Microstructure and mechanical properties of rapidly quenched L1₁ alloys in Ni-Al-X systems

Abstract

Ductile Ll₂-type y′ compounds with rather high strengths and large elongations have been found in rapidly quenched Ni-Al-X (X = Cr, Mn, Fe, Co, or Si) ternary systems. The y′ compounds consist of a metastable phase which contains numerous APD with a size as small as about 50 to 75 nm. Further, the ductile y′ wires with circular cross section have been manufactured directly from the liquid state by an in-rotating-water spinning method. The wire diameter is in the range of 85 to 190 µm and the average grain size is about 2 to 5 µam. The H_v, σ_y, σ_f, and ε_ρ of the y′ wires are about 240 to 400 DPN, 390 to 590 MPa, 580 to 910 MPa, and 4 to 10 pct, respectively, for Ni-Al- (Cr, Fe, Co, or Si) systems and about 220 DPN, 260 MPa, 440 MPa, and 27 pct, respectively, for the Ni-Al-Mn system. A cold drawing causes a significant increase in σ_y and σ_f and the attained values are about 2450 MPa and 2480 MPa, respectively, for Ni-20Al-10Cr wire drawn to about 90 pct reduction in area. Around the temperatures where the APB disappear on annealing, the H_v, σ_y, σ_f, and ε_gr of the y′ wires decrease significantly accompanied with a drastic change in fracture surface morphology from a transgranular type to an intergranular type. It has been therefore inferred that the high strengths and good ductility of the melt-quenched y′ compounds are due to the structural changes to a low degree of ordered state containing a high density of APB and the suppression of grain boundary segregation.