Grain Boundary Segregation and Fracture in Al-Li Alloy.

对时效硬化铝锂二元合金低延展性的原因进行了实验研究。实验包括对充氢合金的室温力学性能的测量，对未充氢合金的低温拉伸性能的测量以及对断口表面的化学分析。结果如下：(1) Al-2.6wt%Li合金的力学性能受阴极充电的影响；抗压应力增大，伸长率减小，这一趋势在低时效条件下最为明显。(2)对断口表面的SEM（扫描电子显微镜）观察发现，在欠时效条件下，阴极带电合金经常发生穿晶脆性断裂。(3)通过透射电镜（TEM）观察到带电合金晶界上有气泡的应变对比，认为这是由稳定的< δ > -相和氢气气泡引起的。(4)在低温下，淬火态合金具有延展性，晶粒断裂。时效合金以晶界脆性断裂为主，延伸率明显降低。(5)通过SIMS分析，在断口表面检测到相当高浓度的Na和K。这在时效合金中更为明显。但在所有老化条件下均未发现明显的S和Li偏析。上述实验结果有力地表明，<H_2>、Na和K在测试温度范围内对3wt%Li Al-Li合金的力学性能有不利影响。

An experimental study was made on the origins of low ductility of age-hardenable Al-Li binary alloy. The experiment includes the measurement of room temperature mechanical properties of hydrogen-charged alloy, the measurement of low temperature tensile properties of non-charged alloy and chemical analysis of fracture surfaces by SIMS(Secondary Ion Mass Spectrometry). Results are as follows.(1) Mechanical properties of Al-2.6wt%Li alloy are influenced by cathode-charging; proof stress increases and elongation decreases, the tendency being most pronounced in underaged conditions.(2) SEM(Scanning Electron Microscope) observation of fracture surfaces revealed a frequent occurrence of transgranular brittle fracture in the cathode-charged alloy in underaged conditions.(3) Strain contrasts accompanied with bubbles are observed by TEM(Transmission Electron Microscopy) along grain boundaries of the charged alloy, that are believed to be due to the stable <delta> -phase with hydrogen bubbles.(4) At low temperatures, the as-quenched alloy is ductile and fractures in grains. While, grain boundary brittle fracture is predominant in the aged alloy, corresponding to the much reduced elongation.(5) By the SIMS analysis, appreciably high conconcentrations of Na and K were detected on the fracture surfaces. This is more pronounced for the aged alloy. But no considerable segregation of S and Li was detected for all the aging conditions for all the test temperatures.The above experimental results strongly suggest the detrimental effects of <H_2> , Na and K on the mechanical properties of Al-Li alloy ( 3wt%Li) in the temperature range tested.