等轴细晶TC4钛合金应变补偿本构关系及热加工图的研究

本工作对等轴细晶TC4钛合金进行了热压缩实验,研究了变形温度为800～950℃、应变速率为0.01～10 s-1下TC4钛合金的变形行为,并建立相应的Arrhenius型本构方程和热加工图,再基于实验获得的真应力应变曲线对本构方程进行应变补偿修正.结果表明:合金的真应力值随温度升高、应变速率下降而减小;修正后本构方程真应力预测值与实验值相关系数R为0.985,相对误差ARRE为6.8％.结合热加工图和相应区域的电子背散射衍射(EBSD)分析可知:失稳区的温度为875～950℃,应变速率为0.3～10 s-1,组织特征表现为长条状晶粒;最适宜加工区的温度为800～875℃,应变速率为0.01～0.3 s-1,组织特征表现为等轴细晶.     

超细晶纯铝微塑性变形本构关系的建立

为了研究超细晶纯铝热变形行为,利用等通道挤压工艺制备了超细晶纯铝,并采用INSTRON 5965材料电子万能试验机对超细晶纯铝进行了高温微压缩实验,分析了超细晶纯铝在150~250℃、应变速率3.3!10-4~1!10-1s-1条件下流动应力的变化规律.研究表明,流动应力随试样尺寸的减小呈下降趋势,且变形温度和应变速率对流动应力有显著影响,流动应力随变形温度的升高和应变速率的降低而降低.基于双曲正弦形式修正的Arrhenius关系的本构模型,建立了超细晶纯铝高温微压缩变形本构关系.实验验证表明,所建立的本构关系能够较好地反映超细晶纯铝高温微塑性变形行为特征.     

应变速率循环法构建TC4-DT钛合金本构方程

采用应变速率循环法在超塑拉伸机上对TC4-DT钛合金进行三组高温超塑性拉伸实验,变形温度为850~890℃,应变速率为3.3×10-5~3.3×10-3s-1。通过对拉伸实验数据的分析计算出TC4-DT钛合金动态再结晶激活能,并利用Arrhenius模型构建TC4-DT高温条件下的超塑性本构方程。结果表明:TC4-DT钛合金的流动应力对变形温度较为敏感,随着温度的升高,流变应力逐渐减小,软化机制愈发明显,870℃附近的超塑性较好,伸长率达到554%。     

TC21 钛合金高温变形本构方程研究

目的研究变形温度、应变速率等热力参数对TC21钛合金流动应力的影响规律,并构建出TC21钛合金本构方程。方法在热模拟试验机上对TC21钛合金进行了等温恒应变速率压缩实验,分析其真应力-真应变曲线。结果获得了该合金在变形温度范围为760~920℃、应变速率范围为0.001~10 s-1的流动应力数据,采用多元线性回归法建立了该合金的本构方程。结论误差分析表明,该本构方程具有较高精度,可为TC21钛合金锻造过程中的数值模拟和锻造热力参数的合理制定提供理论依据。     

纯镍N6挤压坯料的高温变形行为及加工图

目的以来源于挤压坯料的纯镍N6作为研究对象,分析其高温压缩变形行为及微观组织演化,为挤压工艺参数设计提供有效依据。方法利用Gleeble-1500D热-力模拟试验机,对实验样品在温度900~1200℃和应变速率0.01~10 s-1范围进行热压缩变形,获得了材料真应力-真应变曲线,利用光镜观察了变形后的微观组织。结果建立了双曲正弦函数形式的本构方程,得到材料的本构参数为Q=272.77 k J/mol,α=0.01024 MPa-1,n=4.045。基于动态材料模型建立了材料的热加工图,判断材料具有低温低应变速率和高温高应变速率2个适宜的加工区间。结论纯镍N6极易发生晶粒长大,温度和应变速率对变形组织影响显著,在变形条件为1200℃/1 s-1时,晶粒平均尺寸已经到达84.83μm。加工图失稳区间内的微观组织呈现不均匀性,为粗大变形晶粒与细小再结晶晶粒混杂的两级结构,因此在热挤压加工工艺设计中需要避开相应的参数区域。     

