The By martensite has recently been found to be stress-induced from the 7; martensite in Cu-Al-Ni alloys near the composition Cu-lJ.OAl-INi (wt.!<). The martensite is metastablr 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 = 0.443Y, h = 0.530,. c = 3.81, (nm) and jl = 8Y.2,' referring to the 18 layers' conventional cell. This is equivalent to a reduced primitive cell with lattice parameters a = 0.443,. b = 0.530,, c = 1.278, (nm) and p = 95.8s'. The space group is P2,'m (Ci& --The stacking sequence was found to be AB'AB'CA'CA'CA'BC'BC'BC'AB' [ 18R (I 13l),] 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 useiul for the structure analysis of bulk single crystals under special esperimen*a! conditions.
RGsum&On a trouve recemment que la martensite /I; se formait sous contrainte a partir de la martensite $, dans les alliages Cu-AI-X dont la composition est voisine de Cu-lJ.OAl-4Ni (en poids). Cette martensite est metastable sous la contrainte et elle prtsente un comportement pseudoelastique associe B la transformation. Xous avons analyse la structure de cette martensite par diffraction de neutrons sous contrainte de traction. Nous avons trouve une maille tltmentaire monoclinique. de parametres n = O,j13,, b = 0.530,. c = 3.81J (nm) et d'angle B = 89.2& degris. si I'on se rapporte i la maille conventionnclle de 18 couches atomiques. Ceci est equivalent a une maille primitive riduite de paramttres a = O.J-l3;, b = 0,530, c = 1,278, nm et dangle B = 95.8s degrts. Le groupe d'espace est PZ,!m (Ci,). Nous avons trouve une sequence d'empilement AB'AB'CA'CA'CA'BC'BC'BC'AB'