粗晶Ti40合金超塑性变形时的动态软化行为研究

采用单向拉伸试验对粗晶Ti40合金进行了超塑性能测试,并结合TEM和EBSD分析技术研究了该合金超塑性变形过程中的动态软化行为及机制。结果表明：粗晶Ti40合金在所选实验条件下具有良好的超塑性能并在840oC、1×10-3s-1条件下获得最大延伸率436%;基于形变Z因子和断裂延伸率并结合微观组织分析可将变形条件划分为无超塑性、动态回复、动态再结晶三个区域;分别基于Sellars模型和KM方程建立了Ti40合金超塑性变形的动态再结晶临界应变模型和位错密度演变模型;粗晶Ti40合金超塑性变形过程中的动态回复以位错运动—位错胞—多边形化—形成亚晶的机制为主;动态再结晶机制主要为亚晶持续转动导致大角度晶界形成的连续动态再结晶。     

一种新型镍基粉末高温合金等温热压缩过程中的超塑性变形行为研究

基于Gleeble等温热压缩试验,并结合OM、SEM、EBSD等分析手段,研究了一种新型热挤压态第3代镍基粉末高温合金FGH4113A(WZ-A3)在超塑性压缩变形过程中的流动特性和微观组织演化规律。给出了在温度1050、1100℃,应变速率0.001、0.005 s^-1,不同变形量(应变)条件下的超塑性变形行为及其变形机制。结果表明：FGH4113A(WZ-A3)合金在热压缩变形过程中表现出良好的超塑性,未发现孔洞或裂纹;1100℃/0.001 s-1大变形后期(60%～80%变形量)晶粒有长大趋势,其余变形条件晶粒尺寸变化不大;在超塑性压缩变形过程中,累积的位错主要通过动态回复和动态再结晶所湮灭,晶界滑移是发生大变形而未开裂的主要原因。该研究结果为新型的FGH4113A(WZ-A3)合金超塑性等温锻造工艺制定奠定了良好的基础。     

基于L-J位错密度模型及CA法的45Cr4NiMoV合金动态再结晶行为北大核心CSCD

基于热压缩实验得到的数据,获得了45Cr4NiMoV合金在改进的Laasraoui-Jonas(L-J)位错密度模型中的应变软化参数及应变硬化参数,建立了45Cr4NiMoV合金的动态再结晶模型。采用元胞自动机(CA)方法,应用DEFORM-3D软件对45Cr4NiMoV合金热成形过程中的动态再结晶行为进行有限元分析,并与实验得到的微观组织进行对比。结果表明:当应变速率及变形量一定时,较高的变形温度促使45Cr4NiMoV合金位错及晶界迁移运动加剧,其动态再结晶晶粒尺寸随着变形温度的升高而增大,模拟结果与实验结果较为吻合,验证了所建立的L-J位错密度模型的合理性,说明修正的L-J位错密度模型结合CA能准确预测45Cr4NiMoV合金的动态再结晶微观组织演变过程。     

NiAl-25%Cr合金的高温蠕变行为

研究挤压NiAl-25Cr合金在1073~1123K温度区间的蠕变行为。结果表明:在实验温度区域内,挤压NiAl-25Cr合金表现出两种明显不同的蠕变机制;在低温区域中,由位错的滑移和攀移控制蠕变变形过程;而在高温区域中,位错粘滞滑移控制蠕变变形过程。在研究蠕变的温度范围内,合金的蠕变表观激活能高于Ni的晶格自扩散能,这可以解释为在蠕变瞬时变形过程中,第二相粒子沿着基体相运动的结果,第二相粒子可能是γ'-Ni3Al相或α-Cr相。     

基于L-J位错密度模型及CA法的45Cr4NiMoV合金动态再结晶行为

基于热压缩实验得到的数据,获得了45Cr4NiMoV合金在改进的Laasraoui-Jonas(L-J)位错密度模型中的应变软化参数及应变硬化参数,建立了45Cr4NiMoV合金的动态再结晶模型。采用元胞自动机(CA)方法,应用DEFORM-3D软件对45Cr4NiMoV合金热成形过程中的动态再结晶行为进行有限元分析,并与实验得到的微观组织进行对比。结果表明:当应变速率及变形量一定时,较高的变形温度促使45Cr4NiMoV合金位错及晶界迁移运动加剧,其动态再结晶晶粒尺寸随着变形温度的升高而增大,模拟结果与实验结果较为吻合,验证了所建立的L-J位错密度模型的合理性,说明修正的L-J位错密度模型结合CA能准确预测45Cr4NiMoV合金的动态再结晶微观组织演变过程。     

