NMR INVESTIGATION OF THE MARTENSITIC-TRANSFORMATION IN FINE PARTICLESOF NI-AL ALLOYS

Al-27 NMR results are presented for three Ni-Al alloys with compositio n 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 fo r samples having different particle sizes (1 mm-50 mum). We observe an incomplete transformation in the fine particle sample (less than 150 mum). The effect can be explained as due to the presence of austenite trapped in intramartensitic regions near the center of the particles.