Estudo da influência do teor de Ni nas transformações de fases das ligas com memória de forma Ni100-xTix via dinâmica molecular

Abstract

Ni100-xTix (45 < x < 55) alloys were studied by using molecular dynamics simulations, under the modified embedded atom method (MEAM) interatomic potential to determine the compositional limit at which alloys exhibit martensitic transformations. Initially equiatomic composition alloys were produced from the B2 crystalline structure, minimized and relaxed to 100 K for 10 ps, under the canonical ensemble, NVT. Subsequently, the alloy was heated and cooled at rates of 7.0, 5.0, 3.5 and 2.0 K/ps in the temperatures range of 100 to 500 K. Depending on the alloy composition, was observed the appearance of thermal hysteresis between phases B2 and B19' and the reverse transformation or even none of these behaviors. The structural characterization was computationally realized by using both the X-ray and electron diffraction. The processing of the Ni45Ti55, Ni47.5Ti52.5, Ni52.5Ti47.5 and Ni45Ti55 alloys followed the same computational procedure described above, however, the thermal rates used were 5.0 and 2.0 K/ps. The Ni45Ti55 alloy does not show hysteresis, but it was observed, in the heating and cooling steps, a slight slope in the graph of potential energy (EP) x temperature (T) and a change in the edges of the crystal structure, in the graph of Li x T, (Li = edges of the x, y and z axes simulation box) indicating a transformation from B2 to tetragonal phases. The alloy Ni47.5Ti52.5 showed change in EP, in the EP x T graph indicating the existence of phase transformation (Tetragonal → B2). Furthermore, the curves do not overlap, indicating the existence of hysteresis. The Li x T graph showed an abrupt change in the dimensions of the crystal structure indicating that the phase transformations occurred in a single step. Similarly, to the Ni50Ti50 alloy presented thermal hysteresis, in the EP x T graph, however, the R phase appeared only during cooling. The Ni52.5Ti47.5 alloy displayed, in the EP x T graph, a martensitic transformation in two stages (B2→R→B19') and a well-defined hysteresis, similarly to the equiatomic alloy, however, it presented the phase transitions in both heating and cooling steps. Finally, the EP x T curve of the e Ni45Ti55 alloy showed a phase shift in the heating step, at the rate of 5 K/ps, a behavior that was not repeated at the rate of 2K/ps and in the cooling step regardless of the thermal rate, evidencing an atypical and complex behavior. Thus, it was estimated that the compositional range to observe martensitic transformations is approximately 47.5 to 52.5% at. of Ni.