Influence of the temperature and strain rate on the deformability of low-alloy carbon steel

The influence of the temperature, strain, and strain rate on the deformability of low-alloy carbon steel is studied experimentally. Samples are tested on the STD 812 torsional plastometer at Czestochowa Technological University. The hardening of low-alloy carbon steel at different strain rates and temperatures is plotted on the basis of the results for its resistance to deformation. For all the curves, the rate of hardening is high in the initial section, with no relaxation processes. The influence of the strain rate and temperature on the maximum resistance to deformation is quantitatively determined; this is important for practical purposes. The resistance to deformation declines on account of relaxation. The influence of the strain rate and temperature on the mean hardening rate in the range 0 < ε_u < ε_u*is also studied.     

Influence of temperature and strain rate on slip and twinning behavior of zr

The stress-strain behavior of pure Zr was studied systematically at various temperatures and strain rates. At 76 K, Zr deforms predominantly by twinning, whereas above room temperature (RT), slip is the controlling deformation mode. A transition in the rate-controlling deformation mode from slip to twinning has been observed to occur at intermediate temperatures during the course of plastic deformation. Above 373 K, slip dominates the entire course of deformation. The transition from slip to twinning in the stress-strain behavior is linked to differing strain-hardening rates and temperature sensitivities of the two deformation modes.     

The effect of strain rate and depressed temperature on the low cycle deformation behavior of alpha iron

The low cycle deformation saturation stress in Ferrovac-E a-iron was studied using diametral plastic strain (0.001 ≤ Δε_dp/2 ≤ 0.0135) as the control variable. Increasing strain rate (6 × 10^•5 s^•1 • 4 × 10^•3 s^•1) and decreasing temperature (295 to 173 K) increased the saturation stress levels. The cyclic work hardening coefficient decreased from 0.18 at 295 K to 0.10 at 173 K, which is consistent with previous studies of monotonie deformation. The temperature dependence of both the saturation stress and the strain rate sensitivity, as measured during cyclic deformation, were similar to that measured during monotonic tensile tests. The temperature dependence of the dislocation velocity indexm* was in good agreement with published values from high cycle fatigue and monotonie tensile tests. Thus the same deformation mechanisms are believed to occur in both monotonie and large plastic cyclic deformation (Δε_dp/2 ≥ 0.001) of a-iron.     

Effect of grain size on high strain rate deformation of copper

Experiments were performed to observe the deformation characteristics of oxygen-free high-conductivity (OFHC) copper at high strain rates (up to 40,000 s⁻¹) and to relate differenc in grain size with differences in deformation behavior. The rod impact and torsional Hopkinson bar test methods were used in these experiments. Results show that grain size reductions substantially reduce surface irregularities that develop during deformation. The effect of grain size on the yield stress and on the strain-hardening behavior of copper is small and is similar to the effect of grain size in copper at quasistatic strain rates. The observation that grain size has a substantial effect on surface irregularities may have important implications for applications in which stable deformation of thin sections is of concern.     

Tensile deformation behavior of mechanically stabilized Fe-Mn austenite

The tensile deformation behavior of mechanically-stabilized austenite is investigated in Fe-Mn binary alloys. A 30 pct thickness reduction by rolling at 673 K (above the A_f temperature) largely suppresses the austenite (γ) to hcp epsilon martensite (ε) transformation in 17Mn and 25Mn steels. However, the deformation behavior of the mechanically stabilized austenite in the two alloys differs significantly. In 25Mn steel, the onset of plastic deformation is due to the stress-induced γ→ ε transformation and results in a positive temperature dependence of the yield strength. The uniform elongation is enhanced by the γ → ε transformation during deformation. In 17Mn steel, bccα′ martensite is deformation-induced along with e and a plateau region similar to Lüders band deformation appears at the beginning of the stress-strain curve. The mechanical stabilization of austenite also suppresses the intergranular fracture of 17Mn steel at low temperatures. M. STRUM, formerly Candidate for Ph.D. at the University of California at Berkeley     

