Possible ways of enhancing high temperature ductility in CuO doped cubic zirconia (8Y–CSZ)

Abstract

The effect of initial density and rapid prestraining on superplastic ductility of 1 wt-%CuO doped cubic zirconia (8Y–CSZ) was investigated. To obtain a range of initial densities, the tensile test specimens were slip cast to net shape and pressureless sintered over a range of temperatures in air. The specimens were then superplastically tested at a temperature of 1500 K and at a constant strain rate of 1×10^-4 s^-1. The results showed that specimens with low initial densities had lower flow stresses and higher superplastic elongations to failure than higher density specimens. The reasons for the ductility change were discussed with reference to the presence of porosity and grain growth. For the prestraining test, a specimen with an initial density of 95% was prestrained to 30% at a temperature of 1550 K and at a prestrain rate ? · ₁ of 1×10^-3 s^-1, followed by elongation to failure at a slower test strain rate ? · ₂ of 1×10^-4 s^-1. It was seen that prestraining at the above test conditions considerably improved superplastic ductility. The reasons for this ductility enhancement were explained in terms of suppression of grain growth.     

Microstructural evolution during superplastic deformation of a 7475 Al alloy

Grain refinement of a superplastic 7475 Al alloy is observed at strain rates of 10-2s-1 or higher. Metallographic observation shows that the average grain size is changed from 14 m to 10 m after 100% elongation. Two-stage strain-rate tests were performed on the 7475 Al alloy to correlate grain refinement with an improvement of superplasticity. The optimum first strain rate and strain in the first stage were determined through tensile superplastic tests. Superplasticity was improved significantly through two-stage strain-rate testing. This is believed to be related to the refinement of the initial grains at high strain rate. The specimen tested at a strain rate of 2.1×10-4s-1 revealed dispersoid-free zones (DFZs) near grain boundaries normal to the stress axis. When a higher strain rate was applied to the specimens with DFZs, no grain refinement was observed. The absence of grain refinement is due to the concentration of plastic deformation in the weak DFZs. © 1998 Kluwer Academic Publishers     

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。     

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 in an Aluminum Alloy 6061/A12O3p Composite

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Effect of prestrain on the superplastic behaviour of a fine grained 7475 AI alloy

The effect of prestraining at a fast strain rate (region III) on the subsequent superplastic behaviour (region II) of a 7475 AI alloy has been studied. The results show that prestraining causes a decrease in the elongation to failure as compared to the non-prestrained (as-received) samples. This decrease in elongation is postulated to be associated with the growth of cavities formed during prestraining as well as grain growth during deformation in region II. Prestraining in region III did not lead to any observable inhomogeneities in strain distribution during subsequent deformation.     

Microstructural evolution and threshold stress for superplastic flow in the Zn-Al eutectoid

The microstructural evolution and the stress-strain rate behaviour of superplastic Zn-Al eutectoid alloy were investigated by prestraining specimens at two strain rates corresponding to Regions I and II. Even though the scale of microstructure was similar, the stress-strain rate curves of differently prestrained specimens were distinctly different in the lower strain-rate regime. While Region I of low rate sensitivity was more prominent when prestrained at a lower strain rate of Region I, it was less distinct because of prestrain in Region II. The threshold stress for superplastic flow, as assessed by an extrapolation procedure, varied with the nature of prestrain. The interphase boundaries were more rounded (higher mean curvature) on prestraining on Region II, compared to Region I. The correlation between the changes in the mean curvature of phase boundaries and the threshold stress arising from the nature of prestrain was consistent with the boundary-migration controlled sliding mechanism to interpret the threshold stress for superplastic flow.     

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.     

