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.     

基于最大m值法和恒应变速率法的Ti_3Al基合金超塑变形行为研究

分别采用最大m值法和恒应变速率法对Ti-24Al-15Nb-1.5Mo合金板材进行超塑拉伸,研究了940~1000℃、5.5×10-5~1.7×10-3s-1和不同拉伸轴方向的超塑性变形行为。结果表明:采用最大m值法获得的伸长率均高于恒应变速率法的,分别在980℃、垂直轧制方向获得了1596%的最大伸长率和960℃、3.3×10-4s-1、与轧制方向成45°获得了932%的伸长率。原始纤维组织经过超塑变形后发生等轴化,并且等轴晶粒随着应变速率的减小和温度的升高,长大程度逐渐增大。最大m值法超塑拉伸可以明显减少孔洞的产生。     

热轧AZ31B镁合金板材超塑成形性能研究

研究了工业态热轧AZ31B镁合金板材的基本成形性能,其室温塑性较差且存在各向异性,而在应变温度为400-490℃,应变速率为1×10-4～1×10-3s-1的实验条件下均表现出良好的超塑性.其最大断裂延伸率达到216%,应变速率敏感指数达0.36.因此,对不具有典型等轴细晶的工业态热轧AZ31B镁合金板材,无需经过复杂的预先热处理,同样可以得到良好的超塑性,更具有经济实用价值.     

加工方式对AZ31变形镁合金热拉伸流变应力的影响

针对不同方法制备的AZ31镁合金薄板,利用热拉伸试验机和金相显微镜对其在不同温度和变形速率下的流变应力进行了实验研究.结果表明:挤压、交叉、热轧和冷轧等方法制备的AZ31镁合金薄板的应力-应变曲线基本特征是相同的.峰值流变应力随变形温度的升高和应变速率的降低而降低,在低温时具有明显的厚度效应;当温度大于350℃时峰值流变应力几乎不随板材厚度变化而变化;应变速率小于1.0×10-2s-1,变形温度大于150℃下所有AZ31薄板的延伸率均δ≥45%;单向轧制薄板的各向异性随温度提高减小.     

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。     

纳米Ni和Ni/SiCp纳米复合材料的超塑性

研究了用电沉积方法制备的纳米Ni和Ni/SiCp纳米复合材料的超塑特性,在试验温度410℃和450℃,应变速率为8.3×10-4s-1～5×10-2s-1的条件下,纳米Ni和Ni/SiCp纳米复合材料均表现出超塑性.当温度为450℃、应变速率为1.67×10-2s-1时,在Ni/SiCp中获得最大延伸率为836%;在同样的温度下应变速率为1.67×10-3s-1时纳米Ni获得最大延伸率为550%.对超塑性变形后组织的分析表明,晶界滑移是主要变形机制,晶粒长大至亚微米/微米量级后,变形机制是位错协调晶界滑移和位错滑移塑性.     

Ti6A14V合金的低温超塑性拉伸变形行为

在700℃-850℃的温度范围内对Ti-6%Al-4%V(质量分数)合金板材进行超塑性拉伸试验,研究了应变速率为3×10-4-5×10-38-1条件下的拉伸变形行为.结果表明:Ti6A14V合金在空气中表现出良好的低温超塑性变形能力.在800℃初始应变速率ε=5×10-4s-1条件下,延伸率达到536%.在较低的700℃下变形(ε=5×10-4s-1),延伸率仍然超过了300%.在整个变形温度区间内,应变速率敏感性指数m均为0.3左右,最大值为0.63.在850℃变形激活能与晶界自扩散激活能十分相近,表明晶界扩散控制的晶界滑动是超塑性变形的主要机制.在700-750℃,变形激活能远大于晶界自扩散激活能,位错运动是激活能升高的原因.在800℃变形的激活能介于两者之间,表明随着温度的降低变形机制逐渐发生改变.     

镁合金板材超塑性成形极限的实验研究

通过超塑性刚性凸模胀形实验研究了AZ31B镁合金板材的超塑性成形极限.在变形温度为573K,初始变形速率为3.3×10-4s-1的条件下,建立了AZ31B镁合金板料成形极限实验曲线(FLC),并且得到无论在拉压变形方式或是在双向受拉变形方式下超塑性变形时,AZ31B镁合金板料发生集中性失稳的条件均是dε2=0.     

轧制AZ91镁合金超塑性研究

研究了轧制态AZ91镁合金在实验温度为350℃－425℃（0．67Tm-0.76Tm）以及应变速率为10^-3s^-1－10^0s^-1下的超塑性变形能力及其特征。实验发现，轧制态AZ91镁合金在350℃（0.67Tm）以及应变速率为10^-3s^－1时获得最大延伸率455．05％，应变速率敏感系数达到0．64。通过分析表明，高应变速率下的超塑性变形过程中主要的变形机制为晶界滑移机制，但其主要的协调机制则是孔洞扩散聚集机制。     

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。     

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%.     