高碳钢线材轧制工艺参数对晶粒尺寸的影响

结合高碳钢线材的生产实际，利用物理冶金理论和实验模型，对奥氏体的变形-再结晶过程进行模拟计算，计算出各道次的奥氏体晶粒尺寸并分析变化规律。模拟计算结果表明，亚动态再结晶在粗轧和中轧阶段起主要作用，在轧制后半程，静态再结晶起主要作用。分析改变生产工艺参数对晶粒尺寸的影响以及晶粒细化的途径。为进一步的相变、性能预测提供指导依据。     

石墨化碳素钢室温压缩过程中的不均匀变形行为

将0. 46%含碳量(质量分数)的石墨化碳素钢在万能材料试验机上进行室温压缩变形,试验钢表现出良好的压缩变形性能.根据载荷-位移曲线的变化特点,试验钢的压缩变形过程以位移7. 0 mm(对应相对压下量为58. 3%)为节点分为两个阶段:在位移≤7. 0 mm的压缩阶段,载荷呈线性增加,压缩试样的鼓度值逐渐增加而达到一个极大值(14. 6%),压缩试样中心位置的维氏硬度增幅最大,为38. 1 HV,至位移7. 0 mm时试样端面径向伸长率的增幅为34%;而在位移> 7. 0 mm的压缩阶段,载荷呈指数增加,压缩试样的鼓度值从极大值开始逐渐减小,至位移为10. 72 mm时(相对压下量为89. 3%),试样端面的径向伸长率相比于位移7. 0 mm时增加了83. 1%,压缩试样的中心位置的维氏硬度增幅最小,为32. 7 HV.上述试验数据表明,在位移≤7. 0 mm的压缩过程中,压缩试样内的三个不均匀变形区的位置与传统压缩模型一致,但是当压缩变形进入位移> 7. 0 mm的压缩过程中,试样中心位置已不再是传统压缩模中变形程度最大的变形区了,即在这个阶段试样中的3个不均匀变形区的变形程度发生了改变.正因这种不均匀变形区变形程度的改变导致了变形过程中载荷的急剧增加和鼓度值的减低.另外,在压缩变形过程中,三个不均匀变形区中石墨粒子的微观变形量总是高于铁素体基体,其原因之一可以归结为石墨粒子中层与层之间容易于滑动的结果.     

Effects of strain rate on austenite stability and mechanical properties in a 5Mn steel

The austenite stability and the mechanical properties in a typical medium Mn grade steel,i.e.,5Mn steel,were investigated under a wide range of strain rates thr...     

Effects of austenitizing temperature and austenite grain size on the formation of athermal martensite in an iron-nickel and an iron-nickel-carbon alloy

The effects of austenitizing conditions on the kinetics at the start of martensite formation in Fe-31Ni and Fe-31 Ni-0.28C alloys have been studied using electrical-resistance measurements during cooling of the specimens to follow the course of the transformation. The primary object of the study was to decide whether or not a change in austenitizing temperature, in the absence of a change in austenite grain size, has any effect on the Ms temperature or the burst characteristics of athermal martensite. It is concluded that it does not, suggesting that the potential nuclei (embryos) of martensite are mechanically stable crystal defects. Another interesting observation is that when the austenite grain size is small, the M_b temperature increases with increasing grain size and the burst is always small. When the austenite grains are coarse, the M_b temperature is independent of the grain size and the burst is large. It is suggested that this phenomenon is a result of the elastic shear stress concentration being related to the size of the first martensite plate and, in turn, to the size of the austenite grain. M. Umemoto, formerly a Graduate Student in the Department of Materials Science at Northwestern University W. S. Owen, formerly at Northwestern University     

Influence of austenite strength on martensite start temperature Ms

It is known that austenite strength determines the morphology of the new phase during martensitic transformation. As the strength of austenite influences the growth of a martensite crystal, i.e. the movement of the austenite/martensite interface, a correlation between strength of the parent phase and M_s has to exist. M_s depends on thermodynamical and mechanical properties of the alloys. To distinguish the individual variables, austenite strength was changed by different hardening mechanisms: solid solution hardening, plastic deformation or both.     

