Decomposition processing and precipitation hardening of rapidly solidified Al-Cr-Y-Zr alloy

1　ＩＮＴＲＯＤＵＣＴＩＯＮＦｏｒｌａｓｔｔｗｏｄｅｃａｄｅｓｒａｐｉｄｌｙｓｏｌｉｄｉｆｉｅｄＡｌ Ｃｒ Ｚｒ( Ｍｎ)ｈｅａｔ ｒｅｓｉｓｔａｎｔａｌｕｍｉｎｕｍａｌｌｏｙｓｙｓｔｅｍｈａｓｂｅｅｎｐａｉｄａｐｕｂｌｉｃａｔｔｅｎｔｉｏｎｂｅｃａｕｓｅｏｆｉ

Decomposition processing and precipitation hardening of rapidly solidified Al-Cr-Y-Zr alloy

Foil powders of Al-5.0Cr-4.0Y-1.5Zr (%) were prepared by using a multi-stage atomization-rapid solidification powder-making device. The obtained powders were exposed thermally at various temperatures. Variation of microstructures and properties of the alloy powders was investigated by micro-hardness measurement, X-ray diffraction, differential thermal-analysis, and transmission electron microscopy with energy disperse X-ray analyses. The results show that cubic Al20Cr2Y (a=1.437nm) and metastable Ll2 Al3Zr (FCC, a=0.4051nm) or equilibrium DO23 Al3Zr (tetragonal structure, a=0.4091nm, b=1.730nm) are main second phases precipitated from supersaturated solid solution of the rapidly solidified foil powders during thermal exposure. The cubic dispersion precipitates prior to the two other Al3Zr type intermetallic phases in the course of the decomposition. Precipitation of incoherent Al20Cr2Y results in softening of foil powder, and coherent Ll2 Al3Zr has intensive precipitation strengthening effect. The Al20Cr2Y phase is structurally stable, but it is prone to coarsen and polygonize above 450℃. Both Al3Zr type intermetallic phases have much smaller coarsening rate than Al20Cr2Y at temperature higher than 450℃. These two phases are able to keep their fine spherical morphologies up to 550℃, but Al3Zr transforms into DO23 structure from Ll2 structure during thermal exposure above 550℃.