超塑性Y-TZP的高温塑性流动行为

通过恒位移速度压缩试验研究了不同晶粒大小,不同致密度的３Ｙ－ＴＺＰ的高温塑性流动行为。结果表明,塑性流动应力随着温度和应变速率的提高及晶粒尺寸的增大而提高,随着气孔率的增大而下降,对于晶粒较粗的试样,当应力水平较高时,由于材料内部产生孔穴化,引起屈服后流动应力下降。细晶３Ｙ－ＴＺＰ在高温塑性形变过程中产生应变硬化,这是由于形变过程中晶粒动态生长和应变速率提高而引起。     

TC11钛合金热变形特性分析及其本构关系的建立

对具有原始β转变组织的TC11钛合金在温度1090～800 ℃,应变速率10-3～10-1 s-1条件下进行了热力模拟压缩实验,获得了该合金不同热变形条件下的应力-应变曲线.通过分析该合金的热变形行为,获得了应力指数、应变速率敏感指数和热变形激活能等表征其变形特性的重要特征参数,并采用所得参数建立了热变形本构关系.对比结果表明,所建立函数关系与实验结果吻合较好.     

TB8钛合金的热变形行为及加工图

采用Gleeble-1500热模拟机在变形温度为750～1100℃、应变速率为0.01～1s-1范围内对TB8钛合金进行了单道次热压缩变形试验,研究了其高温变形力学行为.结果表明:随着变形温度的升高和应变速率的降低材料的峰值应力和稳态应力显著降低;高温变形条件下TB8合金流变应力本构关系可以用双曲正弦方程和Z参数描述;建立并初步分析了基于动态材料模型的TB8钛合金热加工图,当温度为950～1100℃、应变速率为0.01s-1时TB8合金的能量耗散效率较高.     

TC18钛合金的超塑性行为与变形机制

通过高温拉伸实验研究TC18钛合金在温度为720~950℃,初始应变速率为6.7×10~(-5)~3.3×10~(-1)s~(-1)时的超塑性拉伸行为和变形机制。结果表明:TC18钛合金在最佳超塑性变形条件下(890℃,3.3×10~(-4)s~(-1)),最大伸长率为470%,峰值应力为17.93MPa,晶粒大小均匀。在相变点Tβ(872℃)以下拉伸,伸长率先升高后下降,在温度为830℃,初始应变速率为3.3×10~(-4)s~(-1)时取得极大值373%,峰值应力为31.45MPa。TC18钛合金在两相区的超塑性变形机制为晶粒转动与晶界滑移,变形协调机制为晶内位错滑移与攀移;在单相区的超塑性变形机制为晶内位错运动,变形协调机制为动态回复和动态再结晶。     

TC21钛合金不同变形条件下的显微组织研究

在国内率先完成了新型八元系高强韧钛合金TC21的热模拟压缩变形实验,研究了TC21合金在β相区、(α β)相区及相变点Tβ共8个温度点及0.01～50s-1等5个应变速率值条件下的单向热加工变形特性,重点对加工态金相组织、尤其是0.01s-1和50s-1条件下的组织进行了研究,结果发现,在试样的不同部位存在变形组织的不均匀现象,该合金在不同温度区域变形时分别发生重结晶和动态再结晶.重结晶导致晶粒粗化(约100～200μm),而动态再结晶致使晶粒细化(最小在1～2μm以下).     

Superplasticity of Ti2448 Alloy with Nanostructured Grains

Ti-24Nb-4Zr-8Sn, abbreviated as Ti2448 from its chemical composition in weight percent, is a multifunctional β type titanium alloy with body centered cubic (bcc) crystal structure, and its highly localized plastic deformation behavior contributes significantly to grain refinement during conventional cold processing. In the paper, the nanostructured (NS) alloy with grain size less than 50 nm produced by cold rolling has been used to investigate its superplastic deformation behavior by uniaxial tensile tests at initial strain rates of 1.5×10-2, 1.5×10-3 and 1.6×10-4 s-1 and temperatures of 600, 650 and 700℃. The results show that, in comparison with the coarse-grained alloy with size of 50 μm, the NS alloy has better superplasticity with elongation up to ～275% and ultimate strength of 50–100 MPa. Strain rate sensitivity (m) of the NS alloy is 0.21, 0.30 and 0.29 for 600, 650 and 700℃, respectively. These results demonstrate that grain refinement is a valid way to enhance the superplasticity of Ti2448 alloy.     

