The aging feature of Al-Li-Cu-Mg-Zr alloy containing Sc

The aging behaviors of Al-1.42% Li-2.41% Cu-0.93% Mg-0.073% Zr-0.17% Sc (mass fraction, the same below) alloy at room temperature, 160 °C, and 160°C after 8% pre-deformation were studied respectively by hardness measurement. The microstructure of the alloy in various aging conditions was observed by TEM. The results show that the main precipitations of the alloy in quenching condition are the particles containing Sc and Zr which have certain coherent relation with the matrix. Addition of Sc in Al-Li-Cu-Mg-Zr alloy will be favorable to promoting precipitation. The particles can serve as preferred nucleation sites for δ′ phases which accelerate the aging hardening rate at initial aging. The main hardening phases of the alloy aged at 160 °C are δ′ and δ′/β′ composite precipitates. The size of the composite precipitates is very small (nanometer size). The composite precipitates will preclude efficiently the formation concentrative slip location and will improve the mechanical properties of the alloy. S′ phase will occur in the alloy aged at 160 °C after 8% pre-deformation. It is found that 8% pre-deformation has no obvious influence on the precipitation of the composite phase. Foundation item: The Key Program of the 9th Five-year Plan of China (No. 95-YS-001) Biography of the first author: TAN Cheng-yu, associate professor, born in 1963, majoring in materials and their suface performances.     

Mechanical model for yield strength of nanocrystalline materials under high strain rate loading

To understand the high strain rate deformation mechanism and determine the grain size,strain rate and porosity dependent yield strength of nanocrystalline materials,a new mechanical model based on the deformation mechanism of nanocrystalline materials under high strain rate loading was developed.As a first step of the research,the yield behavior of the nanocrystalline materials under high strain rate loading was mainly concerned in the model and uniform deformation was assumed for simplification.Nanocrystalline materials were treated as composites consisting of grain interior phase and grain boundary phase,and grain interior and grain boundary deformation mechanisms under high strain rate loading were analyzed,then Voigt model was applied to coupling grain boundary constitutive relation with mechanical model for grain interior phase to describe the overall yield mechanical behavior of nanocrystalline materials.The predictions by the developed model on the yield strength of nanocrysatlline materials at high strain rates show good agreements with various experimental data.Further discussion was presented for calculation results and relative experimental observations.     

快凝PM Al—Li合金高应变速率超塑性变形机制

对PM－Al-Li合金在高应变速率超塑性变形行为进行了研究,对其变形机制进行了讨论,研究表明,PM－Li合金在超塑变形过程中晶粒会发生形变诱发动态长大,其长大速率主要受应变速率所控制;在变形过程中,随着就变速率的增加,晶界滑动的协调机制由晶内位错滑移协调逐步向晶粒变形协调转变;PM－Al-Li合金在570℃以上变形时,绝热加热现象的发生会导致液相的产生,并且液相的体积分数随应速率的增加而增大,合金的延伸率随着液相体积分数的增加而急剧下降。     

温度与应变率对Cu70 Zn30孪晶变形的影响

为了研究孪晶变形过程中孪晶间距与孪晶片层厚度随温度与应变率变化情况,建立一个关于Cu70 Zn30的流动应力本构方程,其中流动应力分为短程应力与长程应力,短程应力用Johnson Cook模型描述,长程应力采用幂次强化法则,运用Matlab软件模拟了Cu70 Zn30在不同温度与不同应变率的条件下的孪晶变形,得出了孪晶变形过程中孪晶间距与孪晶片层厚度在不同条件下的演化曲线,通过对比实验结果,证实了低温与高应变率均能促进孪晶变形,其效果随着温度的降低与应变率的升高而增强;相对于应变率的影响,温度的降低更能促使孪晶的生长,孪晶间距的大小与孪晶片层的厚度随着温度的降低与应变率的升高而减小.     

应变速率对AZ31B变形镁合金力学性能的影响

为了研究应变速率对AZ31B变形镁合金力学性能的影响,试验温度为室温、150℃时,对AZ31B变形镁合金进行拉伸试验,并记录抗拉强度和屈服强度,计算延伸率.通过扫描电镜观察拉伸断口形貌,结果表明,随着应变速率的提高,AZ31B变形镁合金的抗拉强度和屈服强度都随之提高,而延伸率却逐渐降低;随着温度的升高,同一应变速率下的抗拉强度和屈服强度降低,而延伸率大幅度升高.通过观察扫描断口形貌发现,合金表现为韧性断裂,且随着应变速率的降低,韧窝逐渐增多.     

高应变速率下载荷方向对镁合金力学性能的影响

为了研究高应变速率下AZ31B镁合金动态力学行为各向异性,采用分离式霍普金森压杆（SHPB）对沿挤压方向（ED）、法向（ND）及横向（TD）加工的AZ31B镁合金在应变速率为1.058~2.500s-1范围内进行冲击压缩实验,并采用扫描电镜观察压缩断口形貌.结果表明：高应变速率下AZ31B镁合金沿ED方向和ND方向有明显的应变强化现象,而沿TD方向应力对应变速率不敏感;随应变速率的增加,不同载荷方向屈服强度与最大应力值的差值变化程度有所不同,其中沿ND方向差值变化最大,沿TD方向差值变化最小;通过观察扫描断口形貌发现,在高应变速率下AZ31B镁合金在不同载荷方向下的断裂方式均为解理断裂.     

Effects of strain rate on dislocation for TA15 alloy during hot compressive deformation

The effects of strain rate on the dislocation type and dislocation configure of TA15 alloy were investigated.The experimental results show that the operating dislocation type changes from c type to c and a+ c type with increasing strain rate under the deformation condition of 900℃,60% strain.Under the condition of 900℃,60% strain and 0.001/s strain rate,lots of orientate dislocation cellular configurations and sub-grains,many dislocations pile up before sub boundary.When the strain rate increases to 0.1/s,some dislocations exhibit curved and dislocation tangles and pile-ups can be found,suggesting more dislocations and much stronger interactions among dislocations.     

I-II-III复合型裂缝应力强度因子与能量释放率的关系

基于断裂力学理论,应用复合型断裂判据中的最大周向应力判据和最大拉应变判据,以单一型裂缝应力强度因子K与能量释放率G的关系为基础,推导出I-II-III复合型裂缝应力强度因子K_I、K_II、K_III与能量释放率G_I-II-III关系公式;并应用有限元软件进行I-II-III复合型裂缝的有限元模拟,模拟值与理论值之间相差为1.14%,拟合良好,分析验证了复合型裂缝应力强度因子K_I、K_II、K_III与能量释放率G_I-II-III关系公式的合理性。     

Effect of strain hardening and strain softening on welding distortion and residual stress of A7N01-T4 aluminum alloy by simulation analysis

The effect of strain hardening and strain softening behavior of flow stress changing with temperature on welding residual stress,plastic strain and welding distortion of A7N01-T4 aluminum alloy was studied by finite simulation method.The simulation results show that the weld seam undergoes strain hardening in the temperature range of 180-250 ℃,however,it exhibits strain softening at temperature above 250 ℃ during welding heating and cooling process.As a result,the strain hardening and strain softening effec...