Effect of Indentation Size and Strain Rate on Nanomechanical Behavior of Ti-6Al-4VAlloy

Nanoindentation experiments were carried out at strain rates of 0.05, 0.10, 0.15 and 0.20/s at indentation depths of 1000, 1500 and 2000 nm to investigate the nanomechanical behaviour of Ti-6Al-4V alloy. The strain rate had little influence on nanohardness, however, nanohardness as well as Young’s modulus gradually decreased with the increase of indentation depth indicating strong indentation size effects. The relation between H² and 1/h exhibited a good linear relationship, and it is observed that the effect of strain gradient on σ/σ₀ is significant at high strain rates according to Nix and Gao strain gradient model. The analysis of plastic behaviour revealed that, strain rates had no significant effect on strain hardening exponent, but had little influence on yield stress.     

Modeling Grain Size and Strain Rate in Linear Friction Welded Waspaloy

The high-temperature deformation behavior of the Ni-base superalloy, Waspaloy, using uniaxial isothermal compression testing was investigated at temperatures above the γ′ solvus, 1333 K, 1373 K, and 1413 K (1060 °C, 1100 °C, and 1140 °C) for constant true strain rates of 0.001, 0.01, 0.1, and 1 s^?1 and up to a true strain of 0.83. Flow softening and microstructural investigation indicated that dynamic recrystallization took place during deformation. For the investigated conditions, the strain rate sensitivity factor and the activation energy of hot deformation were 0.199 and 462 kJ/mol, respectively. Constitutive equations relating the dynamic recrystallized grain size to the deformation temperature and strain rate were developed and used to predict the grain size and strain rate in linear friction-welded (LFWed) Waspaloy. The predictions were validated against experimental findings and data reported in the literature. It was found that the equations can reliably predict the grain size of LFWed Waspaloy. Furthermore, the estimated strain rate was in agreement with finite element modeling data reported in the literature.     

Effect of Strain Rate on Deformation Behavior of Semi-Solid Dendritic Alloys

The behavior of semi-solid Sn-Pb alloys was studied in compression between two parallel plates. Small dendritic samples were deformed at cross-head speeds leading to initial strain rates ranging from 1.3 × 10^-3 s^-1 to 1.2 × 10³ s^-1 in the semi-solid state at a temperature just above the eutectic. At the lower rates of deformation, breakdown of the dendrite structure occurs, at strains of 0.2 to 0.4, and a high degree of segregation of the liquid phase occurs. For higher rates the segregation no longer occurs to such a great extent and the alloy deforms more homogeneously. Some related experiments involving compression over a filter are presented to obtain stress-strain relations in bulk compression for later analysis. The behavior in compression of alloys in the semi-solid state may be used as a refining process in the low strain-rate range where segregation of the liquid is large. It may also prove useful in the high strain-rate range as a forming method. M. SUERY, formerly Visiting Scientist, Materials Processing Center, Massachusetts Institute of Technology.     

Size Effects of Hardness and Strain Rate Sensitivity in Amorphous Silicon Measured by Nanoindentation

Metallurgical and Materials Transactions A - In this work, dynamic mechanical properties of amorphous silicon and scale effects were investigated by the means of nanoindentation. An amorphous...     

Effect of Strain Rate on the Yield Stress of Ferritic Stainless Steels

The effect of strain rate on the yield stress of ferritic stainless steel sheet was experimentally determined and a previously developed model was applied to the data. Five ferritic stainless steel alloys, including one in two thicknesses, were mechanically tested at room temperature in uniaxial tension at strain rates ranging from 0.001 to 300 s⁻¹, and low-strain-rate tests were selectively performed at nonambient temperatures. The hypothesis that ferritic stainless steels react similarly to strain rate as mild steels was investigated by the application of a widely accepted strengthening model, based on body-centered-cubic (bcc) crystal lattice deformation mechanisms, to the experimental data.[1] Yield stresses were compared to model predictions and good agreement was found. The results allow for the prediction of yield stresses for these materials over strain rate ranges of 0.001 to 300 s⁻¹, and as a function of test temperature. Model parameters for the ferritic stainless steels were reasonable relative to those previously reported for pure bcc ferritic iron.[1] A correlation between the effect of alloying additions on solid solution strengthening and the athermal component of shear stress is also suggested. The results allow prediction of yield stress of ferritic stainless steels over a wide range of strain rates and temperatures. This article is based on a presentation made in the symposium entitled “Dynamic Behavior of Materials,” which occurred during the TMS Annual Meeting and Exhibition, February 25–March 1, 2007 in Orlando, Florida, under the auspices of The Minerals, Metals and Materials Society, TMS Structural Materials Division, and TMS/ASM Mechanical Behavior of Materials Committee.

