Materials Science Forum Vols. 738-739

Abstract: Nb-rich precipitates in the matrix of as-cast and annealed Ni_45.5Ti_45.5Nb₉ alloys are investigated by scanning and scanning transmission electron microscopy, including slice-and-view and geometric phase analysis (GPA). The Nb-rich bcc nano-precipitates in the as-cast alloy have a 10% lattice parameter difference with the B2 matrix and reveal compensating interface dislocations. The 3D reconstruction of the configuration of small Nb-rich precipitates in the annealed alloy reveals a wall-like distribution of precipitates, which may increase the thermal hysteresis of the material.

Abstract: Under rolling of Ti-48%Ni-2%Fe single crystals in the phase B2 at 350oC the twinning is the main mechanism of plastic deformation by many initial orientations of these single crystals, as texture data show clearly. Splitting of initial maxima in the stereographic projection of single crystal into several new ones is an evident manifestation of twinning. But this mode of reorientation is observed only at initial stages of rolling, when deformation degrees do not exceed ~10-15%. A geometrical analysis of pole figures for rolled single crystals showed, that at the used rolling temperature the preferable twinning planes there were {114} and {118}.

Abstract: Intermetallic compounds of ZrNi-ZrCo cross-section undergo martensitic phase transformation, which is accompanied by the imperfect shape memory effect at elevated temperatures. Martensitic transformation is also taking place in one of the constituents for the in-situ composites of ZrCo-Zr₂Co-Zr₂Ni-ZrNi region. High temperature shape memory behavior of Zr-Ni-Co composite material shows improvement in shape recovery comparing to quasibinary intermetallic compounds of ZrNi-ZrCo cross-section.

Abstract: The electronic energy structure of Ti-Ni and TiNi-Cu alloys has been studied experimentally by XPS and NEXAFS and theoretically by the full-potential local-orbital minimum-basis code. The study has revealed formation in the valence band of the Ti50Ni50 alloy of a common d-band through hybridization of the d-states of Ti with those of Ni, which is localized within a narrow binding-energy interval. This brings about an intra-atomic redistribution of Ti electron density in the Ti50Ni50 alloy compared to the pure metal: decrease of the occupancy of the Ti d-shell is countered by an increase in the number of Ti p-electrons. The occupancy of the Ti d-shell in the TiNi-Cu alloys, where Ni atoms are partially substituted by Cu, and in the Ti-Ni alloys with an excess of the Ni is more than in the equiatomic Ti50Ni50 alloy. The occupancy of the Ni and Cu d-shells in the Ti-Ni and TiNi-Cu alloys is independent of the atomic composition. The Ti d-shell sensitivity to variation of the nearest atomic environment is apparently due to their relatively itinerant behavior in the studied alloys. The results obtained suggest that the increase of the stiffness of an interatomic bond in the TiNi-Cu alloys with increasing Cu content and in the Ti-Ni alloys with increasing Ni content is possibly associated with the filling of Ti bonding d-states.

Abstract: The dynamical behavior of the reverse martensitic transformation has been numerically simulated with an atomistic model and compared with experiments in Cu-Zn-Al alloys. Starting from different configurations of the martensitic variants (varying mainly their mean size), the transformation to austenite was studied as a function of the heating speed. Both, experimental and numerical results show that at low velocities there is no dependence of the transition temperatures, whereas at higher speeds they gradually increase. Simulations allow us to have an insight of the underlying processes during the transition to austenite. They also show that a heating speed independent transition can only be obtained when a microstructure of very small variants is present.

Abstract: A twin boundary model was established to describe the multi-variant interface in the martensitic materials. The modified semi-implicit Fourier-spectral method was proposed to solve the 3-D phase-field equation. Self-accommodation plays an important role in the micro-structural evolution during the loading and unloading. The external compressive stress can cause the rearrangement of martensites from three variants to one variant. After releasing the loading, another variant can nucleate and grow in one variant at the twin boundary. Cyclic stress may lead to the redistribution of martensite variants besides the rearrangement.

Abstract: A vibroisolation of a payload connected to a vibrating housing by two helical shape memory alloy (SMA) elements is considered. A microstructural theory is used for the simulation of the mechanical behavior of the SMA. The simulations have shown that the resonant frequency and the mitigation of the external vibrations depend on the shape memory alloy state. The maximum reduction of the acceleration amplitude for harmonic excitation is reached when the SMA is in the martensitic pseudoplastic state or in the two-phase state. Variation of temperature allows changing the resonance frequency and thus escaping from resonance. The acceleration of the payload at impact is the smallest when the SMA elements are in the pseudoelastic state.

Abstract: We study the effect of anisotropy and disorder on the morphology of precursor microstructures within a Ginzburg-Landau free energy framework. In addition, with increasing disorder at low anisotropy we find a crossover from a twinned state to a non-transforming frozen state. Results are compared with available experimental data.

Abstract: Shape memory alloys have become very popular over the past few decades, mainly as actuators or superelastic devices. Their complex behavior complicates the design process of these applications, and several models have been developed to assist design engineers in this endeavor. One of these models, the structure-analytical theory proposed by Likhachev, is particularly attractive because it is physically grounded and capable of dealing with tensorial stress and strain states. Unfortunately, its stress-controlled formulation has hindered its implementation in displacement-based finite element programs. This paper presents an adaptation of Likhachev’s model leading to a strain-controlled formulation based on an iterative algorithm and a proportional controller. The resulting model is implemented in ANSYS and a simple finite element analysis is carried out to illustrate its appropriate functioning.

Abstract: In the present work an investigation on CuZr based shape memory alloys was proposed. In particular, this study has been addressed the effect of the addition of Cr and Co on the martensitic transformation behaviour. The characterization was performed using DSC in terms of evolution of characteristic temperatures. The analysis of the proposed alloys was completed with the evaluation of the microhardness and the microstructure, observed by means of a scanning electron. Moreover, X-rays diffraction analysis was also carried out to check the crystal structures in the different alloys. It was shown how the addition of Co can improve thermal stability and the thermal hysteresis of the martensitic transformation by the first thermal cycles, even if the characteristic temperatures were significantly decreased.