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.

THE SUPERPLASTICITY AND DEFORMATION MECHANISM OF ULTRALIGHT BINARY Mg-8wt%Li ALLOY

The maximum elongation-to-failure of 960% has been achieved in the Mg-8wt%Li alloy. It is shown by measurement of true stress-strain curve and microstructure observation that dynamic recrystallization takes place during superplastic deformation, which turns the banded grains into equiaxed grains. It is postulated by theoretical analysis that during superplastic deformation,continuous introduction of lattice dislocations into the phase interfaces contributes to the superplasticity in this alloy.During superplastic deformation,grain growth and cavity nucleation at α phase grain boundaries have also been observed.

Flow curve correction and processing map of 2050 Al-Li alloy

Hot compression tests of 2050 Al-Li alloy were performed in the deformation temperature range of 340-500°C and strain rate range of 0.001-10 s-1 to investigate the hot deformation behavior of the alloy.The effects of friction and temperature difference on flow stress were analyzed and the flow curves were corrected.Based on the dynamic material model,processing map at a strain of 0.5 was established.The grain structure of the compressed samples was observed using optical microscopy.The results show that friction and temperature variation during the hot compression have significant influences on flow stress.The optimum processing domains are in the temperature range from 370 to 430°C with the strain rate range from 0.01 to 0.001 s-1,and in the temperature range from 440 to 500°C with the strain rate range from 0.3 to 0.01 s-1;the flow instable region is located at high strain rates(3-10 s-1)in the entire temperature range.Dynamic recovery(DRV)and dynamic recrystallization(DRX)are the main deformation mechanisms of the 2050 alloy in the stable domains,whereas the alloy exhibits flow localization in the instable region.

DYNAMIC RECRYSTALLIZATION IN SUPERPLASTIC DEFORMATION OF TiAl BASED ALLOY

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Superplasticity Enhanced by Two-stage Deformation in a Hot-extruded AZ61 Magnesium Alloy

A two-stage strain rate deformation method is proposed to enhance the superplasticity in a hot extruded AZ61 alloy. In the stage-one of deformation, a relatively high strain rate was applied in order to obtain fine grains through dynamic recrystallization. The optimum strain rate for DRX at 300℃ was identified as -5×10-3s-1. Stage-two is conducted at relatively low strain rate in order to utilize the fine grains refined by DRX during stage-one to make the grain boundary sliding operate more smoothly, which resulting in enhanced superplastic elongation from 350% to 440%.

Microstructure and texture characterization of superplastic Al-Mg-Li alloy

A novel thermomechanical processing was developed for producing fine grained Al-Mg-Li alloy sheets. The influences of static recrystallization annealing on the grain structure and superplastic behavior were investigated. The results show that the refined microstructure has a variation in the distribution of grain size, shape and texture across the normal direction of the sheet. The surface layer （SL） has fine, nearly equiaxed grains with a rotated cUbeND {001 }（310） orientation, whereas the center layer （CL） has coarse, elongated grains with a portion of a fiber orientation. Increasing static recrystallized temperature results in grain growth in the full thickness, decreasing of grain aspect ratio in the center layer, texture sharpening in the surface layer, but weakening in the center layer as well as decreasing of superplastic elongation. Increasing the annealing temperature also produces an sharpening of the rotated cube {001}（310） component and a decreasing of the a fiber texture in the full thickness of the sheet. The formation mechanisms of recrystallization texture at various temperatures and layers were discussed.

Superplasticity of fine-grained AZ31 Mg alloy sheets

Superplastic mechanical properties of fine-grained AZ31 Mg alloy sheets in the temperature range of 250450 ℃ and strain rate range of 0.7×10-31.4×10-1 s-1 were investigated by uniaxial tensile tests. The microstructure evolution during the superplastic deformation of AZ31 Mg alloy was examined by means of metallurgical microscope and transmission electronic microscope （TEM）. It is shown that, fine-grained AZ31 Mg alloy starts to exhibit superplasticity at 300 ℃ and the maximum elongation of 362.5% is obtained at 400 ℃ and 0.7×10-3 s-1. The predominate superplastic mechanism of AZ31 Mg alloy in the temperature range of 300400 ℃ is grain boundary sliding （GBS）. Twinning caused by pile-up of dislocations during the early stage of superplastic deformation is the hardening mechanism, and dynamic continuous recrystallization （DCRX） is the important softening mechanism and grain stability mechanism during the superplastic deformation of the alloy.

