超细晶纯锆热压缩变形行为及组织演变研究

采用Gleeble-3500热模拟机对等通道转角挤压(ECAP)+旋锻复合细化工艺制备的超细晶纯锆,在温度为300～450℃、应变速率为1×10~(-3)～1×10~(-1) s~(-1)的条件下进行压缩实验,分析了超细晶纯锆在热变形过程中流变应力特点及显微组织演变规律。并使用人工神经网络建立了超细晶纯锆压缩本构模型。结果表明:变形初期流变应力迅速升高,达到峰值应力后逐渐进入稳态流变阶段。随着温度的升高和应变速率的降低,稳态流变应力降低。超细晶纯锆屈服强度显著高于粗晶纯锆;超细晶纯锆的应变速率敏感指数m值为0.028～0.132,高于粗晶纯锆,而且低应变速率和高温有利于提高超细晶纯锆的塑性;透射电镜(TEM)结果表明:随着变形温度的升高和应变速率的降低,超细晶纯锆呈现明显的动态回复与再结晶,晶内位错密度减小,晶界逐渐清晰,晶粒尺寸逐渐增大。基于人工神经网络的压缩本构模型预测结果表明:预测值与实验值的平均相对误差(AARE)为0.5385%,相关系数(R)为0.9991,模型预测精度高。     

初始晶粒尺寸对2024铝合金热变形行为和组织的影响

利用永磁搅拌近液相线铸造和普通铸造方法制备不同晶粒尺寸的2024铝合金铸锭,利用Gleeble-1500热模拟试验机研究初始晶粒尺寸对不同压缩变形条件下2024铝合金的热变形行为和变形后显微组织的影响。研究表明:2024铝合金的热变形行为依赖于变形条件和初始组织。初始晶粒尺寸对流变应力的影响是:当应变速率小于0.1 s~(-1)时,流变应力随晶粒尺寸减小而减少;当应变速率为10 s~(-1)时,流变应力随晶粒尺寸减小而增大。降低变形温度会弱化晶粒尺寸对流变应力的影响。热压缩流变应力随应变速率增大而增大,随变形温度升高而减小。应变速率为10 s~(-1)时,热压缩应力应变曲线呈现周期性波动;只在粗晶2024铝合金中发现变形剪切带。     

可压缩纯钼粉末烧结材料的塑性变形行为及影响因素

基于单轴压缩实验,研究纯钼粉末烧结材料的塑性变形行为及其影响因素。结果表明:可压缩纯钼粉末烧结材料的塑性变形行为对初始相对密度、温度和应变速率的变化相当敏感,其流动应力随应变速率的增加而增加,随温度的升高而减小;高温条件下材料对应变速率不太敏感,但初始相对密度在低温状况下对流动应力的影响更甚;对压缩后试样的微观组织分析显示:初始平均粒径为44.0μm的粗大等轴晶组织经过约35%的单轴压缩后,其中心主变形区域得到平均粒径为1.45μm完全致密的超细晶组织;初始相对密度越大,材料屈服强度越低,出现破裂的时间越早;其硬度增加速率对温度变化不敏感,而提高温度则有利于降低屈服强度。     

工业纯钛压缩变形与损伤行为的温度效应

利用材料万能试验机、扫描电子显微镜等仪器系统分析了粗晶工业纯钛在高、低温环境下的变形及损伤行为.结果表明:工业纯钛的压缩变形及损伤行为与温度密切相关.随着温度升高,压缩屈服强度、峰值应力以及流变应力逐渐减小.表面变形及损伤特征发生相应的变化,在200℃以下表现出反常的随温度升高塑形变形能力下降的现象.     