Evolution of dynamic recrystallisation in AISI 304 stainless steel

Abstract

The nucleation and development of dynamic recrystallisation (DRX) has been studied via hot torsion testing of AISI 304 stainless steel. The DRX behaviour was investigated with microstructural analysis and slope changes of flow stress curves. The characteristics of serrated grain boundaries observed by SEM, electron backscattered diffraction and TEM indicated that the nucleated DRX grain size was similar to that of the bulged part of the original grain boundary. The DRX of the alloy was nucleated and developed by strain induced grain boundary migration and by the necklace mechanism. Before the steady state in the flow curve at 1000 ° C and 0.5 s^-1, the dynamically recrystallised grains did not remain a constant size and gradually grew to the size of fully DRX grains at steady state (30 μm). The calculation of the grain size was based on X _DRX (volume fraction of dynamically recrystallisation) under the assumption that the nucleated DRX grains grow to the steady state continuously. It was found that the calculated grain size of the alloy was good agreement with that of the observed grain size. It is expected that a fine grained steel can be obtained by controlling hot deformation conditions on the basis of newly developed equations for predicting DRX behaviour.     

Analysis of high flow stress and microstructural evolution of TC6 titanium alloy during isothermal forging

Abstract

Elevated temperature true stress – strain curves have been determined for the isothermal deformation of a TC6 titanium alloy using hot compression testing in the deformation temperature range 800 – 1040°C, strain rate range 0.001 – 50 s^-1 and reduction in height of 30 – 50%. The experimental results show that the flow stress of TC6 titanium alloy is strongly dependent on process parameters, especially on the deformation temperature and strain rate. The peak stress and steady stress of such an alloy have the same characterisation, which increases with higher strain rate and lower deformation temperature. During isothermal forging, microstructural characterisation, including volume fraction, grain size, and grain pattern of prior α phase, varies with different temperatures, height reductions, and strain rates.     

Mechanical behavior and wear characteristics of fine grained ZE41 magnesium alloy

ZE41 magnesium alloy was successfully produced by friction stir processing and grain refinement was achieved from a starting size of 107 μm±6.7 μm to 3.5 μm±1.5 μm. MgZn intermetallic which was appeared as network like structure at the grain boundaries before friction stir processing was greatly affected due to the severe plastic deformation and broken as small particles as observed from the microstructural studies. Higher hardness (≈30 %) was measured for the fine grained ZE41 magnesium alloy compared with the base alloy due to the grain refinement. From the tensile tests, yield strength and ultimate tensile strength was significantly increased at the cost of decreased ductility reflected in lower strain for the fine grained ZE41 compared with the base alloy. Wear studies showed higher coefficient of friction and lower mass loss for the grain refined ZE41 magnesium alloy. From the results, it can be understood that the grain refinement achieved by friction stir processing has a profound influence on enhancing the mechanical and tribological properties of ZE41 magnesium alloy.     

Superplasticity of Fe3Al(Cr)

Abstract

The superplasticity of an Fe₃Al based intermetallic alloy with 3 at.-% chromium has been investigated in the strain rate range 10^-5-10^-2 s^-1 at test temperatures between 700 and 900°C. The composition of the iron aluminide was Fe–28Al–3Cr (at.-%) with additions of titanium and carbon. After thermomechanical processing the material possessed a coarse grained microstructure with an average grain size of 55 ± 10 μm. Strain rate exponents of 0·33≤m≤0.42 were recorded at strain rates of approximately 10^-5-10^-3 s^-1 in the temperature range 750-900°C. Superplastic elongations of 350% and more were achieved. From thermal activation analysis of superplastic flow, an activation energy of 185 ± 10 kJ mol^-1 was derived. This value is comparable to activation energies of superplastic flow in Fe₃Al(Ti) alloys. However, in unalloyed Fe₃Al the activation energy is higher, ~ 263 kJ mol^-1. Optical microscopy showed grain refinement to ~ 30 ± 5 μm in size in superplastically strained tensile specimens. Transmission electron microscopy gave evidence of the formation of subgrains of 0·3–0·5 μm in size. Superplasticity in this iron aluminide is mainly attributed to viscous dislocation glide, controlled by solute drag in the transformed B2 lattice at the deformation temperatures. During superplastic deformation, subgrain formation and grain refinement in the gauge length were revealed. From this it is concluded that dynamic recrystallisation makes an important contribution to the deformation mechanism of superplastic flow in this material.     