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应变速率的影响与带钢热轧模型预报精度改进

轧制力预报模型是带钢热轧模型的核心，而变形阻力模型作为轧制力预报模型的核心，直接决定轧制力预报精度。从解决调试生产中存在的实际问题出发，对变形阻力模型的结构以及自适应方法进行分析。指出应变速率的影响、存在的问题以及相应的改进措施，并进一步阐明在实践中应用所取得的良好效果。     

冷形变量和加热速率对IF钢罩式退火再结晶晶粒尺寸分布的影响

研究了在两种加热速率、3种冷形变量（60％、70％和80％）条件IF钢罩式退火再结晶晶粒尺寸的分布规律。在冷形变量大于或等于70％时，其再结晶晶粒尺寸分布符合对数正态分布。在此基础上采用非线性回归法求出再结晶晶粒尺寸的分布函数（SDF）。研究结果表明，形变量对再结晶晶粒尺寸的分布有明显的影响，而加热速率和影响较弱。     

应变速率对X70管线钢应力腐蚀行为的影响

 通过波动拉伸载荷试验（F-SSRT）,研究了不同应变速率对X70管线钢在阴极电位下近中性介质应力腐蚀开裂（SCC）行为的影响。结果表明：X70钢在特定的近中性介质中,在波动拉伸载荷和阴极电位（-850mVSCE）条件下,存在着一定的SCC敏感性。在波动拉伸载荷试验（F-SSRT）条件下,SCC裂纹的扩展机理与匀速慢拉伸试验（SSRT）时的裂纹扩展机理明显不同,SCC裂纹可以从材料表面产生的微孔处萌生并沿主应力面扩展,当应变速率处于一定范围时,SCC裂纹转变为沿45°最大剪应力面扩展。分析了氢在F-SSRT试样SCC发生发展过程中的作用和机理。     

应变速率对钛合金室温拉伸性能的影响

拉伸速率对金属材料的室温拉伸性能影响很大,文章介绍了不同应变速率对钛合金Ti6Al4V(TC4)、TA15和Ti1023室温拉伸性能影响的研究进展,并通过分析和归纳,阐述了第一应变速率和第二应变速率对钛合金的抗拉强度、屈服强度、断后伸长率及断面收缩率的影响程度和规律,最后对应变速率对钛合金室温拉伸性能影响研究的发展方向进行了展望。     

应变速率对Fe-Mn-Si-Cr-Ni合金组织和力学性能的影响

目前的TRIP钢由于Mn、Si含量较低,室温条件下仅保留约10%的残余奥氏体,结果使变形过程中材料产生的相变诱发塑性量有限,不能满足一些要求产生更大相变诱发塑性场合的需要。研究表明：提高钢中Mn、Si含量并辅以适量其它合金元素（如铬和镍）是提高材料相变诱发塑性的有效途径。研究了Fe-17Mn-5Si-10Cr-4Ni合...     

The Contribution of Dislocation Density and Velocity to the Strain Rate and Size Effect Using Transient Indentation Methods and Activation Volume Analysis

Constant load indentation creep and load relaxation tests were performed on several FCC Al, Ag, and Ni metals that exhibit indentation size effect (ISE) to examine the coupled relationship between the activation volume V* at specific loads, the dislocation density ρ, and the dislocation velocity (v) from kinetics-based perspective. The influence of the ISE on the dislocation velocity and the activation volume is thoroughly examined using the two independent indentation creep and load relaxation experiments. This study is carried out based on the general experimental and theoretical hypothesis that the ISE is driven by a dislocation mechanism, specifically the increase in the geometrically necessary dislocation density at shallow depth of indentation due to the presence of a large strain gradient. Geometrically necessary dislocations dominate the material’s propensity to work harden when their density exceeds the density of statistically stored dislocations and are primarily considered responsible for the size effects observed in indentation. Based on the preestablished bilinear behavior and the decrease in the activation volume with hardening due to dislocation–dislocation interaction in indentation creep experiments by Elmustafa and Stone, 2003, we demonstrate that the dislocation velocity exhibits a bilinear behavior when plotted vs hardness using the Orowan’s relation. Ag and Ni distinctively depict a bilinear behavior in the dislocation velocity with hardness, whereas Al exhibited a rather linear behavior. This can be explained by the fact that aluminum’s work-hardening rate is higher due to the increase in the rate and intensity of cross-slip and dislocation climbing.