SUPERPLASTIC DEFORMATION OF TiAl BASED ALLOY

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Property and microstructure analysis of ZK60 alloy superplastic sheet

It was investigated that the superplastic mechanical properties of fine-grained ZK60 magnesium alloy sheets at the temperature range of 200-420 ℃ and strain rate range of 5.56 × 10-4 -5.56 ×10-2 s-1 by tensile tests.And the microstructure evolution during the superplastic deformation of ZK60 magnesium alloy was examined by metallurgical microscope and transmission electronic microscope (TEM).The results showed that fine-grained ZK60 magnesium alloy starts to exhibit superplasticity from 250 ℃ and the maximum elongation is about 1106% at 400 ℃ and 5.56 × 10-4 s-1.The strain rate sensitivity is significantly enhanced with the increase of temperature and with the decrease of strain rate.The predominate superplastic mechanism of ZK60 magnesium alloy is grain boundary slide (GBS) at the temperature range of 300-400 ℃.The grains of ZK60 alloy remain equaxial after superplastic deformation,and dynamic continuous recrystallization (DCRX) is an important softening mechanism and grain stability mechanism during the superplastic deformation of the alloy.The curved grain boundaries and crumpled bands at grain boundaries after deformation prove GBS generates during superplastic deformation of ZK60 magnesium alloy.

Al-Li-Cu-Mg-Zr合金的超塑性研究

Al-1.91Li-1.25Cu-0.46Mg-0.21Zr合金超塑性变形结果表明,最佳的时效工艺是400℃8h;冷轧工艺得到了比温轧(550％)更高的超塑性延伸率(630％);最佳超塑性变形工艺是T=500℃,(?)_i=3.33×10^(-3)s^(-1)(起始拉伸速度)。研究指出,Al-1.91Li-1.25Cu-0.46Mg-0.21Zr合金的超塑性预处理的时效工艺和轧制工艺影响了合金超塑变形初期的应变诱发再结晶,从而影响超塑性性能。超塑变形中的动态回变和动态再结晶是晶界滑动的重要协调机制之一。

轧制镁合金超塑性和超塑胀形

对轧制态MB15镁合金进行了超塑性拉伸实验 ,结果表明 :晶粒尺寸为 5 .9μm的MB15镁合金板材 ,在温度为 5 73K、初始应变速率为 5 .5 6× 10 -4s-1的变形条件下 ,获得的最大延伸率为 30 9% ,应变速率敏感指数为0 .34;当真应变为 0 .3时 ,试样的晶粒尺寸为 4 .5 μm ,说明在拉伸初始阶段轧制镁合金可以获得细晶组织 ,同时发生了部分动态再结晶。利用扫描电镜观察断口发现典型的超塑性空洞形貌特征。通过胀形实验可以看出 ,该镁合金板材的超塑成形性能好 。

Ti-24Al-15Nb-1.5Mo合金的超塑性能及超塑变形时的组织演化

研究了Ti-24Al-15Nb-1.5Mo合金在900～1020℃,3.3×10-4～3.3×10-2s-1条件下进行的超塑性拉伸性能结果表明:除温度900℃,应变速率3.3×10-2s-1外,合金都显示出超塑性,延伸率范围为105%～1570%,最佳变形温度为980℃,最佳应变速率为3.3×10-4s-1,在此条件下拉伸时,延伸率达到最大值1570%。应变速率对Ti-24Al-15Nb-1.5Mo合金的组织演化有显著影响。在较高应变速率下变形,α2相尺寸先随温度升高至940℃有所减小,之后则随温度的升高有所粗化;而在较低的应变速率下变形,α2相呈粗化且不均匀的趋势,高的延伸率与大晶粒周围镶嵌许多小颗粒能有效协调变形。