7B04铝合金超塑性变形行为

针对7B04铝合金开展了变形温度为470~530℃,应变速率为0.0003~0.01s~(-1)的高温超塑性拉伸实验,研究了材料的超塑性变形行为和变形机制。结果表明,7B04铝合金的流动应力随着变形温度的升高和应变速率的降低而逐渐减小,伸长率随之增加;在变形温度为530℃,应变速率为0.0003s~(-1)时,7B04铝合金的伸长率达到最大1105%,超塑性能最佳;应变速率敏感性指数m值均大于0.3,且随变形温度的升高而增加;在500~530℃的变形温度范围内,m值大于0.5,表明7B04铝合金超塑性变形以晶界滑动为主要变形机制;变形激活能Q为190kJ/mol,表明7B04铝合金的超塑性变形主要受晶内扩散控制;7B04铝合金超塑性变形中在晶界附近有液相产生,且适量的液相有利于提高材料的超塑性能。     

Low-temperature superplasticity of β-stabilized Ti-43Al-9V-Y alloy sheet with bimodal γ-grain-size distribution

The superplasticity of Ti-43Al-9V-0.2Y alloy sheet hot-rolled at 1100 ℃ was systematically investigated in the temperature range of 750-900 ℃ under an initial strain rate of 10-4 s-1.A bimodal γ grain-distribution microstructure of TiA1 alloy sheet,with abundant nano-scale or sub-micron γ laths embed-ded inside β matrix,exhibits an impressive superplastic behaviour.This inhomogeneous microstructure shows low-temperature superplasticity with a strain-rate sensitivity exponent of m =0.27 at 800 ℃,which is the lowest temperature of superplastic deformation for TiAl alloys attained so far.The maximum elongation reaches ～360％ at 900 ℃ with an initial strain rate of 2.0 × 10-4 s-1.To elucidate the softening mechanism of the disordered β phase during superplastic deformation,the changes of phase composi-tion were investigated up to 1000 ℃ using in situ high-temperature X-ray diffraction (XRD) in this study.The results indicate that β phase does not undergo the transformation from an ordered L20 structure to a disordered A2 structure and cannot coordinate superplastic deformation as a lubricant.Based on the microstructural evolution and occurrence of both y and β dynamic recrystallization (DR) after tensile tests as characterized with electron backscatter diffraction (EBSD),the superplastic deformation mecha-nism can be explained by the combination of DR and grain boundary slipping (GBS).In the early stage of superplastic deformation,DR is an important coordination mechanism as associated with the reduced cavitation and dislocation density with increasing tensile temperature.Sufficient DR can relieve stress concentration arising from dislocation piling-up at grain boundaries through the fragmentation from the original coarse structures into the fine equiaxed ones due to recrystallization,which further effectively suppresses apparent grain growth during superplastic deformation.At the late stage of superplastic de-formation,these equiaxed grains make GBS prevalent,which can effectively avoid intergranular cracking and is conducive to the further improvement in elongation.This study advances the understanding of the superplastic deformation mechanism of intermetallic TiAl alloy.     

AZ31B镁合金管材热态内压成形性能的研究

为了研究变形镁合金AZ31B管材的热态内压成形性能,通过单向拉伸测试了不同温度和应变速率下其力学性能的变化,通过胀形实验研究了温度对内高压成形性能的影响,以及相应变形条件下微观组织的变化.实验结果表明:在20～300℃时,AZ31B的屈服强度和抗拉强度随着温度的升高而降低,总伸长率随着温度的升高而提高,均匀伸长率随着温度的升高先增大后减小;当应变速率在0.001～0.1s-1时,屈服强度和抗拉强度随应变速率的增大而升高,总伸长率随着应变速率的增大而减小,均匀伸长率随着应变速率的增大先增加后减小;当温度在20～250℃时,镁合金管材的极限胀形率随温度的升高先增大后减小,在175℃时达到最大值.微观组织观察表明,175℃下不完全动态再结晶和孪晶两种微观组织的出现是使镁合金管材极限胀形率提高的主要原因.     

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.     

Cavitation Behavior during Superplastic Deformation of AZ31 Magnesium Alloy

Superplasticity of AZ31 Mg alloy at the temperature range of 250～450℃ and stain rate range of 0.7x10-3～ 1.4x 10-1 s-1 was examined through uniaxial tensile test. Optical microscopy (OM) and scanning electron microscopy (SEM) were employed to investigate the morphology of cavities and surface relief near fracture surface, respectively.It is shown that AZ31 Mg alloy starts to exhibit superplasticity from 300℃. The maximum elongation of 362.5%was obtained at 400℃ and strain rate of 0.7×10-3 s-1. There exist many O-shaped cavities and filaments at the boundaries near fracture surface. The fracture of filaments results in intergranular cavity and the model for the formation of intergranular cavities is proposed. The growth of cavities is plasticity-controlled and the serrated boundaries of intergranular cavities agree with the results of surface relieves.     

Ti6213合金的热加工图研究

在Gleeble-1500热模拟机上进行了Ti6213合金热模拟压缩试验,变形温度范围为800-1050℃,应变速率范围为0.001-10 s-1,最大变形量为60%,并根据动态材料模型建立了加工图。结果表明,合金在高温变形时主要有2个合适的加工区域,一个是变形温度800-950℃,应变速率0.01 s-1以下区域;另一个在相变温度以下40℃内,应变速率10 s-1以上区域。在900-930℃和0.001 s-1的变形条件下,出现耗散率峰值为65%,高m值,S形应力和应变速率对数曲线的现象,合金表现出超塑特性。拉伸实验进一步表明,延伸率可达512%,组织为两相混合组织。另外,合金在800-930℃和大于0.01 s-1的条件下出现集中变形带,表现为局部流变特征。     

镁合金 AZ40M 再结晶晶粒尺寸与硬度模型研究

目的研究变形温度及变形速率对镁合金AZ40M再结晶晶粒尺寸以及硬度的影响。方法在gleeble-1500D热模拟机上进行热物理模拟压缩实验,变形温度为250~400℃,变形速率为0.001~1 s-1,通过金相法观测AZ40M镁合金在不同变形条件下的组织形貌,采用维氏硬度计测出镁合金热变形后的硬度值。结果当升高变形温度或降低变形速率时,材料的晶粒尺寸增大且硬度减小。结论得出了再结晶晶粒尺寸的变化规律,建立了AZ40M镁合金的晶粒尺寸与硬度的关系模型。