Crystallography of grain refinement in cast zinc–copper alloys

Abstract

Adding the peritectic forming element Cu effectively reduced the average grain size of cast Zn by over 85%. At a specified cast condition, the smallest grain size was obtained at 2 wt% Cu addition. A further increase in Cu content led to grain coarsening in the cast Zn–Cu alloys. Although the solute effect of Cu was predominately responsible for the grain refinement through restriction of the grain growth, it was found that the variation of grain size is also closely related to the formation of the pro-peritectic phase, ɛ-CuZn4. Crystallographic calculations using the edge-to-edge matching model showed low interatomic misfit and interplanar mismatch between Zn and the ɛ-CuZn4 phase. In addition, a reproducible h.c.p.–h.c.p. (h.c.p. denotes hexagonal close-packed) orientation relationship between Zn and the ɛ-CuZn4 particles (located within the Zn grain centres) was also experimentally determined using the electron backscattered diffraction method. This indicated the high potency of the pro-peritectic ɛ-CuZn4 particles as effective heterogeneous nucleation sites for η-Zn, which further refined the Zn grains. However, when the Cu content was over 2.0 wt%, formation of large ɛ-CuZn4 particles resulted in grain coarsening of the cast alloys.