 s−1. The MDF was carried out up to large cumulative strains with changing the loading direction during decreasing temperature from pass to pass. The structural changes are characterized by the generation of many mutually crossing kink bands at low strain, followed by the development of very fine grains at large strains. The dynamic change in grain size can be expressed by two different power law functions of flow stress for the region of flow stress above or below 100 MPa. The MDF with decreasing temperature can accelerate the uniform development of much finer grains and the improvement in plastic workability, leading to the minimal grain size of 0.23 μm at 423 K. The mechanism of grain refinement is discussed in detail.

High strain rate superplasticity of hot extruded and hot rolled AA 6013/20 vol.-%SiCp composite

High strain rate superplasticity(HSRS)of an AA 6013/20SiCpcomposite, produced by powder metallurgy and then hot extruded or hotrolled, was evaluated by means of tensile tests carried out over a range of initial strain rates from 1 × 102 to 3.8 × 10-1 s-1 and temperatures from 520 to 590 ° C. A maximum elongation to failure of 370% was achieved in a hot rolled composite deformed at 1 × 10-1 s-1 and 560 ° C. Substantially lower elongations were achieved in hot extruded composites, with a maximumof200% at1 × 10-2 s -1 and 580 ° C. The lower elongations in the hot extruded composite could be related to the large quantity of intermetallic compounds, shown by TEM analyses, which probably hinder large superplastic elongations. In both hot extruded and hot rolled composite, the flow stress was strongly dependenton temperature and strain rate; a steady state flow stress region was observed in the specimen that exhibited the maximum elongation to failure. The strain rate sensitivity index m reached a maximum ofabout 0.4 for the hot rolled composite, and about 0.35 for the hot extruded composite. Analyses of the fracture surfaces of hot rolled composite deformed at the maximum elongation, were characterised by the presence of many filaments or 'whiskers', which are generally considered as evidence of a liquid phase present at grain boundaries or interfaces.

Flow state in molecular-dynamics-simulated deformed amorphousNi0.5Zr0.5

Results are reported from molecular dynamics simulations about the deformation behavior of amorphous metallic ${\mathrm{Ni}}_{0.5}{\mathrm{Zr}}_{0.5}$ alloys near their glass transition temperature. Constant shear strain rate and constant shear stress deformations are analyzed. Stress-strain curves from constant shear strain rate deformations reveal different stages of deformation, first a regime with approximately linear stress-strain relation, then a transient regime with pronounced strain softening, and finally, for large strains, a nearly strain-independent flow stress region. The strong stress overshoot in the transient regime is ascribed to a delayed transition under steady-state deformations from the initial glassy state into a structurally and dynamically different one. This flow state shows a decreased viscosity, an increased potential energy per atom, and a loss of short-range order, the latter being deduced from the radial distribution functions. In the flow state, the self-part of the intermediate scattering function reveals an unchanged vibration spectrum of the system but a faster decay of density correlations compared to the starting structure. Creep curves from constant shear stress deformations show a nearly linearly increasing strain for moderate shear stresses and moderate deformations. The strain rate depends in a non-Newtonian manner on stress with an activation volume of $7--8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}29}{\mathrm{m}}^{3}.$ Deformation simulation with large constant shear stress leads to an instability, identified as the transition into the flow state.

Extension of the viscoplasticity theory based on overstress (VBO) to capture non-standard rate dependence in solids

Abstract The viscoplasticity theory based on overstress (VBO) can model unusual loading rate effects. Included are rate-insensitivity and positive and negative rate sensitivity. An augmentation function is required that operates on the dynamic recovery term of the growth law for the equilibrium stress, which is a state variable in VBO. The augmentation function does not affect the initial, quasi-elastic region. Its full effect appears when the long-time, asymptotic solution, which corresponds to the flow stress region is reached in an experiment. The flow stress of the VBO model consists of monotonic, rate-independent hardening affected by the kinematic stress, the viscous, rate-dependent hardening through the overstress and the rate-independent hardening represented by the isotropic stress. With the augmentation function it is possible to have only rate-dependent contributions to the flow stress. Through a proper choice of the augmentation function the rate-dependent and the rate-independent contributions can be apportioned to yield positive, zero and negative rate dependence in a consistent manner. As a demonstration, the stress-strain curves of Copper, a fcc metal, are reproduced. In fcc metals the positive rate dependence increases with deformation. The rate-dependent stress-strain curves of polymethylmethacrylate (PMMA) exhibiting a distinct yield point are also reproduced. Numerical creep tests performed at the same stress level before and after the yield point lead to unlimited creep strain and primary creep, respectively. The modeling of these very different types of material behavior requires only different material constants and augmentation functions within the same general constitutive equation.

