Physical metallurgy of Ti–Ni-based shape memory alloys

Recent developments in the research of shape memory alloys

https://repository.library.northeastern.edu/files/neu:331186/fulltext.pdf

A survey of the Van Hove scenario for high-tc superconductivity with special emphasis on pseudogaps and striped phases

Single crystal elastic constants of Ti–Ni alloys without (quenched) and with (aged) Ti 3 Ni 4 precipitates were measured systematically by rectangular parallelepiped resonance method as a function of composition and temperature, and compared with Ti–Ni–Cu and Ti–Ni–Fe alloys, in an attempt to answer some long-standing questions as to the origin of the unique monoclinic B19′ martensite, and why composition and thermomechanical treatment greatly changes the path of martensitic transformation. The results showed that softening in c 44 , in additional to c ′, is a common feature for all Ti–Ni binary (both quenched or aged) and ternary alloys. This general feature just corresponds to the fact that all these alloys ultimately transform into B19′, suggesting that softening in c 44 is responsible for the unique B19′ martensite, which found no analogy in other β phase alloys. We also found an interesting correspondence between the temperature dependence of anisotropy factor and transformation path. Prior to B2–B19′ transformation anisotropy shows a decrease with lowering temperature; prior to B2–B19 an anisotropy increase, while prior to B2–R transformation a constant anisotropy. We further showed that three possible martensite candidates (R, B19, B19′) are rooted in anomalies in specific phonon modes and elastic softening. We showed that the multi-stage transformations are restricted by a general rule: multi-stage transformation occurs in the sequence of increasing transformation strain. With this rule we can explain all known transformation paths by considering the effect of alloying addition and fine precipitates/dislocation network on relative stability of different martensites. We further predict that there may exist a new transformation path in Ti–Ni-based alloys: B2–R–B19–B19′.

A comparative study of elastic constants of Ti-Ni-based alloys prior to martensitic transformation

A crystal-mechanics-based constitutive model for polycrystalline shape-memory materials has been developed. The model has been implemented in a finite-element program. In our finite-element model of a polycrystal, each element represents one crystal, and a set of crystal orientations which approximate the initial crystallographic texture of the shape-memory alloy are assigned to the elements. The macroscopic stress–strain responses are calculated as volume averages over the entire aggregate. Pseudoelasticity experiments in tension, compression, and shear have been performed on an initially textured polycrystalline Ti–Ni alloy. In order to determine the material parameters for Ti–Ni, the stress–strain results from a finite-element calculation of a polycrystalline aggregate subjected to simple tension have been fit to corresponding results obtained from the physical experiment. Using the material parameters so determined, the predicted pseudoelastic stress–strain curves for simple compression and thin-walled tubular torsion of the initially textured Ti–Ni are shown to be in good accord with the corresponding experiments. Our calculations also show that the crystallographic texture is the main cause for the observed tension–compression asymmetry in the pseudoelastic response of Ti–Ni. The predictive capability of the model for the variation of the pseudoelastic behavior with temperature is shown by comparing the calculated stress–strain response from the model against results from experiments of Shaw and Kyriakides (J. Mech. Phys. Solids 43 (1995) 1243) on Ti–Ni wires at a few different temperatures. By performing numerical experiments, we show that our model is able to qualitatively capture the shape-memory effect by transformation. We have also evaluated the applicability of a simple Taylor-type model for shape-memory materials. Our calculations show that the Taylor model predicts the macroscopic pseudoelastic stress–strain curves in simple tension, simple compression and tubular torsion fairly well. Therefore, it may be used as a relatively inexpensive computational tool for the design of components made from shape-memory materials.

Polycrystalline shape-memory materials: effect of crystallographic texture

The authors present measurements of the elastic constants on a single crystal of the shape-memory alloy NiTi in a wide temperature range. Step-like anomalies and hysteresis are detected at the austenitic-martensitic phase transition which provide evidence for the strain-order parameter coupling in the pre-martensitic phases. The results are discussed in the framework of existing Landau theory models.

https://iopscience.iop.org/article/10.1088/0953-8984/3/48/004/pdf

Elastic properties of NiTi

Magnetic resonances in CsCuCl3

We present experimental data on elastic constants, magnetic susceptibility, and magnetization for single-crystal ${\mathrm{La}}_{2}$${\mathrm{NiO}}_{4}$. Strong acoustic anomalies especially for the ${\mathit{c}}_{66}$ mode are observed for the structural tetragonal-orthorhombic transition at 360 K, whereas for the lower structural transition at 80 K the acoustic effects are rather smeared out. With the help of our magnetic measurements, we can construct a spin model describing the essential features of the magnetic structure in ${\mathrm{La}}_{2}$${\mathrm{NiO}}_{4}$.

Magnetic and elastic properties of La2NiO4+x.

Magnetic resonance experiments in the frequency range 55–404 GHz in magnetic fields up to 14 T are presented. Comparison with an existing field theoretical model leads to gap parameters and critical fields that are only slightly different from those obtained by neutron scattering experiments at q=π. Temperature dependent resonance signal intensities of various branches are compared to calculated intensities in the framework of Boltzmann statistics for temperatures between 1.5 K and 18 K. In this way resonance branches are assigned to transitions within a scheme of energy levels.

Magnetic resonances in the one-dimensional s=1 spin system nenp

We present magentic resonance and magnetization measurements in the low dimensional spin system CsNiCl3 for temperatures down to 1.5 K and in magnetic fields up to 14 T. The frequency range from 35 GHz to 380 GHz is much larger than for previous experiments. The outstanding features of these experiments are a spreading of the magnetic resonance branches in high fields belowT

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 and an upturn of the magnetization at higher fields and for temperatures below 20 K. Wheras we can interpret our resonance data with a six sublattice model invoking a large interchain exchange, the magnetization points to typical one dimensional behaviour in higher fields. A theory for a consistent description of resonance data, spinwave spectra and magnetization is required.

B. Lüthi

Papers

Elastic properties of NiTi

Magnetic resonances in CsCuCl3

Magnetic and elastic properties of La2NiO4+x.

Magnetic resonances in the one-dimensional s=1 spin system nenp

Magnetic resonances and magnetization in the low-dimensional spin system CsNiCl3