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。

超塑性TiAl基合金的显微组织特征

采用金相显微镜、扫描电镜和透射电镜，对超塑性拉伸变形后的Ti－33Al－3Cr－0．5Mo（质量分数）合金的显微组织进行了观察和分析。研究发现，TiAl基合金在超塑性变形过程中发生了动态再结晶现象，动态再结晶使晶粒显著细化。透射电镜观察结果表明，γ晶粒内有位错运动．位错运动导致γ晶粒内形成位错墙、位错网。这些显微组织特征与TiAl基合金的超塑性变形机理有着密切的关系。

供应态LC9铝合金的超塑性试验研究

通过拉伸实验研究了供应态LC9铝合金经退火处理后的超塑性变形特性。在初始应变速率3.3×10-4s-1,拉伸温度410～510℃时,合金均具有超塑性,平均伸长率为106%～181%。最佳超塑性温度为450℃,最佳初始应变速率为3.3×10-4s-1,在此温度和应变速率条件下,合金平均伸长率达到181%,m值为0.41,流动应力仅为14.4MPa。显微组织和断口观察表明,在超塑性变形过程中发生了明显的动态再结晶,再结晶晶粒等轴、细小、均匀。空洞在晶界处形核、长大,最后连接,导致试样断裂。

Ti-24Al-14Nb-3V-0.5Mo合金的压缩超塑性

对Ti 2 4Al 14Nb 3V 0 5Mo合金的超塑压缩变形行为进行了研究·结果表明 ,该合金的最佳超塑温度为 980℃ ,最佳应变速率范围为 (2× 10 -4 ～ 2× 10 -3 )s-1· 在超塑变形过程中 ,条状O相和α2 相中发生了动态再结晶和等轴化过程 ;B2相中发生了动态回复·新形成的硬相等轴晶粒可在软的基体中滑动和转动 ,造成的应力集中由B2相中的位错运动松弛·

含Sc铝镁合金超塑变形行为与显微组织特征

研究了含微量Ｓｃ元素的ＡｌＭｇ合金在超塑变形过程中的力学行为和显微组织．结果表明：合金可在较宽温度和应变速率范围内获得良好的超塑性．在温度为５２０℃，初始应变速率为１６７×１０－３ｓ－１条件下拉伸变形时最大延伸率可以达到３９６％．显微组织分析发现，合金的超塑性效应是由变形初期的动态再结晶诱发产生，其超塑变形过程可分为亚晶超塑性阶段、过渡阶段和细晶超塑性阶段．

TiAl 基合金超塑性变形中的位错运动

研究了细晶 Ｔｉ３３ Ａｌ３ Ｃｒ０ ．５ Ｍｏ （ 质量分数） 合金在超塑性变形过程中的位错运动。透射电镜分析结果表明， 超塑性变形时， 合金中存在大量位错运动， 在晶界附近和晶粒内形成了位错网， 并在晶界上出现位错塞积。大多数位错是从晶界及α２ 相粒子处发射出来的。还发现了动态再结晶现象， 这种动态再结晶是由于位错的低可动性导致位错密度不断升高造成的。分析认为， 位错运动是 Ｔｉ Ａｌ 基合金超塑性变形过程的主要协调机理。

Ti-15-3在β相区的两段超塑性行为研究

研究了应变速率变化时粗晶Ti-15-3板材在β相区的超塑性。第一段的应变速率分别为1×10-1和1×10-2 s-1,应变量为0.2～0.6。在第二段的应变速率为3×10-4 s-1时,获得345%最大伸长率,比在恒应变速率下的伸长率大93%。而两段应变速率下,应变速率敏感系数m值无太大变化。研究了第一段变形结束时动态再结晶晶粒大小和后续超塑性的关系。

TC4-DT合金的超塑性变形及其本构方程

采用CMT4104电子万能拉伸试验机分别进行温度为870℃,应变速率为3.3×10-4s-1的恒应变速率和温度为850~890℃,应变速率为3.3×10-5~3.3×10-3s-1的应变速率循环法超塑性拉伸实验。结果表明:在变形过程中存在动态回复与动态再结晶现象,并采用Avrami方程描述了动态再结晶动力学行为;基于应变速率循环法获得了TC4-DT合金的本构模型,再通过1stopt软件加以回归拟合,得到较为精确的TC4-DT合金超塑性变形本构方程。