Intrinsic relationship between crystallization mechanism of metallic glass powder and microstructure of bulk alloys fabricated by powder consolidation and crystallization of amorphous phase

Ti64Nb12Cu11.2Ni9.6Sn3.2 bulk alloys were fabricated by consolidation of metallic glass (MG) powder synthesized by mechanical alloying and crystallization of amorphous phase. The Johnson-Mehl-Avrami-Kolmogorov method was employed to investigate non-isothermal crystallization kinetic of the synthesized MG powder. The average local Avrami exponent n determined from crystallization kinetic increases from 1.7 to 2.9 and then decreases to 1.8-2.0 when the crystallized volume fraction a is in the range of 0.1-0.3, and around 0.5 and 0.9, respectively. This indicates the corresponding crystallization mechanism is diffusion-controlled three-dimensional growth of nuclei with decreasing nucleation rate, volume diffusion-controlled three-dimension growth with increasing nucleation rate, and diffusion-controlled growth with decreasing nucleation rate, respectively. Meanwhile, the values of n imply that the nucleation rate increases with increasing heating rate. Microstructure analysis indicates that the fabricated bulk alloys mainly consist of three kinds of crystallized phases, beta-Ti, (Cu, Ni)-Ti-2, and Nb3Sn, but exhibit different microstructures and mechanical properties. The effect of the heating rate on the microstructure and mechanical properties can be explained based on different crystallization mechanisms. Especially, obvious plasticity is resulted from formation of ductile (Ti, Nb)(3)Sn from the Nb3Sn at relative high sintering temperature. The results obtained provide insight into fabrication of large-sized crystallized phase-containing bulk alloys with excellent mechanical property by powder metallurgy. (C) 2013 Elsevier B. V. All rights reserved.