NMR investigation of the martensitic transformation in fine particles of Ni-Al alloys

Al27 NMR results are presented for three Ni-Al alloys with composition 62%, 62.5%, and 63% Ni undergoing a thermoelastic martensitic phase transformation below room temperature. By a method of analysis of the NMR data recently developed by us [Phys. Rev. B 46, 10 563 (1992)], we measure the growth of the volume fraction of martensite on cooling for samples having different particle sizes (1 mm-50 ?m). We observe an incomplete transformation in the fine-particle sample (less than 150 ?m). The effect can be explained as due to the presence of austenite trapped in intramartensitic regions near the center of the particles.