Relaxation behavior and modeling

Abstract Load relaxation tests deliver several orders of magnitude of inelastic strain rate data while elastic strains are converted into inelastic strains [see Lemaitre and Chaboche, 1994. (Mechanics of Solid Materials, Oxford University Press, Cambridge p. 264)]. Hart used this test for providing information on the inelastic deformation behavior for modeling purposes. Characteristic relaxation curves were obtained with ductile metals and alloys at room and high temperature showing a scaling relation derived from Hart's theory. Subsequent testing with servo-controlled testing machines and strain measurement on the gage length showed that an increase of prior strain rate also increased the average relaxation rate. For relaxation tests starting in the flow stress region, the relaxation curves can be independent of the stress and strain at the start of the relaxation tests. For the modeling of these newly found relaxation behaviors and other phenomena the viscoplasticity theory based on overstress (VBO) has been introduced. It is shown that VBO admits a long-term (asymptotic) solution that can be used with sufficient accuracy for the flow stress region of the stress–strain diagram. The long-term solution predicts the observed relaxation behaviors and that the relaxation curves coincide when shifted along the stress axis. This behavior is observed for the recently obtained data and is confirmed by two sets of the Hart-type data when they are plotted according to the new method.

The effect of grain size and temperature on the superplastic deformation behavior of a 7075 Al alloy

The high-temperature deformation behavior of a 7075 Al alloy has been investigated within the framework of a recently proposed internal-variable theory for structural superplasticity (SSP). The flow curves were obtained by performing a series of load relaxation tests for specimens with various grain sizes, at temperatures ranging from 445 °C to 515 °C. The overall flow curves were then separated into two parts, according to the respective physical mechanisms, viz., the grain-boundary sliding (GBS) and the accommodating dislocation glide processes, contrary to the conventional approach which uses a single power-law relation. These individual curves were then analyzed based on the internal-variable theory. Much valuable information has been obtained in this way, providing new physical insight as well as a more comprehensive understanding of SSP. The GBS curve could be described as a Newtonian viscous flow, signified by the power-index value of Mg=1.0 for this alloy. The unresolved issue of threshold stress is also clarified and identified as a critical stress required for the GBS. The role of grain refinement is found to shift the grain-matrix deformation (GMD) curve into a higher stress and strain-rate region, while the GBS curve into a lower stress and higher strain-rate region along the respective characteristic scaling line to bring both curves into a common flow-stress region, in which the GMD and GBS can operate simultaneously, resulting in the usual superplastic deformation behavior.

Effect of multipeak dynamic recrystallisation on structure of deformed and annealed copper

Compression tests were carried out on fine grained copper at 870 K and at a constant true strain rate of 1·4×10−3 s−1. Under these conditions, well defined flow stress oscillations followed by steady state flow stress are obtained. Grain size measurements of as deformed material revealed non-monotonic grain coarsening when stress oscillations appear. It was found that grain coarsening is most effective when the flow stress decreases after the first flow stress peak. Periodic flow stress is accompanied by periodic grain coarsening until the latter becomes practically independent of strain when the steady state flow stress region is attained. The structural effects of static processes (recovery and recrystallisation) in dynamically recrystallised material were examined closely. According to the model of periodic dynamic recrystallisation, one would expect periodic changes of the driving force for static restoration processes (mainly metadynamic and/or static recrystallisation). From the present work, conclusions are drawn that are contrary to this concept of structural softening. The critical strain leading to grain coarsening during post-deformation annealing of hot deformed copper was found to be significantly less than the strain corresponding to the first flow stress peak. For higher strains, the grain size of dynamically recrystallised copper was found to be highly stable during annealing for 7 h at 870 K.MST/978

Mechanical Properties of Ga1–xInxAs

ABSTRACTSubstantial solid solution strengthening of GaAs by In acting as InAs4 units has recently been predicted. This strengthening could account for the reduction of dislocation density in GaAs single crystals grown from the melt. High temperature hardness measurements up to 700ºC have been carried out on (100) GaAs and Ga0.9975 In0.0025 As wafers. Results show a significant strengthening effect in In—doped GaAs even at concentration levels of about 0.2 wt%. A temperature independent flow stress region is observed for both these alloys. The In—doped GaAs shows ahigher plateau stress level compared to the undoped GaAs. The results are consistent with the solid solution strengthening model.

Further observations on the plastic deformation of a normalized low carbon steel

The plastic deformation of a commercial grade low carbon steel has been investigated using microhardness and grain strain measurement techniques. Two distinct modes of deformation during plastic flow in low carbon ferritic steel have been identified. The initial stage involves the propagation of the Luder's band along the gauge length of the sample by slip strain in the surface and near surface grains only, the strain accommodation in the interior of the material being attained by a predominantly grain translation mode. The second stage involves the propagation of a strain hardening front through the cross-section of the material as the macro-strain is increased through the flow stress region.