Stress-induced martensitic transformations in NiTi and NiTi–TiC composites investigated by neutron diffraction

Abstract

Superelastic NiTi (51.0 at.% Ni) specimens reinforced with 0, 10 and 20 vol.% TiC particles were deformed under uniaxial compression while neutron diffraction spectra were collected. The experiments yielded in-situ measurements of the thermoelastic stress-induced transformation. The evolution of austenite/martensite phase fractions and of elastic strains in the reinforcing TiC particles and the austenite matrix were obtained by Rietveld refinement [1] during the loading cycle as the austenite transforms to martensite (and its subsequent back transformation during unloading). Phase fractions and strains are discussed in terms of load transfer in composites where the matrix undergoes a stress-induced phase transformation.

Development of crystallographic texture during high rate deformation of rolled and hot-pressed beryllium

“…This late hardening behavior is also found in Zr and Mg [3,6] and is frequently associated with deformation twinning or a stress-induced phase transformation as in superelastic nickel-titanium alloys. [26] Again, the calculation satisfactorily reproduces the observed mechanical response, including the increased hardening above 15 pct deformation. Reduced or elevated temperature testing (75 or 673 K) does not significantly affect the yield point of the beryllium in any of the texture-load combinations; however, the subsequent hardening behavior is a strong function of the temperature.…”

Fatigue crack-growth in shape-memory NiTi and NiTi–TiC composites

“…The lack of significant fatigue differences is surprising since the uniaxial tensile stress-strain response of NiTi and NiTi-TiC are significantly different, with the composite showing lower ductility and higher twinning stresses. This could be due to a combination of factors such as microcracking, crack-deflection and the accommodation observed previously in shape-memory and superelastic composites [8,12,25]. Thus, the reinforcement concept can be used, for dilute concentrations at least, to tailor properties such as elastic modulus and transformation temperatures in NiTi without altering their fatigue crack growth characteristics.…”

“…Eshelby's inclusion theory was used to determine the residual stress due to the thermal expansion mismatch between the matrix and TiC [25,26]. The matrix has a mean tensile stress of 26 and 51 MPa (for NiTi -10TiC and NiTi-20TiC, respectively) due to this mismatch.…”

“…Since NiTi and NiTi-10TiC both were tested below M s , it is likely that the matrix is dominating and the particles do not significantly influence the crack growth behavior in the composites. It has been established that in the case of shape-memory [8] and superelastic composites [12,25] the mismatch between reinforcement and matrix (thermal, elastic and allotropic) is largely accommodated. Hence given the low volume fraction of TiC and the accommodation of the mismatch it is understandable that both NiTi and NiTi-10TiC exhibit similar crack-growth behavior.…”

Phase-stress partition and stress-induced martensitic transformation in NbTi/NiTi nanocomposite

“…However, little is known about the phase-stress partition in the MMCs having the matrix exhibiting alternative deformation mechanisms (e.g., stress-induced martensitic transformation (SIMT) or mechanical twinning). [8][9][10] Here, the phase-stress partition in a NbTi/NiTi nanocomposite where the matrix is capable of deforming by SIMT was studied by using in situ synchrotron x-ray diffraction during tensile loading/unloading. It was found that, after the initial elastic deformation, the stress carried by the NbTi nanowires increased significantly while the stress taken by the NiTi matrix decreased with increasing macroscopic strain, which is different from that previously reported in the MMCs.…”