Microstructure design and control of intermetallics

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1. Microstructure control of Ni_3Al polycrystalsMicrostructures of Ni_3Al ternary alloys were controlled to consist of equiaxd, recrystallized grains by isothermal forging under appropriate deformation conditions and ductility at room temperature was investigated. Ductilities of Ni_3Al alloyed with ternary elements of more than 2% are explained by ordering energy, and ductilization of Ni_3Al microalloyed with Zr or Hf is ascribed to scavenging of impurities. The latter conclusion is consistent with the fact that high purity Ni_3Al prepared by using zone-refined Ni and Al in a ultra-high vacuum exhibits ductility.2. Microstructure control of TiAlUtra-fine grained TiAl was obtained by mechanical alloying in H_2 followed by hot pressing or mechanical alloying in Ar followed by HIPing. Ultra-fine grained TiAl deforms under lower stress compared with coarse grained TiAl. Ultra-fine grained TiAl+Ti_3Al exhibits superplasticity and heat treatment after superplastic deformation improves high temperature strength.Compressive deformation at high temperatures products deformation texture for TiAl and for lamellar structured TiAl/Ti_3Al.Ternary additions to TiAl Produce precipitates consisting of intermetallic compounds, carbides or nitrides, thereby increasing high temperature strength.3. Microstructure control of Nb_3AlNb_3Al+Nb two phase alloys were prepared by isothermal forging of ingots, HIP consolidation of alloyed powders or reactive sintering in hot pressing. High temperature strength and fracture toughness at room temperature are improved by controlling alloy composition and microstructure.