Mechanical behavior of zirconium and hafnium in tension and compression

The mechanical behavior and substructural evolution of highly textured hafnium (Hf) has been examined in tension and compression and compared to the mechanical response of zirconium (Zr). The quasi-static work-hardening rate as a function of strain for both metals exhibits a compression-tension asymmetry. Both Zr and Hf exhibit a downward work-hardening response in tension, while each displays a parabolic and then concave upward work-hardening behavior in compression. Additionally, Hf displays higher flow stresses than Zr both in tension and compression. The stress-strain and strain-hardening curves for Zr and Hf have been characterized in terms of their propensity for deformation twinning and evolution of substructure with strain. Differences in the work-hardening rates and flow stresses as a function of the sense of the applied load and material are discussed in terms of slip-twin interactions during deformation.     

Comparison of plane-strain and tensile work hardening in two sheet steel alloys

Extended tensile tests and newly-devised plane-strain tests have been performed on two commercial sheet steels: an aluminum-killed (A-K) steel and a cold-rolled, dual-phase (D-P) steel. Up to uniform strain in uniaxial tension, the average work-hardening rate of both steels was found to be independent of strain state (isotropic hardening). The planestrain behavior of both steels was in good agreement with predictions of Hill’s old theory of normal anisotropic plasticity. At larger strains, the work-hardening rate in tension is greater than in plane strain. All of the tests were adjusted to reflect constant strain rates and all results were well represented by work hardening of the Hollomon type.     

The effects of combined strain-path and strain-rate changes in aluminum

The effect of changes in both the direction of tensile stress and the strain rate upon the plastic behavior and microstructure of aluminum has been investigated. The reduction in strain-hardening rate following a strain-path change was influenced by the strain rate prior to and after the change; higher strain rates in the first stage and lower rates in the second stage decreased the initial transient hardening rate. The introduction of a large path change significantly reduced the effect of strain-rate changes on the strain-hardening rate observed in testing without changing the tensile axis. Evidence of cell wall disruption following a path change was found, and this mechanism combined with effects of cell wall orientation can qualitatively explain the removal of the rate sensitivity of hardening rate because the usual recovery events in cell walls no longer have a dominant influence on flow stress development.     

The effects of combined strain-path and strain-rate changes in aluminum

The effect of changes in both the direction of tensile stress and the strain rate upon the plastic behavior and microstructure of aluminum has been investigated. The reduction in strain-hardening rate following a strain-path change was influenced by the strain rate prior to and after the change; higher strain rates in the first stage and lower rates in the second stage decreased the initial transient hardening rate. The introduction of a large path change significantly reduced the effect of strain-rate changes on the strain-hardening rate observed in testing without changing the tensile axis. Evidence of cell wall disruption following a path change was found, and this mechanism combined with effects of cell wall orientation can qualitatively explain the removal of the rate sensitivity of hardening rate because the usual recovery events in cell walls no longer have a dominant influence on flow stress development.     

Mechanical Behavior of Carbide-free Medium Carbon Bainitic Steels

The effect of bainitic transformation time on the microstructure and mechanical properties was investigated in a steel containing 0.4 pct C-2.8 pct Mn-1.8 pct Si. The microstructure was characterized using optical and transmission electron microscopy; it consisted of bainitic ferrite, martensite, and retained austenite. The volume fraction of bainite increased from 0.4 for the shortest bainitic transformation time (30 minutes) to 0.9 at the longest time (120 minutes). The above microstructures exhibited an extended elasto-plastic transition leading to very high initial work-hardening rates. The work-hardening behavior was investigated in detail using strain-path reversals to measure the back stresses. These measurements point to a substantial kinematic hardening due to the mechanical contrast between the microstructural constituents. The onset of necking coincided with the saturation of kinematic hardening. Examination of the fracture surfaces indicated that the prior austenite grain boundaries play an important role in the fracture process.     

On the influence of strain-path changes on fracture

Abrupt changes in strain path between uniaxial and equibiaxial tension are shown to have a large effect on plane-strain ductility. Data for titanium sheets, both with and without hydrides, show that a significant ductility enhancement occurs at a final strain state of plane-strain tension following multi-stage deformation sequences comprised of uniaxial and equibiaxial tension. While the dependence of ductile fracture on both accumulated damage and strain hardening suggests that failure strains should be sensitive to a nonproportional strain-path history, the detailed cause(s) of the present effect is not known.     

Effects of Anisotropy in Drawing and Extrusion Processes of Bulk Metal Forming

The effects of anisotropy of axisymmetric materials (round bars, tubes) on metal forming processes are discussed. These effects are strongest for thin‐walled hollow materials in metal forming processes when the wall thickness is not predetermined by the die (tube drawing without mandrel, free extrusion of hollow components). Similarly to the normal anisotropy of sheet metal, a high radial anisotropy increases the resistance against a variation of wall thickness in tube drawing. There are also effects in forming solid materials such as forward extrusion of bars whereby the buckling of cross sections is influenced through the variation of radial anisotropy with the distance from the axis. The favourable anisotropy properties depend on the actual priorities. If, for example, for a metal forming process the material anisotropy results in high compressive stresses this may be favourable for increasing the ductility of the material whereas the increase of the load acting on the tool reduces tool life.     

