Electronic structure and the martensitic transformation in beta -phase Ni-Al alloys: 27Al NMR and specific-heat measurements.

Abstract

$^{27}\mathrm{Al}$ NMR line shape, Knight shift, relaxation rates together with low-temperature specific heat and magnetic susceptibiltiy were measured as a function of temperature and external magnetic-field in two Ni-rich \ensuremath{\beta}-phase Ni-Al alloys (62% Ni and 63% Ni) undergoing a martensitic phase transformation (MPT) below room temperature. For the purpose of comparison, two further alloys not undergoing the MPT were investigated (50% Ni and 55% Ni). Regarding the electronic properties of the alloys, we find an increase in the density of states at the Fermi level and a decrease of s-character of the conduction electron wave function as a function of the increase of Ni content from the 1:1 Ni-Al alloy. Furthermore, the inhomogeneous broadening of the $^{27}\mathrm{Al}$ linewidth indicates that the additional Ni replacing Al at the ``wrong'' site introduces quasilocalized magnetic states. Regarding the MPT we find a slight decrease of the Knight shift and of the Korringa product (${\mathit{T}}_{1}$T${)}^{\mathrm{\ensuremath{-}}1}$ going from the austenite to the martensite. In the transformation-temperature region, the $^{27}\mathrm{Al}$ NMR line is broadened by the effect of superposition of two signals arising from the austenite and the transforming phase. The deconvolution of the spectrum gives information about the nucleation and growth of the martensite, which appears to be continuous and involving a succession of intermediate states, in contrast with the abrupt nucleation of fully transformed martensite observed in Cu-Zn-Al. No anomalous enhancement of the relaxation rate is observed above ${\mathit{M}}_{\mathit{s}}$ or during the MPT. Although the NMR cannot rule out the presence of static precursor effects a few degrees above the transformation temperature ${\mathit{M}}_{\mathit{s}}$, no evidence was found for the formation of martensitic regions well above ${\mathit{M}}_{\mathit{s}}$ where tweed patterns are observed by electron microscopy.