A DESCRIPTION OF DEFORMATION TWINNING IN DYNAMICALLY DEFORMED AND SHOCK-LOADED POLYCRYSTALLINE NI3AL

Deformation twinning has been documented by means of transmission electron microscopy (TEM) in the gamma'-phase in two nickel aluminide alloys, single-phase Ni3Al + B and two-phase Ni-20 at.% Al-30 at.% Fe. The first intermetallic was shock loaded to a stress of 14 GPa, and the second was dynamically deformed at a strain rate of 3000 s-1 to a total accumulated strain of 16%. In both materials, twins were found to form solely in grains with the L1(2) superlattice structure and were observed to extend completely across the grains. In the two-phase alloy, no twins were found to occur in grains with the B2 superlattice structure. Analysis of the twins show that the twinning plane K1 is {111} with a twinning direction eta1 of [112], such as is commonly found in disordered f.c.c. metals. It is proposed that the eta2 direction in the second undistorted plane K2 is [110], which negates the need for atomic shuffling. The twinning mechanism necessary for this twin plane and the alternate eta2 direction of [112] requiring shuffles is discussed. Extensive planar slip on {111}-type planes was also seen in both materials, and in both the B2 and the L1(2) grains in the two-phase alloy. A high density of stacking faults was observed in the shock-loaded single-phase alloy.