平面应变压缩工业纯钛的动态再结晶研究

利用Gleeble—3800型热模拟实验机,在变形温度650~800℃、应变速率1s~(-1)和5s~(-1)条件下对工业纯钛进行多道次平面应变压缩实验,模拟实际热轧生产。通过真应力、应变、软化率和加工硬化率等数据,研究工业纯钛多道次变形过程的软化现象和动态再结晶临界应变。结果表明,工业纯钛间歇时间内的软化率随着变形温度升高和应变速率降低而增加;再结晶临界应变量随着变形温度升高而减小,且动态再结晶临界应变ε_c与峰值应变ε_p存在线性关系:ε_c=0.467 6ε_p。     

Cu-Cr-Zr合金的热变形行为

采用Gleeble-3500热模拟实验机对Cu-Cr-Zr合金进行了压缩变形实验,分析了在变形温度为25~700℃、应变速率为0.0001~0.1000s-1的条件下流变应力的变化规律,利用扫描电镜及透射电镜分析合金在热压缩过程中的组织演变及动态再结晶机制。结果表明:Cu-Cr-Zr合金在热变形过程中发生了动态再结晶,且变形温度和应变速率均对流变应力有显著的影响,流变应力随着变形温度的升高而降低,随着应变速率的增加而升高,说明该合金属于正应变速率敏感材料;当变形温度为400~500℃时,低应变速率(0.0001~0.0010 s-1)的真应力-真应变曲线呈现动态再结晶曲线特征,高应变速率(0.01~0.10 s-1)的真应力-真应变曲线呈现动态回复特征;在真应力-真应变曲线的基础上,采用双曲正弦模型能较好地描述Cu-Cr-Zr合金高温变形时的流变行为,建立了完整描述合金热变形过程中流变应力与应变速率和变形温度关系的本构方程,确定了合金的变形激活能为311.43 kJ·mol-1。     

工业纯钛TA2热变形过程的流变行为本构方程

利用Gleeble-3800热模拟实验机研究了工业纯钛TA2的热变形行为.变形温度为750~1000℃，步长50℃，应变速率分别为0.01、0.1、1和10 s-1.实验结果表明，TA2在热压缩变形过程中发生了加工硬化以及动态回复、动态再结晶.随着变形温度的降低和应变速率的增加，流变应力逐渐增加.为了准确预测TA2的高温流变行为，基于实验数据和双曲正弦Arrhenius模型构建了考虑应变影响的本构方程，本构方程中材料常数α、n、Q、lnA与应变之间存在6阶多项式关系.本文所提出考虑应变影响的本构方程可以用于研究工业纯钛TA2的高温流变行为.      

纯镍的高温塑性变形行为及本构方程

利用Gleeble-3800热模拟试验机,采用正交实验的方法,进行了单道次压缩试验,研究了纯镍N6板材在不同变形温度和不同应变速率条件下的大变形量热变形行为。结果显示,纯镍N6板坯在大的应变量条件下,出现了稳态流变之后,流变应力再升高的现象;在应变速率一定的条件下,流变应力随变形温度的升高而降低;在相同的变形温度条件下,流变应力随应变速率的增大而增大。说明流变应力与变形温度和应变速率的关系敏感。在变形速率为40 s-1时,流变应力曲线失稳,呈波浪形,说明在较大的应变速率条件下,纯镍N6板材呈现明显的流变失稳特征。并对实验测得的数据进行线性回归,得出双曲正弦函数形式本构方程中的材料参数,将材料参数对应变进行二次拟合,建立了纯镍N6板坯热变形过程中流变应力与变形温度、应变速率及应变的本构关系。经验证,所建立的本构关系能够很好地反应纯镍N6板坯的实际热变形行为特征。     

纯钼高温塑性变形及流变应力行为

采用MMS-300热模拟实验机研究纯钼在变形温度为900~1 300℃和应变速率为0.004~1 s-1条件下的高温塑性变形行为。分析了纯钼流变应力与应变速率、变形温度之间的关系,计算了纯钼高温塑性变形时的变形激活能。研究结果表明:纯钼在热变形过程中流变应力随应变速率的增加而增加,随温度的升高而降低,且其高温塑性变形行为可以用Zener-Hollomon参数的流变应力方程进行描述。该纯钼在实验条件范围内发生了明显的动态回复与动态再结晶。     

