Crystal structure and lattice defects of the Ni-36.8 at % Al martensite have been investigated mainly by transmission electron microscopy. A structure of the martensite determined in the present work has a normal Llo type (CuAu I type) structure which is different from a modified Llo type (not CuAu I type) structure suggested by Rosen and Goebel. Transformation twins in the martensite have been found on the {1111}t<112>t system. Brief considerations are given to the (111)t[111]t twinning modes. (Received July 11, 1972) temperatures. It was first observed by Guard and Turkalo(2) that the alloy near this composition range undergoes a transformation on quenching to an acicular structure. Afterwards, Ball(3) also has reported a similar martensitic structure in compound NiAI. Rosen and Goebel(4) investigated the structure of the martensite of alloys containing 62 to 68 at % Ni using the X-ray powder method, and showed that the structure of martensite has a modified L1o type (not CuAu I type) structure with an axial ratio cla = 0.865 (in the case of the alloy containing 66 at % Ni) as given in Fig. 1(a). However, this modified Llo type structure cannot form directly from the high-temperature CsCl type structure martensitically. Therefore they hypothesized "ordered nickel-rich NiAI" (see Fig. 3 in ref. 4) in the high temperature region. This hypothetical "ordered NiAI" has not yet been proved experimentally. If the Llo type structure is formed from the CsC1 type structure martensitically, that is, this structure is obtained by the Bain distortiont of bcc to fct transformation, it may be such structure as shown in Fig. 1(b). Enami and Nenno(s) have recently shown that the Ni-36.8 at % Al martensite has a shape memory effect. It is necessary to obtain more detailed information about the crystal structure or lattice defects of this martensite for the understanding of the transformation mechanism and the shape memory effect found in this alloy.In the present study a new crystal structure of the Ni-36.8 at % Al martensite having a normal Llo type (CuAu I type) structure (see Fig. 1(b)) has been found which is different from the modified Llo type (not CuAu I type) structure suggested by Rosen and Goebel, and it is shown that there exist {111}<112> internal twinstt in the Ni-36.8 at % Al martensite.
II. Experimental ProceduresA Ni-36.8 at % Al alloy was, prepared by vacuummelting electrolytic nickel (99.9 %) and pure aluminium (99.99 %) in a high frequency induction furnace. The for electron microscope observation were cut out from the ingot to about 0.3 mm in thickness.To obtain
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