Calculation of Grain Boundary Segregation in Cu–Based Alloys by CALPHAD Method

Abstract

Compositions of the grain boundary （GB） segregation of solute elements in Cu–based alloys were calculated by CALPHAD （Calculation of Phase Diagrams） method considering the grain radius and volume fraction of grain boundary as well as the stagnant diffusivity of solute elements at low temperatures. The calculations were performed based on the parallel tangent law proposed by Hillert, in which the GB phase and its Gibbs energy were assumed to exist in polycrystalline microstructures. The Gibbs energies of the fcc–Cu and the GB phase, which was supposed as a supercooled liquid phase, were calculated by Thermo–Calc using a thermodynamic database of Cu–based alloys （TCCU2） and incorporated into the calculation program of the GB segregation via TQ–Interface. Under the equilibrium condition in the Cu–X （X＝Ag, Al, As, Be, Co, Cr, Fe, Mg, Mn, Mo, Ni, Pb, Si, Sn, Ti, Zn, Zr） binary systems, segregated compositions of X except for Ni increase monotonously with decreasing temperature. The negative segregation of Ni was changed to the positive one by addition of Si, which indicates the co–segregation of Ni and Si might be precursors for the nucleation of the Ni₂Si phase in Cu–Ni–Si alloys. It was suggested that considerable amount of B as well as Ti segregates on GB in Cu–4Ti–0.03B （at.％） alloy, which could enhance the precipitation of TiB₂ on GB and suppress the discontinuous precipitation of Cu₄Ti. The calculated GB segregation of P and Zr in Cu–Ni–Si alloys is suggested to suppress the discontinuous precipitation of Ni₂Si.