不同状态的Mg-6Gd-2Y合金的显微组织特征

Microstructure Characteristics of Mg-6Gd-2Y Alloy at Different States

综合运用OM、XRD、FEGSEM和HRTEM手段深入分析了Mg-6%Gd-2%Y(质量分数)(记为GW62)合金铸态、固溶态和时效态的显微组织特征及演变过程。GW62合金铸态组织主要由α(Mg)和呈不连续网状的Mg_5(Gd,Y)相组成,在紧邻Mg_5(Gd,Y)处有少量非平衡凝固相Mg_2(Gd,Y);对合金进行520℃固溶处理,随着固溶时间延长,半连续状Mg_5(Gd,Y)相尺寸逐渐缩小,直至溶解,并在晶界形成大量细小fcc结构的富稀土相Mg(Gd,Y)_2,明显阻止了α(Mg)晶粒的长大;合金在175~225℃时效处理时,175℃时效硬化效果最明显,时效析出过程包括:时效初期(4~32 h),过饱和α(Mg)析出β″相;快速析出期(32~100 h),α(Mg)析出β′相,100 h达到峰值时效,析出相为β″、β′相;过时效期区(>100 h),β′相尺寸逐渐增大,并转化成β_1和向稳定的β相转变。

The microstructures characteristics and evolution rule of the Mg-6wt%Gd-2wt%Y （GW62） alloy at as-cast, homogenized and aging states were investigated by OM, SEM, EDS, XRD and HRTEM. The microstructure of the as-cast GW62 alloy is composed of isometric a（Mg） crystals, equilibrium phase Mg5（Gd, Y） and non-equilibrium phase Mg2（Gd, Y）. With the increase of solution treatment time, the sizes of the semicontinuous Mg5（Gd,Y） phases gradually decrease until dissolved. Rare-earth-rich phases Mg（Gd,Y）2 with fcc structures are newly generated in the alloy at grain boundaries and inside grains after solution treatment, which could hinder the growth of isometric a（Mg） crystals in the solution process. The maximum hardness of the GW62 alloy is significantly different for different aging temperatures （175-225 ℃）. The age-hardening effect is most apparent at 175 ℃. The microstructural evolution of the alloy after aging at 175℃is as follows： the β″ phase precipitates from a（Mg） in the under-aging stage （4-32 h）; then, theβ′ phase precipitates from a（Mg） until peak aging of 100 h （32～100 h） and the precipitated phases are the β and β″ phases; finally, during over-aging （〉 100 h）, the size of β′ phase continually increases and it transforms intoβ and β 1phases.