Caracterização térmica e estrutural de ligas Ni50Ti50-XHfX .at% com efeito memória de forma.

Abstract

The objective of the present study was to obtain Ni50Ti50-XHfX .at% alloys with EMF by means of three types of processes: (i) plasma arc casting (SPP method) on EDG Discovery camera All Metal ; (ii) fast solidification by the melt spinning method; (iii) arc melting. Then, it continues with the thermal characterization (by means of the DSC technique) and microstructural characterization (by the XRD, OM and SEM/EDS tests). In the thermal characterization were recorded and discussed the transformation temperatures [TTs], heat flows [W/g], energy densities [J/g] and thermal hystereses [ΔP and ΔT]. These results were correlated with qualitative and quantitative phase analyzes that was detected by XRD in the 5 substituting compositions studied (Hf = 8, 11, 14, 17 and 20 .at%). The Rietveld method was used to make structure refinement and to determinate the variables such as specific density, percentage of phases, network parameters [a b c; α ȕ Ȗ], indexing of crystallographic planes (h k ), calculation of the relative degree of crystallinity [Xc] and amorphous percentage, in addition to measuring the chemical compositions of the observed phases. Based on non-linear regressions, the results of the X-ray diffraction experiments were also used as a statistical tool to establish a mathematical relationship between the interplanar distance [d, Å] & crystalline identification fraction 1/d2

with the Bragg angle [2ș, o]. Afete that, the cold-embedded samples were chemically attacked by optical microscopy (OM) and scanning electron microscopy (SEM) with the objetive to observe surface aspects, textures, contours and grain sizes (width, length, perimeter and area). The EDS technique was used to determine the atomic percentage (.at%) of the elements of the alloys in specific areas. It was demonstrated in this work that although Ni-rich alloys were prepared using the same methodology, materials and nominal compositions, the thermal and microstructural results were considered very distinct, with peculiarities that direct us to the individualized discussion in each of the 3 processes.