2124铝合金高温压缩流变形行为研究

采用圆柱试样在Gleeble-1500热/力模拟试验机上进行高温压缩变形试验,研究了2124铝合金在高温塑性变形过程中流变应力的变化规律.试验在变形温度为350～480 ℃、应变速率0.04～10 s-1的条件下进行.结果表明:应变速率和变形温度的变化对合金稳态流变应力有明显的影响,在低应变速率条件下,流变应力开始随应变增加而增大,达到峰值后趋于平稳,表现出近稳态特征;而在高应变速率条件下,应力出现强烈锯齿波动,达到峰值后随着应变的增加锯齿波动趋于平缓;2124铝合金高温塑性变形时的流变行为可用Zener-Hollomon参数的双曲正弦函数来描述.     

Effect of silicon carbide nanoparticles on hot deformation of ultrafine-grained aluminium nanocomposites prepared by hot powder extrusion process

The flow behaviour of Al–SiC nanocomposites prepared by mechanical milling and hot powder extrusion methods was studied at different temperatures (350–500°C) and strain rates (0.005–0.5 s^?1). The flow of the Powder metallurgy nanocomposites exhibited a peak stress followed by a dynamic flow softening behaviour. It was shown that mechanical milling increased high-temperature strain rate sensitivity of ultrafine-grained (UFG) aluminium while decreasing its flow dependence to temperature. Constitutive analysis of the hot deformation process by Zener–Hollomon parameter (Z) also indicated a remarkable increase in the deformation activation energy (about 40%). Likewise, SiC nanoparticles (up to 2vol.-%) were shown to contribute in the high-temperature strengthening of UFG aluminium with a significant effect on its thermal stability. The findings were explained based on the pinning effect of hard nanoparticles on grain boundaries and mobile dislocations as well as microstructure stabilisation at elevated temperatures.     

Analysis of Strain Rate Sensitivity of Ultrafine-Grained AA1050 by Stress Relaxation Test

Commercially pure aluminum sheets, AA 1050, are processed by accumulative roll bonding (ARB) up to eight cycles to achieve ultrafine-grained (UFG) aluminum as primary material for mechanical testing. Optical microscopy and electron backscattering diffraction analysis are used for microstructural analysis of the processed sheets. Strain rate sensitivity (m-value) of the specimens is measured over a wide range of strain rates by stress relaxation test under plane strain compression. It is shown that the flow stress activation volume is reduced by decrease of the grain size. This reduction which follows a linear relation for UFG specimens, is thought to enhance the required effective (or thermal) component of flow stress. This results in increase of the m-value with the number of ARB cycles. Strain rate sensitivity is also obtained as a monotonic function of strain rate. The results show that this parameter increases monotonically by decrease of the strain rate, in particular for specimens processed by more ARB cycles. This increase is mainly linked to enhanced grain boundary sliding as a competing mechanism of deformation acting besides the common dislocation glide at low strain rate deformation of UFGed aluminum. Recovery of the internal (athermal) component of flow stress during the relaxation of these specimens seems also to cause further increase of the m-value by decrease of the strain rate.     

Comparison of compressive deformation of ultrafine-grained 5083 Al alloy at 77 and 298 K

The compression behaviors of well-annealed coarsegrained (CG) and ultrafine-grained (UFG) 5083 Al alloys at 77 and 298 K were compared. For the CG alloy, stage II and III strain hardening were dominant at 77 and 298 K, respectively, depending on the completeness of dislocation cell formation. The UFG alloy exhibited the elastic-near perfectly plastic behavior without distinctive dislocation cell formation at both temperatures. For both alloys, the flow stress at 77 K was much higher than that at 298 K. This work was supported by the Basic Research Program (Grant No. R01-2003-000-10202-0) of the Korea Science & Engineering Foundation.     