TC18钛合金的热压缩行为及组织演变规律

利用热压缩法研究了TC18合金的热变形行为,并利用TEM和EBSD研究了热压缩过程的显微组织演化。结果表明,TC18合金650℃流变曲线呈现单一峰值的动态再结晶特征,变形温度的提高使单一峰值的特征逐渐减弱直至消失。650℃,0.001 s-1时出现大量细小的再结晶晶粒,说明这时以再结晶为主;温度提高到810℃时,出现了规则的位错列和位错胞及介于位错胞之间的无位错区,说明这时出现回复和再结晶共存的现象。EBSD观察结果表明,在各个变形条件下β相亚结构比例都要高于α相,说明β相比α相更易发生回复现象;随变形温度提高和变形速率降低,β相发生回复的体积分数增加。     

Al-Cu-Mg-Ag合金热压缩变形的流变应力行为和显微组织

采用热模拟实验对Al-Cu-Mg-Ag耐热铝合金进行热压缩实验,研究合金在热压缩变形中的流变应力行为和变形组织.结果表明:Al-Cu-Mg-Ag耐热铝合金在热压缩变形中的流变应力随着温度的升高而减小,随着应变速率的增大而增大;该合金的热压缩变形流变应力行为可用双曲正弦形式的本构方程来描述,其变形激活能为196.27 kJ/mol;在变形温度较高或应变速率较低的合金中发生部分再结晶,并且在合金组织中存在大量的位错和亚晶;随着温度的升高和应变速率的降低,合金中拉长的晶粒发生粗化,亚晶尺寸增大,位错密度减小,合金的主要软化机制逐步由动态回复转变为动态再结晶.     

AA7005铝合金的热加工变形特性

研究了AA7005合金高温压缩变形时的流变应力、动态回复与再结晶以变形组织变化特征。合金稳态变形时,应变速度、温度和流变应力之间满足包含热激活材料常数的Arrhenius项的双曲正弦关系,变形过程为受位错增殖和相互销毁速率控制的热激活过程,螺型位错的交滑移和刃型位错的攀移为主要动态回复机制。动态回复时,形成典型的变形亚晶组织,亚晶尺寸随1nZ的减小而增大。高温低速变形条件下,合金发生局部几何动态再结晶,流变曲线呈现连续下降的特征,形成与原始纤维组织不同的细小等轴大角度再结晶晶粒。     

Ti-62A合金热变形过程中的流变应力特性及组织演变

通过 Gleeble-3800 热模拟试验机的热压缩实验,研究了 Ti-62A 合金在 800、850、900 和 950℃,应变速率为 0.001、0.01、0.1 和 1s-1 下的热变形行为和动态再结晶（DRX）规律。结果表明：Ti-62A 合金的流变应力受应变速率和变形温度的影响显著;流变应力随着变形温度的升高和应变速率的降低而降低;在 900~950℃、应变速率 0.01~1s-1 条件下,Ti-62A 合金的热变形应力-应变曲线属于动态回复型;该合金的热变形机制主要由位错运动控制,其动态软化机制包括晶界滑动和位错对消、攀移机制;Ti-62A 合金在热变形过程中,动态再结晶更有可能发生在较高的温度和较低的应变速率下,即 950℃ 和 0.001s-1;基于经典位错密度理论和 DRX 动力学理论,建立了加工硬化—动态回复和 DRX 软化效应的两阶段本构模型。DEFORM-3D 软件的仿真模拟结果证实,基于 DRX 软化效应的本构模型对 Ti-62A 合金在动态再结晶阶段的热变形行为的预测具有较高的准确性,能够为实际生产工艺的制定提供技术参考。     

ZK60镁合金热变形过程中的动态再结晶动力学

采用Gleeble-1500热模拟机对ZK60镁合金在温度为200～400℃、应变速率为0.001～10s-1、最大变形量为60%的条件下进行恒应变速率高温压缩实验,研究高温变形过程中合金的动态再结晶行为;采用EM模型描述合金的动态回复曲线,以此为基础,得出ZK60合金热压缩过程中的动态再结晶动力学Avrami方程.利用有限元模拟合金热压缩过程中的动态再结晶.结果表明ZK60合金热压缩过程中由于存在动态再结晶的软化作用,流变应力达到峰值后逐渐减小,并最终达到稳态;随着变形量的增加和变形温度的升高,动态再结晶体积分数增加,合金变形更加均匀;随着应变速率的增加,动态再结晶分数有所减小,且.变形也更不均匀.     

