STRUCTURE-ANALYSIS OF STRESS-INDUCED BETA''-1 MARTENSITE IN A CU-AL-NI ALLOY BY NEUTRON-DIFFRACTION

The β′ 1 martensite has recently been found to be stress-induced from the γ′1 martensite in Cu-Ai-Ni alloys near the composition Cu-14,0Al-4Ni (wt.%). The martensite is metastable only under stress and exhibits pseudoelastic behavior associated with the transformation. The structure analysis of this martensite has been carried out by neutron diffraction under tensile stress. The unit cell was found to be monoclinic with lattice parameters a = 0.4437, b = 0.5301, c = 3.814 (nm) and β = 89.2°4 referring to the 18 layers' conventional cell. This is equivalent to a reduced primitive cell with lattice parameters a = 0.4437, b = 0.5301, c = 1.2783 (nm) and β = 95.8°8. The space group is P21 m (C22h. The stacking sequence was found to be AB′AB′CA′CA′CA′BC′BC′BC′AB& [18R (11̄31̄)3]consistent with Martynov and Khandros' report, but the stacking positions were found to have deviated from the ideal positions. The amount of deviation has been determined by the least-squares method, and the origin of such deviations has been discussed. It has been proved that the neutron diffraction technique is very useful for the structure analysis of bulk single crystals under special experimental conditions. © 1979.