CRYSTAL-STRUCTURES AND MICROSTRUCTURES OF (NI,CU)3SN ALLOYS

A phase diagram for (Ni, Cu)3Sn alloys containing 10-22 at.% Cu has been proposed in a previous paper. In this paper, the microstructure and crystal structure observations of as-quenched and furnace-cooled alloy specimens containing 14, 17.5 and 20 at.% Cu are made by transmission electron microscopy and electron diffraction. A high temperature phase having a DO3 structure transforms in part to a phase with a triclinic structure when furnace cooled from 1000 °C for these alloys. This triclinic phase is slightly distorted from a β-Cu3Ti-type 2H structure (denoted by d2H) and appears in an acicular form having uniformly distributed internal faults. The acicular structure is surrounded by the 2H phase having faults on (101) and a low density of dislocations. The high temperature DO3 phase is found to be retained in alloys containing 16-22 at.% Cu when specimens are quenched from 1000 °C into ice-water; on the contrary, a 2H martensite containing (121) twins is formed in an alloy containing 14 at.% Cu. This is contradictory to a recent report claiming that a 2H martensite was formed in alloys containing 8-19 at.% Cu. A possible formation mechanism of the d2H phase is proposed. © 1990.