热变形及工艺参数对C-Mn钢过冷奥氏体转变的影响

以C-Mn钢SS400为研究对象,在Gleeble-1500热模拟试验机上进行了热模拟实验,测得了试验钢静、动态CCT曲线.从CCT曲线可以看出,未变形时试验钢奥氏体向铁素体开始转变温度在800～690℃之间,变形后在810～710℃之间,变形提高了Ar3点.当冷却速度比较低时(0.2～5℃/s),变形奥氏体转变后室温组织的维氏硬度值低于未变形奥氏体转变后的室温组织维氏硬度值,而当冷却速度比较高时(5～20℃/s),趋势正好相反;变形后淬火试验结果表明应变量为0.4、变形速率为1/s 时,试验钢在820℃时已经发生形变诱导铁素体相变.随着应变量的增大和变形温度的降低,原奥氏体晶界的铁素体量增多.     

Mass transport of carbon in one and two phase iron-nickel alloys in a temperature gradient

The flux of carbon atoms induced by an applied temperature gradient on a specimen was investigated for an Fe-32.5 wt pct Ni alloy for six carbon concentrations. Carbon was found to migrate to the higher temperature region in the low carbon single phase alloys. However, in the higher carbon alloys an abrupt jump in carbon concentration results when a portion of the specimen is in a two-phase region while the portion in the one-phase region exhibits the usual solute migration toward the higher temperature. A value of -12.2 +- 0.4 kJ mol^-1 was obtained for the heat of transport of carbon in the γ-phase Fe-Ni alloys for a wide range of carbon concentrations. A model for diffusion and thermotransport in multiphase systems is presented to explain the observed results. I. C. I. OKAFOR, formerly with Ames Laboratory, DOE     

The influence of microstructure and strain rate on the compressive deformation behavior of Ti-6Al-4V

This article reports on a study of deformation of Ti-6Al-4V in compression. In particular, two different microstructures, the equiaxed microstructure and the Widmanstätten microstructure, were generated from the same parent material and their properties were measured. The results show that at small strains, the mechanical response of samples with these microstructures is similar. The yield strength and the flow stress at a 0.05 true strain have similar values; these increase with increasing strain rate over the range of 0.1 to 1000 s^?1. However, samples with the Widmanstätten microstructure failed at a smaller strain than their counterparts with the equiaxed microstructure, and this difference increased with increasing strain rate. Examination of cross sections of samples deformed to different levels of strain showed that the deformation was inhomogeneous. As the sample barreled, the deformation built up on the surfaces of two cones of material whose apices met in the center of the sample. Cracks formed in the corners of the samples and propagated in toward the center. In samples with the equiaxed microstructure, short cracks and voids formed, but they were usually blunted at the grain boundaries. Long cracks were only observed immediately before failure. In samples with the Widmanstätten microstructure, cracks could grow within the laths more easily, and, as a result, longer cracks formed at lower strains. We propose that this difference leads to the differences in the failure strains for these two microstructures. Finally, examination of data in the literature, along with our own results, indicates that the interstitial content plays an important role in determining the yield stress of the material.     

钒微合金化低碳钢高温变形动态再结晶

利用热模拟压缩试验测定了不同钒含量的钒微合金化低碳钢在900～1000℃温度区间和0.1～1s-1变形速率范围内的真应力-真应变曲线.对曲线的分析表明:随钢中钒含量的增加,低碳钢的动态再结晶开始时间延长,变形奥氏体的动态再结晶名义激活能提高.实验钢薄膜试样的TEM观察表明,钢中的微量钒以固溶态存在于奥氏体中,微量的固溶钒对奥氏体动态再结晶起到抑制作用.     

Influence of deformation conditions and texture on the high temperature flow stress of magnesium AZ31

The evolution of hot working flow stress with strain is examined in torsion, uniaxial compression and channel die compression. The flow stress was found to be strongly dependent on texture and deformation mode. At low strains this dependency accounted for a difference in flow stress of up to a factor of two. At higher strains the influence of texture and deformation mode was less marked. The stresses corresponding to an equivalent strain of 0.5 were modelled using a power law expression with an activation energy of 147 kJ/mol and a strain rate exponent of 0.15. The influence of texture and deformation mode on flow stress is rationalised in terms of the influence of prismatic slip, twinning and dynamic recrystallisation on deformation stress and structure.