     

Influence of Strain Rate,Temperature, Plastic Strain,and Microstructure on the Strain Rate Sensitivity of Automotive Sheet Steels

This work identifies the influence of strain rate, temperature, plastic strain, and microstructure on the strain rate sensitivity of automotive sheet steel grades in crash conditions. The strain rate sensitivity m has been determined by means of dynamic tensile tests in the strain rate range 10^?3–200 s^?1 and in the temperature range 233–373 K. The dynamic flow curves have been tested by means of servohydraulic tensile testing. The strain rate sensitivity decreases with increasing plastic strain due to a gradual exhausting of work hardening potential combined with adiabatic softening effects. The strain rate sensitivity is improved with decreasing temperature and increasing strain rate, according to the thermally activated deformation mechanism. The m‐value is reduced with increasing strength level, this decrease being most pronounced for steels with a yield strength below 400 MPa. Solid solution alloying with manganese, silicon, and especially phosphorous elements lowers the strain rate sensitivity significantly. Second phase hardening with bainite and martensite as the second constituent in a ferritic matrix reduces the strain rate sensitivity of automotive sheet steels. A statistical modeling is proposed to correlate the m‐value with the corresponding quasistatic tensile flow stress.     

Effect of Al and Gd Solutes on the Strain Rate Sensitivity of Magnesium Alloys

Pure magnesium and two binary alloys, Mg-1 wt pct Al and Mg-1.4 wt pct Gd, have been prepared with comparable grain sizes and textures. The alloys have been tensile tested at various strain rates and temperatures to examine the strain rate sensitivity (SRS). It has been found that Mg and Mg-Al show increasing SRS with increasing deformation temperatures. The Mg-Gd alloy showed decreasing SRS with increasing deformation temperatures and exhibited a negative SRS at 200 °C and 250 °C. Above these temperatures, the SRS returned to a positive value. The elongation to fracture was not effected by the SRS, and it has been concluded that for the alloys and conditions examined, the influences of mechanical twinning and dynamic recrystallization dominate the elongation behavior, rather than the SRS.     

Effect of Strain Rate on Ductile-to-Brittle Transition Temperature of a Free-Standing Pt-Aluminide Bond Coat

The ductile-to-brittle transition temperature (DBTT) of a free-standing Pt-aluminide (PtAl) bondcoat was determined using the microtensile testing method and the effect of strain rate variation, in the range 10⁻⁵ to 10⁻¹ s⁻¹, on the DBTT studied. The DBTT increased appreciably with the increase in strain rate. The activation energy determined for brittle-to-ductile transition, suggested that such transition is most likely associated with vacancy diffusion. Climb of 〈100〉 dislocations observed in analysis of dislocation structure using a transmission electron microscope (TEM) supported the preceding mechanism.     

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。     

Effect of Temperature and Strain Rate on Dynamic Properties of Low Silicon TRIP Steel

The dynamic tensile test of 0.11C-0.62Si-1.65Mn TRIP steel was carried out at different strain rates and test temperatures. The results show that both temperature and strain rate affect the retained austenite transformation. At high strain rates, the uniform elongation decreases, whereas the total elongation and energy absorption increase. The tensile strength is less strain rate sensitive. With raising test temperature, the tensile strength is reduced and the mechanical properties generally deteriorate, especially at 110℃,However, excellent mechanical properties were obtained at 50℃ and 75℃.     

变形速率对普碳钢中形变诱导铁素体相变的影响

针对普通碳素钢(Q235类型),研究在Ae3～Ar3温度区间内采用形变诱导铁素体机制获得超细晶铁素体的数量与变形速率的相互关系。实验在Gleeble 1500热模拟实验机上进行。实验方案为：1000℃保温2min,以10℃／s的速度冷却到变形温度[Ae3(840℃)至Ar3(780℃)],变形量为30％～50％,变形后立即水淬。结果表明,在840℃变形时,随着变形速率的增大,形变诱导铁素体量增多;在780℃变形时,随着变形速率的增大,形变诱导铁素体量减少;而在840-780℃之间变形时,变形速率存在最佳值,在该值下诱导生成的铁素体量最大。     

热加工中道次间残余应变率对流动应力的影响

采用恒应变速率凸轮式压缩试验机,测定研究了GCr15、16MnNb钢在两道次变形过程中,热加工道次时间间隔对钢流动应力及道次间软化率的影响规律.     

Effect of Strain Rate on Plastic Flow in Zr-Based Metallic-Glass-Reinforced Porous Tungsten Matrix Composites

The rate-dependent deformation of Zr₃₈Ti₁₇Cu_10.5Co₁₂Be_22.5 bulk metallic-glass-reinforced porous tungsten matrix composites was investigated over a wide range of strain rates. The composites were examined in two forms: the as-cast composite and the as-extruded composite by extrusion. In addition to showing greater strain hardening, the as-cast composite also shows much more obvious strain rate dependence of flow stress than the as-extruded composite. Microhardness tests were performed on the tungsten and the metallic glass phase in both composites, respectively. The results from the microhardness measurements indicate that the strain rate sensitivity of the as-extruded composite is primarily a result of strain rate sensitivity of the tungsten phase.     

Effect of Microstructure in TRIP Steel on Its Tensile Behavior at High Strain Rate

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