Superplastic properties of a TiAl based alloy with a duplex microstructure

The superplastic properties of a engineering TiAl based alloy with a duplex microstructure were investigated with respect to the effect of testing temperatures ranging from 950°C to 1075°C and strain rates ranging from 8 × 10^–5 s^–1 to 2 × 10^–3 s^–1. A maximum elongation of 467% was achieved at 1050°C and at a strain rate of 8 × 10^–5 s^–1. The apparent activation energy was calculated to be 345 kJ/mol. Also, the dependence of the strain rate sensitivity values on strain during superplastic deformation was examined through the jump strain rate tests, and microstructural analysis was performed after superplastic deformation. It is concluded that superplasticity of the alloy at relatively low temperature and relatively high strain rate results from dynamic recrystallization, and grain boundary sliding and associated accommodation mechanism is related to superplasticity at higher temperature and lower strain rate.     

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。     

热挤压态SiCP-LY12复合材料的超塑性变形

未经任何热处理的热挤压态SiC_P-LY12复合材料在基体固相点附近获得超塑性变形,在温度495～510C、应变速率2×10-4s-1时,终断延伸率大于150%。对试验数据的分析表明,在前述变形参数范围内,复合材料的变形机制发生了变化,使终断延伸率显著升高,但复合材料超塑性变形过程的应变速率敏感性指数(m)在同一温度下的m值随应变速率降低而逐渐增大,其最高值并未对应于最大延伸率;同时,应力-应变曲线在通常的动态再结晶阶段后出现一个独特的应变硬化阶段,直至最终断裂。微结构观察发现,变形初期复合材料即发生动态再结晶,据此推测,前述异常硬化阶段可能与再结晶组织的长大有关。另一方面,SiC_P-Al界面在变形过程中严重弱化,成为裂纹优先扩展的路径,不利于获得高的延伸率。     

SiC晶须增强铝基复合材料超塑性

采用高温拉伸、透射电镜、X射线衍射仪、差示扫描量热计和超塑性经典理论,对低压浸渗、小挤压和热轧制备的SiC晶须增强2024Al基复合材料超塑性的力学行为和变形机制进行了研究。研究表明:复合材料的晶粒细小,尺寸约为1 μm;在温度为788 K、初始应变速率为3.3×10-3s-1的拉伸条件下,超塑伸长率为370％;DSC曲线上有一小的初期熔化吸热峰,其温度相应于偏晶反应:Al+Al₂Cu+Cu₄Mg₅Si₄Al_<em>x→液相+Mg₂Si,785 K;超塑性变形的主导机制为传统的晶界扩散机制和适量液相共同控制的晶界(界面)滑动。     

Investigation on Superplasticity in SiCp／2024 Cold Rolling Sheet after Heat Treatment

High strain rate superplastic deformation behavior of powder metallurgy (PM) processed 17 vol. pct SiCp/2024 Al composite sheet after heat treatment was investigated over a range of temperature from 753 to 833 K. At 813 K,a maximum elongation of 259% was discovered at a strain rate of 10^-1 s^-1. The activation energy was closed to that for lattice diffusion of Al and increased at temperature upon incipient melting temperature. The mechanism of superplastic deformation for present composites was attributed to lattice diffusion controlled grain boundary sliding.     

电沉积纳米复合材料的超塑性研究

采用脉冲电沉积技术制备了含不同Al2O3颗粒的Al2O3/Ni-Co纳米复合材料,在应变速率从8.33×10-4s-1到1.67×10-2s-1,温度从723K到823K的范围内,研究了它们的超塑性拉伸变形行为,确定了最佳超塑性条件并获得了最大延伸率.采用SEM和TEM对电沉积和超塑变形前后试件的显微结构进行了表征.应用晶粒长大行为和协调机制对合金和复合材料的超塑性进行了对比研究和讨论.     