DYNAMIC RECRYSTALLIZATION IN SUPERPLASTIC DEFORMATION OF TiAl BASED ALLOY

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DYNAMIC RECRYSTALLIZATION AND SUPERPLASTICITY IN Al-Li ALLOY

The behavior of dynamic recrystallization in the superplastic deformation of 8090 and 2091 aluminum-lithium alloys have been investigated.TEM observations indicated that dynamic recrystallization occurs at thetriple junction of grain boundaries.The measurement of grain boundary angle showed that recrystallization indynamic equilibrium exists in the process of superplastic deformation of 8090 Al-Li alloy.It is also indicatedthat,besides the role of refining grains and the grain boundary sliding,dynamic recrystallization playsconcurrently a role of stablizing microstructure.Thus dynamic rccrystallization can be used to induce metalssuperplasticity,which leads to a simplification of pretreatment for superplastic deformation.     

Mechanism of dynamic continuous recrystallization during superplastic deformation in a microduplex stainless steel

Microstructural evolution during the cyclic cold-rolling and annealing process in an (α + γ) microduplex stainless steel, which consists of α subgrains and fine γ particles, has been studied in detail with the aim of clarifying the mechanism of dynamic continuous recrystallization. A continuous increase in α subgrain boundary misorientation is obtained by the present processing where grain boundary sliding does not occur and the effect of increasing boundary misorientation with cumulative strain is comparable to those observed in dynamic continuous recrystallization of superplastic aluminium alloys. The increase in boundary misorientation is accounted for by the absorption of dislocations into subgrain boundaries during annealing, dislocations which had operated to accommodate the plastic strain incompatibility of the neighboring phases undergoing slip deformation. The present results show that grain boundary sliding is not indispensable but the difference in accommodation deformation between adjacent subgrains is of great importance for the dynamic continuous recrystallization during superplastic deformation.     

Effect of minor Sc and Zr on superplasticity of Al-Mg-Mn alloys

The effect of Sc and Zr on the superplastic properties of Al-Mg-Mn alloy sheets was investigated by control experiment. The superplastic properties and the mechanism of superplastic deformation of the two alloys were studied by means of optical microscope, scanning electronic microscope and transmission electron microscope. The elongation to failure of Al-Mg-Mn-Sc-Zr alloy is larger than that of Al-Mg-Mn alloy at the same temperature and initial strain rate. The variation of strain rate sensitivity index is similar to that of elongation to failure. In addition, Al-Mg-Mn-Sc-Zr alloy exhibits higher strain rate superplastic property. The activation energies of the two alloys that are calculated by constitutive equation and linear regression method approach the energy of grain boundary diffusion. The addition of Sc and Zr decreases activation energy and improves the superplastic property of Al-Mg-Mn alloy. The addition of Sc and Zr refines the grain structure greatly. The main mechanism of superplastic deformation of the two alloys is grain boundary sliding accommodated by grain boundary diffusion. The fine grain structure and high density of grain boundary, benefit grain boundary sliding, and dynamic recrystallization brings new fine grain and high angle grain boundary which benefit grain boundary sliding too. Grain boundary diffusion, dislocation motion and dynamic recrystallization harmonize the grain boundary sliding during deformation.     