Plastic behavior of aluminum-killed steel following plane-strain deformation

Aluminum-killed steel sheets have been subjected to plane-strain prestrain in three ways: two-pass rolling, multi-pass rolling, and inplane, plane-strain tension. Subsequent uniaxial tensile tests were performed to evaluate the residual work-hardening behavior. The subsequent hardening curves depended primarily on the relative direction between major strain axes in the two deformation stages and very little on the specific prestrain procedure. These curves showed high initial yield stresses followed by a region of low (or negative) work hardening rate. This behavior contrasted with earlier results for 70/30 brass sheet, and a model of subsequent tensile behavior based on a strain-induced stress transient emerged. Formerly Staff Research Scientist, General Motors Research Laboratories     

Plastic behavior of 70/30 brass sheet

The plastic yield behavior of strip annealed 70/30 brass sheet has been investigated using several experimental techniques. Proportional path, stress-strain relations were measured in two strain states using a recently devised plane-strain test and a standard sheet tensile test. Based on these data, 70/30 brass exhibits a dramatic departure from Hill's plasticity models. Particularly notable is the lower work-hardening rate in plane strain. A second series of tests was carried out by deforming first in plane-strain tension and subsequently in uniaxial tension. The relative orientation of the principal strain directions in the two strain paths strongly affected the transient yielding behavior, but the original work-hardening pattern and plastic anisotropy were approached after an additional effective strain of ∼0.04. These observations are consistent with a two yield-surface model;i.e., one an underlying, proportional path yield surface and one an instantaneous, transient yield surface.     

应变速率对高氮奥氏体不锈钢塑性流变行为的影响

研究了室温拉伸时应变速率对高氮奥氏体不锈钢18%Cr-18%Mn-0.65%N力学性能和塑性流变行为的影响。结果表明,随应变速率的升高,试验钢的屈服强度Rp0.2升高,而抗拉强度Rm及塑性略有降低;在各应变速率下,试验钢的塑性流变行为均可以用Ludwigson模型进行描述;应变速率的升高对试验钢流变方程参数的影响如下：1）强度系数K1、应变硬化指数n1和n2减小,试验钢的加工硬化能力降低;2）真实屈服强度TYS降低;3）瞬变应变εL减小,表明升高应变速率能够促进位错多系滑移和交滑移。     

Probing the Strain Hardening Response of Small Wear Volumes with Nanoindentation

In order to characterize the wear and related mechanical behavior of materials from small volumes, a program employing nanoscratch and nanoindentation was performed. Nanoscratch techniques were used to generate square wear patterns with varying degrees of shear strain followed by nanoindentation tests to measure the mechanical properties within the deformation area. Results show a systematic increase in hardness with both the applied load and number of nanoscratch passes. An analytical approach was used to determine the stress-strain response and strain hardening behavior of electroformed nickel. The strain hardening exponent determined from this method follows the work hardening behavior established from previous tensile tests, supporting the use of a nanomechanics-based approach for evaluating the mechanical properties of wear-tested material.     

Effects of strain path changes on the formability of sheet metals

The effects of a change in strain path on the deformation characteristics of aluminum-killed steel and 2036-T4 aluminum sheets have been studied. These sheets were pre-strained various amounts in balanced biaxial tension and the resulting uniaxial proper-ties and forming limits for other loading paths were determined. In comparison to uni-axial prestrain the steel was found to suffer a more rapid loss in uniform strain upon the strain path change from biaxial to uniaxial. In contrast, the uniform strain in aluminum does not drop as rapidly after the same change. In keeping with this behavior, the form-ing limit diagram of steel is found to decrease with prestrain at a much faster rate than that of aluminum. Such effects can be explained in terms of the transition flow behavior of the metals occurring upon the path change. Thus, the path change produces strain soften-ing and premature failure in steel, while causing additional strain hardening and consequent flow stabilization in aluminum. AMIT K. GHOSH, formerly with General Motors Research Laboratories     

Influence of cold forming on the tensile behavior and properties of low-carbon microalloyed steel

The tensile behavior and properties of cold formed low-carbon microalloyed steel with its microstructure of all ferrite and pearlite (F+P) were investigated.Bending and flattening deformations were carried out in the laboratory on hot-rolled sheets in order to simulate the cold forming process of steel sheets during pipe fabrication and sampling of high frequency straight bead welding pipes.A comparison of the tensile behavior and properties of the material made before and after cold forming indicates that cold deformation alters the tensile behavior and properties of the material to a certain degree depending on the manner of the cold deformation and the degree.The research on the Bauschinger effect indicates that for the steels investigated,when the plastic strain is small,the back stress increases rapidly with the increase of the plastic strain and then rapidly tends to saturation.The finite element analysis indicates that the change in the properties of the steel sheets due to cold forming is a result of the Bauschinger effect and work hardening.The mechanism of the change in the properties is also given in this study.     