High strain rate superplasticity of ultrafined Ti–10V–2Fe–3Al compact prepared by sintering and isothermal forging

Abstract

High strain rate superplasticity was obtained for powder Ti–10V–2Fe–3Al (Ti-1023) alloy prepared by powder sintering and isothermal forging technology. The selected powder was cold isostatic pressed, sintered and isothermal forged to prepare this powder alloy. Tensile testing was conducted at optimum superplastic temperaure of 1023 K with different initial strain rate, and the elongation to failure, the flow stress and the microstructure were analysed. The experiment results exhibited that the microstructure of this powder alloy is extraordinary uniform and fine, resulted in considerable enhancement of optimum initial strain rate increased from 3·3×10^?4 s^?1 of conventional cast and wrought Ti-1023 alloy to 3·3×10^?3 s^?1 of this powder alloy. The elongation to failure increased first and then decreased with initial strain rate from 3·3×10^?4 to 3·3×10^?2 s^?1. The strain rate sensitivity m is about 0·46 near initial strain rate of 3·3×10^?3 s^?1, larger than conventional cast and wrought Ti-1023 alloy. Microstructure observations showed that dynamic recrystallisation and grain growth were present during superplastic deforming.     

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.     

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%.     

High strain rate superplastic flow stress and post-deformation mechanical properties of mechanically alloyed 2024 aluminium alloy reinforced with SiC particles

Abstract

Composites consisting of 2024 aluminium alloys reinforced with volume fractions of 0, 5, 10, and 15 vol.-% of SiC particles were fabricatedfrom the mechanically alloyed powders by an optimised hot compaction and prestraining process. Fine and equiaxed grain structures with grain sizes of <1 μm were observed within the matrix of each alloy. The composite specimens were compressed at temperatures between 733 and 813 K with a wide strain rate range from 10^?3 to 10 s^?1. Two strain rate regions with different slopes from ～ 5 × 10^?1 s^?1 were found in log (true stress–log (strain rate) curves. In the lower strain rate region of each alloy, the strain rate sensitivity values m were 0.03–0.16. The threshold stress σ_th for each alloy was estimated using an extrapolation procedure. A linear relationship was found between <disp-formula><graphic href="splitsection10-m1.tif"/></disp-formula> and σ_th where V_f is the volume fraction of SiC particles. In the higher strain rate region of each alloy, m values greater than 0.3 were obtained at 773 K, which is very close to the solidus temperature of 775 K for 2024 aluminium alloy. Moreover, the maximum yield strength and elongation for each alloy at room temperature were also obtained in the specimens compressed at 773 K. Thus, it was found that the optimum temperature for the high strain rate superplastic processing of the composites was just below the solidus temperature of the 2024 aluminium alloy. The grain coarsening resulted in the decrease of post-deformation strength and ductility as well as the m value in hot compression above the solidus temperature.     

Exceptional superplasticity in an AZ61 magnesium alloy processed by extrusion and ECAP

Experiments were conducted on a commercial AZ61 alloy to evaluate the potential for achieving an ultrafine grain size and superplastic ductilities through the use of the EX-ECAP two-step processing procedure of extrusion plus equal-channel angular pressing. The results show that EX-ECAP gives excellent grain refinement with grain sizes of 0.6 and 1.3 μm after pressing at 473 and 523 K, respectively. The alloy processed by EX-ECAP exhibits exceptional superplastic properties including a maximum elongation of 1320% after pressing through four passes when testing at 473 K with an initial strain rate of 3.3 × 10⁻⁴ s⁻¹. This result compares with an elongation of 70% achieved in the extruded condition without ECAP under similar testing conditions.     

Superplasticity in an aluminum alloy 6061/Al(2)O(3)p composite

The superplasticity of an Al(2)O(3)p/6061Al composite, fabricated by powder metallurgy techniques, has been investigated. Instead of any special thermomechanical processing or hot rolling, simple hot extrusion has been employed to obtain a fine grained structure before superplastic testing. Superplastic tensile tests were performed at strain rates ranging from 10(-2) to 10(-4) s(-1) and at temperatures from 833 to 893 K. A maximum elongation of 200% was achieved at a temperature of 853 K and an initial strain rate of 1.67x10(-3) s(-1). The highest value obtained for the strain rate sensitivity index (m) was 0.32. Differential scanning calorimeter was used to ascertain the possibility of any partial melting in the vicinity of optimum superplastic temperature. These results suggested that no liquid phase existed where maximum elongation was achieved and deformation took place entirely in the solid state.