Formability of Ultrafine-Grained Interstitial-Free Steels

The stretch formability of ultrafine-grained (UFG) interstitial-free steel (IF-steel) produced by equal-channel angular extrusion/pressing (ECAE/P) via various strain paths was investigated with a miniaturized Erichsen test. A coarse-grained (CG) sample demonstrated high formability with an Erichsen index (EI) of 4.5 mm. Grain refinement by ECAE decreased the formability, but increased the required punch load (F _EI) depending on the applied strain paths. The EI values were 0.35, 2.90, and 3.91 mm for 8A-, 8Bc-, and 8C-processed samples, respectively. Decrease in the biaxial stretch formability was attributed to the limited strain-hardening capacity of the UFG microstructure. Also, the grain morphology of the UFG microstructure was found to be very influential on stretch formability. Heavily elongated grain morphology in the 8A-processed microstructure resulted in the lowest formability due to the increased cracking tendency through elongated grain boundaries. However, the UFG microstructures with equiaxed grains obtained after 8C and 8Bc ECAE resulted in better formability compared to 8A. The UFG microstructure reduced the roughness (orange peel effect) of the free surface of the biaxial stretched samples by decreasing the non-uniform grain flow leading to the so-called orange peel effect. It should be noted that the strength and ductility values gained from uniaxial tensile tests are not comparable directly to the EI and F _EI values determined from the Erichsen tests. Finally, it is important to emphasize that the UFG microstructure produced by a suitable strain path leading to equiaxed grains below 1 μm could be highly deformed even under multiaxial stress conditions.     

The effect of grain size, strain rate, and temperature on the mechanical behavior of commercial purity aluminum

Commercial purity aluminum AA1050 was subjected to equal channel angular extrusion (ECAE) that resulted in an ultrafine-grained (UFG) microstructure with an as-received grain size of 0.35 μm. This UFG material was then annealed to obtain microstructures with grain sizes ranging from 0.47 to 20 μm. Specimens were compressed at quasi-static, intermediate, and dynamic strain rates at temperatures of 77 and 298 K. The mechanical properties were found to vary significantly with grain size, strain rate, and temperature. Yield stress was found to increase with decreasing grain size, decreasing temperature, and increasing strain rate. The work hardening rate was seen to increase with increasing grain size, decreasing temperature, and increasing strain rate. The influence of strain rate and temperature is most significant in the smallest grain size specimens. The rate of work hardening is also influenced by strain rate, temperature, and grain size with negative rates of work hardening observed at 298 K and quasi-static strain rates in the smallest grain sizes and increasing rates of work hardening with increasing loading rate and grain size. Work hardening behavior is correlated with the substructural evolution of these specimens.     

Compression along the Easy-Glide Orientation of Ultrafine and Fine-Grained Mg-3Al-1Zn Alloy

The deformation behavior of textured ultrafine-grained (UFG) and fine-grained (FG) Mg-3Al-1Zn alloy compressed along the easy-glide orientation was studied. It was found that severe strain localization within very thin shear bands (SBs) caused the loss of ductility in UFG Mg-3Al-1Zn alloy. Increasing grain size increases the width of SBs, which decreases the degree of strain localization. The reason for the occurrence of severe strain localization in textured UFG Mg-3Al-1Zn alloy is proposed.     

Low-temperature superplasticity of ultra-fine-grained Ti-6Al-4V processed by equal-channel angular pressing

The low-temperature superplasticity of ultra-fine-grained (UFG) Ti-6Al-4V was established as a function of temperature and strain rate. The equiaxed-alpha grain size of the starting material was reduced from 11 to 0.3 μm (without a change in volume fraction) by imposing an effective strain of ∼4 via isothermal, equal-channel angular pressing (ECAP) at 873 K. The ultrafine microstructure so produced was relatively stable during annealing at temperatures up to 873 K. Uniaxial tension and load-relaxation tests were conducted for both the starting (coarse-grained (CG)) and UFG materials at temperatures of 873 to 973 K and strain rates of 5 × 10⁻⁵ to 10⁻² s⁻¹. The tension tests revealed that the UFG structure exhibited considerably higher elongations compared to those of the CG specimens at the same temperature and strain rate. A total elongation of 474 pct was obtained for the UFG alloy at 973 K and 10⁻⁴ s⁻¹. This fact strongly indicated that low-temperature superplasticity could be achieved using an UFG structure through an enhancement of grain-boundary sliding in addition to strain hardening. The deformation mechanisms underlying the low-temperature superplasticity of UFG Ti-6Al-4V were also elucidated by the load-relaxation tests and accompanying interpretation based on inelastic deformation theory.     