Microstructure and texture evolution under strain-path changes in low-carbon interstitial-free steel

The transmission electron microscopy (TEM) microstructure of a low-carbon Ti-killed interstitial-free (IF) steel has been examined after simple-shear/simple-shear and uniaxial-tension/simple-shear strain-path changes, in connection with the crystallographic orientation of the grains. The results are discussed in the context of the inter-relation between the microstructure and texture evolutions and their joint influence on the mechanical behavior. A partial disappearance of prestrain microstructures is shown to cause the stagnation of work hardening at earlier stages of reversed loading during Bauschinger simple-shear sequences. Under progressing reversed deformation, a fragmentation of the grains of unstable orientations still slows down the work-hardening rate. A strong localization of plastic flow within microbands following an orthogonal strain-path change is shown to occur within the grains containing well-developed prestrain dislocation boundaries and belonging to certain orientation groups.     

Plastic behavior of dual phase steel following plane-strain deformation

Dual-phase, high-strength steel sheet has been prestrained in plane-strain tension. Residual hardening and ductility properties were evaluated by performing subsequent uniaxial tensile tests in either co-axial or noncoaxial principal strain axis orientations. In contrast to similar work on aluminum-killed 1008/1010 steel sheet, only minor changes were found in the subsequent flow behavior of dual-phase steel, and no significant difference was found between the two orientations. The small effect of an abrupt strain path change observed in this study is consistent with the low work hardening rate of this alloy.     

The influence of texture,strain rate,temperature, and chemistry on the mechanical behavior of hafnium

The compressive mechanical behavior of highly textured, polycrystalline hafnium has been examined as a function of texture, strain rate, temperature, and material chemistry. The microstructural and substructural evolution in Hf was also examined as a function of texture. Decreasing temperature, increasing strain rate, and increasing impurity concentrations were found to increase the yield-stress and work-hardening rates, as well as increase the amount of twinning in Hf. Crystallographic texture was found to exhibit the most marked effect on the mechanical behavior of Hf. Differences in the orientation of the c-axis with respect to the loading direction were found to affect the yield stress, work-hardening behavior, and anisotropy of the tested specimen, with the highest yield stresses and rates of work hardening and the lowest anisotropies in specimens compressed along the c-axis. The amount of deformation twinning and the slip systems activated during deformation were seen to vary based on texture and are shown to correlate well with the observed yield stresses and work-hardening behaviors.     

Measurement and analysis of plane-strain work hardening

A new technique for measurement of plane-strain work hardening has been developed which uses tensile loading and computer analysis for interpretation, and which eliminates the experimental uncertainties of large strain gradients, friction, and out-of-plane bending inherent in the usual plane-strain deformation modes. Plane-strain and tensile work-hardening curves have been measured for 2036-T4 aluminum alloy using several types of sheet specimens. The work-hardening rate in plane strain is lower than that in uniaxial tension. In each case a Voce-type empirical work hardening law represents the data well. Hill’s theories cannot account for these data because the isotropic hardening assumption is violated. A method of analysis was introduced to determine Hill’s newm parameter as a function of strain andm was found to vary from 1.6 to 2.0 in the strain range 0.02 ≤ ε ≤ 0.18.     

Deep Drawing Behavior of CoCrFeMnNi High-Entropy Alloys

Herein, the deep drawability and deep drawing behavior of an equiatomic CoCrFeMnNi HEA and its microstructure and texture evolution are first studied for future applications. The CoCrFeMnNi HEA is successfully drawn to a limit drawing ratio (LDR) of 2.14, while the planar anisotropy of the drawn cup specimen is negligible. The moderate combination of strain hardening exponent and strain rate sensitivity and the formation of deformation twins in the edge region play important roles in successful deep drawing. In the meanwhile, the texture evolution of CoCrFeMnNi HEA has similarities with conventional fcc metals.     

Tensile stress-strain analysis of single-structure steels

In an effort to establish a universal model to predict the mechanical properties from processing conditions, tensile tests have been conducted of four single-structure steels, namely, ferrite, pearlite, bainite, and martensite; the data obtained were analyzed in terms of the Ludwik, Hollomon, and Swift equations to characterize their work-hardening behavior. It was found that the differential Crussard-Jaoul (C-J) analysis, based on the Ludwik equation, can describe the work-hardening behavior of these steels fairly well. The differential C-J analysis has shown that the ferrite and pearlite steels deform with two stages of work hardening, each stage associated with a distinctive value of the work-hardening exponent n. Martensitic steels exhibit single-stage work hardening. In bainite, the behavior was found to be dependent on transformation temperature; upper and lower bainite exhibit a behavior similar to pearlitic steels and quenched martensite, respectively. This can be well understood in terms of the similarity of the corresponding microstructures. On the basis of these results, the work-hardening behavior of single-structure steels falls into four categories, according to the n value. This classification may serve as a useful guide to predict the flow behavior of steels with a known microstructure or to judge the microstructure merely by stress-strain curves, without microstructural observations.