异步轧制AZ31镁合金板材的超塑性工艺及变形机制

经过异步轧制工艺获得AZ31镁合金薄板。在300~450℃范围内,分别通过5×10-3,1×10-3s-1和5×10-4s-1不同应变速率进行高温拉伸实验研究其超塑性变形行为,计算应变速率敏感指数m值、超塑性变形激活能Q及门槛应力σ0值。通过EBSD分析和扫描电镜观察拉伸断裂后的断口形貌,分析AZ31镁合金的超塑性变形机制。结果表明:AZ31镁合金的塑性变形能力随着变形温度的升高及应变速率的降低而增强。当拉伸温度为400℃、m=0.72、应变速率为5×10-4s-1时,AZ31具有良好的超塑性,伸长率最大为206%。温度为400℃时,异步轧制AZ31镁合金的超塑性变形是以晶格扩散控制的晶界滑移和基面滑移共同完成的。     

AZ61镁合金薄板超塑性变形特征的SEM研究

利用扫描电镜(SEM)和超塑性拉伸实验对一次热挤压加工成型的AZ61镁合金薄板(晶粒尺寸～12μm)超塑性变形特征进行了研究.结果显示,在最佳的变形温度(623K)和应变速率(1×10-4s-1)条件下,可获得的最大的超塑性形变量为920%.在523～673 K实验温度和1×10-2～1×10-5s-1应变速率范围内,材料的应变速率敏感指数(m值)随实验温度升高和应变速率的降低而增加.较高的m值(0.42～0.46)对应于晶界滑动机制(GBS),而较低的m值(0.22～0.25)则对应于位错滑移机制.变形温度和应变速率是影响超塑性变形量和变量机制的主要因素.     

SA508Gr.4N钢热变形过程微观组织演变及流变应力模型

利用Gleeble-1500D热模拟试验机研究Ni-Cr-Mo系低合金SA508Gr.4N钢在变形温度为850~1200℃,应变速率为0.001~1 s^-1,真应变为0.9条件下的等温热变形行为,建立包含动态回复和动态再结晶的基于物象的流变应力模型与动态再结晶晶粒尺寸模型,并提出避免粗大晶粒组织遗传性的适宜锻造工艺。结果表明:随着变形温度的升高,应变速率的降低,动态再结晶体积分数和晶粒尺寸逐渐增加;SA508Gr.4N钢的真应力-真应变曲线具有明显的不连续动态再结晶现象;通过实验值和模型预测值对比可得流变应力模型的相关系数(R)及平均相对误差(MRE)分别为0.998和4.76%,动态再结晶晶粒尺寸模型的相关系数(R)及平均相对误差(MRE)分别为0.991和8.69%,两个模型均具有较高的准确性。     

神经网络预测搅拌摩擦加工TC4超塑性行为

目的 研究搅拌摩擦加工工艺改性的Ti–6Al–4V双相钛合金的超塑性变形行为。方法 对360 r/min、30mm/min工艺条件下搅拌摩擦加工处理的TC4钛合金在不同的变形条件下进行超塑性拉伸实验,在实验数据的基础上构建以变形温度、应变速率和晶粒尺寸为输入参数且以峰值应力为输出参数的3–16–1结构的BP人工神经网络模型。应用所构建的BP人工神经网络模型对不同变形条件的Ti–6Al–4V钛合金的超塑性行为进行预测。结果 BP人工神经网络预测的精准度较高,实验应力值与预测应力值吻合度较高,相关系数R=0.991 3,相对误差为1.91%～12.48%,平均相对误差为5.92%。结论 该模型预测的准确性较高,能够客观真实地描述Ti–6Al–4V合金的超塑性变形行为。