Ti－6Al－4V及Ti－10V－2Fe－3Al合金超塑性变形中各向异性研究

对Ｔｉ－６Ａｌ－４Ｖ挤压管材和厚板以及Ｔｉ－１０Ｖ－２Ｆｅ－３Ａｌ合金板材进行了拉伸和压缩实验，以考察其超塑性变形中的各向异性。这些钛合金超塑变形中的各向异性主要体现在以下几个方面：（１）超塑拉伸或压缩中圆试样横截面变为椭圆形状；（２）由材料不同方向截取的试样在超塑拉伸或压缩中应力一应变速率关系不同；（３）横向试样超塑拉伸中试样表面凹凸不平并逐渐发展为多颈缩。这些各向异性表现，主要与材料原始显微组织及其在变形中的演变相关。本文并对已有的描述各向异性超塑变形的本构方程进行了修正。     

Ti－6Al－4V及Ti－10V－2Fe－3Al合金超塑性变形中各向异性研究

对Ｔｉ－６Ａｌ－４Ｖ挤压管材和厚板以及Ｔｉ－１０Ｖ－２Ｆｅ－３Ａｌ合金板材进行了拉伸和压缩实验，以考察其超塑性变形中的各向异性。这些钛合金超塑变形中的各向异性主要体现在以下几个方面：（１）超塑拉伸或压缩中圆试样横截面变为椭圆形状；（２）由材料不同方向截取的试样在超塑拉伸或压缩中应力一应变速率关系不同；（３）横向试样超塑拉伸中试样表面凹凸不平并逐渐发展为多颈缩。这些各向异性表现，主要与材料原始显微组织及其在变形中的演变相关。本文并对已有的描述各向异性超塑变形的本构方程进行了修正。     

Ti40阻燃合金粗晶超塑性变形行为及机理

借助OM、TEM研究了高温条件下Ti40阻燃合金的粗晶超塑性变形行为及机理。结果表明:在920℃下,应变速率为5×10-5～1×10-2s-1的Ti40合金表现出良好的超塑性行为,拉伸延伸率均超过250%,应变速率敏感指数m大于0.3。超塑变形后,粗大的等轴组织细化。TEM分析表明,在变形过程中,位错运动形成亚晶界,亚晶界通过吸收滑移位错形成小角度晶界甚至大角度晶界。Ti40合金的粗晶超塑性是由动态回复和再结晶共同作用的结果。     

Al-Mg合金动态再结晶诱发超塑性时的空洞行为

在Ａｌ－Ｍｇ合金超塑变形过程中,发现了一类反常的空洞演化规律,也就是空洞随真应变的增加而呈现“波浪型”的演化规律。通过对微观组织特征和力学性能曲线的综合分析,认为这种特殊的空洞演化规律,主要是动态再结晶所致,也就是动态再结晶对空洞生长具有抑制作用。通过对空洞形貌的定量图像分析,还得到了新的空洞演化模型。     

Effect of Cr elimination on flow behavior and processing map of newly developed ECO-7175 aluminum alloy during hot compression

ECO-Al alloys are introduced as a game-changer for the aluminum industry and it is of utmost importance to determine the role of alloying elements in their processing characteristics. In this study, the effects of Cr on the hot deformation behavior of newly-developed ECO-7175 alloy were investigated. ECO-7175 samples with and without Cr were hot-compressed using a Gleeble simulator (temperature range of 350?500 °C and strain rates of 0.001?1 s^?1). The results were used to study the constitutive equations, the processing maps, and the microstructural evolution of the alloys. In Cr-containing alloy, the analysis of the deformation activation energy reveals that the rate-controlling mechanisms of the deformation change gradually from self-diffusion of Al (or diffusion of Mg in Al) to diffusion of Cr in Al by decreasing the Zener?Hollomon parameter. The analysis of the processing maps of Cr-containing alloy shows that the dynamic recrystallization (DRX) zone is limited to the deformation at high temperatures and low strain rates and expands with increasing applied strain. On the other hand, it is found that the self-diffusion of Al (or Mg in Al) is the only rate-controlling mechanism during hot deformation of Cr-free alloy in all processing conditions and its DRX zone is independent of the plastic strain.     

双相NiAl金属间化合物超塑性

研究双相Ni-31Al金属间化合物的高温变形行为。结果表明,该合金在950~1075℃温度范围,1.25×10-4~8×10-3s-1应变速率范围内呈超塑性变形。在温度为1000℃、应变速率为5×10-4s-1时,最大延伸率可达281.3%。显微结构分析表明,超塑性变形过程中两相具有很好的协调变形能力,超塑性变形后原始组织拉长、细化。双相Ni-31Al金属间化合物超塑性变形机制可能为连续动态回复与再结晶。