Microstructure and Mechanical Properties of Ti-15-3 Alloy Gas Tungsten Arc Welds Prepared Using CP-Titanium Filler

Strength and ductility of fusion zone of metastable β titanium alloy welds can be improved by choosing suitable fillers. This paper reports the effects of using CP-Ti filler on the microstructural and mechanical properties of Ti-15-3 weldments. Full penetration autogenous and CP-Ti filler welds were produced by pulsed gas tungsten arc welding. X-ray diffraction analysis revealed small amounts of α-Ti phase in the diffraction pattern obtained for welds prepared using CP-Ti filler. Transmission electron microscopy analysis showed presence of grain boundary and intragranular α in the fusion zone of the welds prepared using CP-Ti filler. The welds prepared with CP-Ti filler showed higher hardness, higher UTS and lower % strain compared to autogenous welds.     

Dynamic Micro-Strain Analysis of Ultrafine-Grained Aluminum Magnesium Alloy Using Digital Image Correlation

Tensile tests were performed in situ on an ultrafine-grained (UFG) Al-Mg alloy using a micro-tensile module in a scanning electron microscope. The micro-strain evolution was tracked and measured using digital image correlation (DIC). A fine random speckle pattern was required to achieve high resolution and accuracy of strain measurement using DIC. To produce the speckle pattern, a patterning method was developed using electron beam lithography to deposit a gold speckle pattern. The nanoscale feature size of this gold pattern (45 nm) was useful for identifying the micro-strain among individual grains of the UFG Al-Mg alloy. Microstructural aspects of the UFG Al-Mg alloy were revealed by analysis of electron backscattered diffraction (EBSD) patterns. Finally, the effect of the UFG Al-Mg alloy microstructure on the nanoscale deformation mechanism was investigated by combining EBSD and DIC data in a contour map. This combined technique provides a method for direct measurement of micro-strain and is potentially useful for deformation studies of a wide range of nanostructured materials.     

Microstructure and Mechanical Properties of Oxide-Dispersion Strengthened Al6063 Alloy with Ultra-Fine Grain Structure

The microstructure and mechanical properties of the ultra-fine grained (UFG) Al6063 alloy reinforced with nanometric aluminum oxide nanoparticles (25 nm) were investigated and compared with the coarse-grained (CG) Al6063 alloy (~2 μm). The UFG materials were prepared by mechanical alloying (MA) under high-purity Ar and Ar-5 vol pct O₂ atmospheres followed by hot powder extrusion (HPE). The CG alloy was produced by HPE of the gas-atomized Al6063 powder without applying MA. Electron backscatter diffraction under scanning electron microscopy together with transmission electron microscopy studies revealed that the microstructure of the milled powders after HPE consisted of ultra-fine grains (>100 nm) surrounded by nanostructured grains (<100 nm), revealing the formation of a bimodal grain structure. The grain size distribution was in the range of 20 to 850 nm with an average of 360 and 300 nm for Ar and Ar-5 pct O₂ atmospheres, respectively. The amount of oxide particles formed by reactive mechanical alloying under the Ar/O₂ atmosphere was ~0.8 vol pct, whereas the particles were almost uniformly distributed throughout the aluminum matrix. The UFG materials exhibited significant improvement in the hardness and yield strength with an absence of strain hardening behavior compared with CG material. The fracture surfaces showed a ductile fracture mode for both CG and UFG Al6063, in which the dimple size was related to the grain structure. A mixture of ductile–brittle fracture mode was observed for the UFG alloy containing 0.8 vol pct Al₂O₃ particles. The tensile behavior was described based on the formation of nonequilibrium grain boundaries with high internal stress and dislocation-based models.