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An experimental study on the indentation hardness of NiTi shape memory alloys (SMAs) by using a spherical indenter tip and a finite element investigation to understand the experimental results are presented in this paper. It is shown that the spherical indentation hardness of NiTi SMAs increases with the indentation depth. The finding is contrary to the recent study on the hardness of NiTi SMAs using a sharp Berkovich indenter tip, where the interfacial energy plays a dominant role at small indentation depths. Our numerical investigation indicates that the influence of the interfacial energy is not significant on the spherical indentation hardness of SMAs. Furthermore, the depth dependency of SMA hardness under a spherical indenter is explained by the elastic spherical contact theory incorporating the deformation effect of phase transformation of SMAs. Hertz theory for purely elastic contact shows that the spherical hardness increases with the square root of the indentation depth. The phase transformation beneath the spherical tip weakens the depth effect of a purely elastic spherical hardness. This study enriches our knowledge on the basic concept of hardness for SMAs under spherical indentation at micro- and nanoscales. [ABSTRACT FROM AUTHOR]

The mechanical response of spherical indentation of superelastic shape memory alloys (SMAs) was theoretically studied in this paper. Firstly, the friction effect was examined. It was found that the friction influence is negligibly small. Secondly, the influence of the elasticity of the indenter was investigated. Numerical results indicate that this influence can not be neglected as long as the indentation depth is not very small. After that, this paper focused on the effect of transformation volume contraction. Our results show that the transformation volume contraction due to forward martensitic transformation can reduce the maximum indentation force and the spherical indentation hardness. These research results enhance our understanding of the spherical indentation responses, including the hardness of the smart material SMAs. [ABSTRACT FROM AUTHOR]

Abstract: Dimensional analysis and the finite element method are applied in this paper to study spherical indentation of superelastic shape memory alloys. The scaling relationships derived from dimensional analysis bridge the indentation response and the mechanical properties of a superelastic shape memory alloy. Several key variables of a superelastic indentation curve are revealed and examined. We prove that the bifurcation force in a superelastic indentation curve only relies on the forward transformation stress and the elastic properties of the initial austenite; and the return force in a superelastic indentation curve only relies on the reverse transformation stress and the elastic properties of the initial austenite. Furthermore, the dimensionless functions to determine the bifurcation force and the return force are proved to be identical. These results not only enhance our understanding of spherical indentation of superelastic shape memory alloys, but also provide the theoretical basis for developing a practicable method to calibrate the mechanical properties of a superelastic material from the spherical indentation test. [Copyright &y& Elsevier]

Abstract: Recent experimental research indicates that superelastic shape memory alloy nickel–titanium (NiTi) is superior to stainless steel against wear and could be applied in tribological engineering. It is believed that the super wear resistance of shape memory alloys is mainly due to the recovery of the superelastic deformation. Our recent wear study indicates that wear rate is very sensitive to the maximum contact pressure. In the present investigation, which involves applying Hertz contact theory and the finite element method, the wear behaviour of shape memory alloys is examined against that of stainless steels through analyzing the maximum contact pressure and the plastic deformation. Our investigation indicates that the contribution of superelasticity to the high wear resistance of NiTi is directly linked to the low transformation stress and the large recoverable transformation strain. Furthermore, the low Young''s modulus of this alloy also plays an important role to reduce the maximum contact pressure and therefore reduce the wear rate. Additionally, the high plastic yield strength of transformed martensite NiTi enhances its wear resistance further. [Copyright &y& Elsevier]

Abstract: Using dimensional analysis and the finite element method, the spherical indentation hardness of shape memory alloys (SMAs) is investigated. The scaling relationship between the hardness and the mechanical properties of a SMA, such as the forward transformation stress, the maximum transformation strain magnitude, has been derived. Numerical results demonstrated that the hardness increases with the indentation depth but there is no three-fold relationship between the hardness and the forward transformation stress. Increasing the maximum transformation strain magnitude would reduce the hardness of the material. These research results enhance our understanding of the hardness from the spherical indentation of SMAs. [Copyright &y& Elsevier]

Stimulated by recent experimental results on superelastic NiTi shape memory alloy, a theoretical study is carried out to quantify the effect of plasticity on stress-induced martensite transformation, using a constitutive model that combines phase transformation and plasticity. A constraint equation is introduced to quantify the phenomenon of the stabilisation of plasticity on stress-induced martensite. The stabilised martensite volume fraction is determined by the equivalent plastic strain. The transformation constitutive model is adopted from a generalised plastic model with Drucker–Prager type phase transformation functions, which are pressure sensitive, while the plasticity is described by the von Mises isotropic hardening model. The martensite volume fraction is chosen as the internal variable to represent the transformation state and it is determined by the consistency transformation condition. An approach to calibrate model parameters from uniaxial tensile tests is explored, as well as the issue of elastic mismatch between austenite and martensite is discussed. Based on the proposed constitutive model, the influence of hydrostatic stress on transformation is examined. As an example of application, this new constitutive model is employed to numerically study the transformation field and the plastic deformation field near a crack tip. [Copyright &y& Elsevier]

• A thermo-mechanically coupled cyclic constitutive model is constructed for super-elastic NiTi shape memory alloys. • New transformation kinetics equations are established by introducing the residual martensite volume fraction. • The evolution equations of critical transformation temperatures are established. • The rate-dependent super-elastic degeneration is reasonably simulated. A 3D thermo-mechanically coupled constitutive model is constructed in the framework of continuum thermodynamics to describe the uniaxial and multiaxial rate-dependent cyclic super-elastic degeneration of NiTi shape memory alloys (SMAs). To capture the anisotropic martensitic transformation, a J 2 - J 3 type phase transformation surface with a correction tensor is introduced into the proposed constitutive model. As an internal variable, the residual martensitic volume fraction is introduced into the transformation kinetics equations to improve the simulation to the shape of hysteresis loop. A new evolution equation of residual martensitic volume fraction is constructed by introducing the saturated residual strain and maximum temperature rise. The accumulation of residual strain with the increasing number of cycles and loading rate can be reasonably described. Evolution equations of critical transformation temperatures during the cyclic loading are established by introducing a maximum temperature rise in current loading history, which reflects the evolutions of critical transformation stresses and the increasing transformation hardening modulus. Comparison of simulated and experimental results shows that the proposed model can reasonably describe the uniaxial and multiaxial rate-dependent cyclic stress-strain curves and temperature oscillations of NiTi SMAs. [ABSTRACT FROM AUTHOR]

Based on the strain-controlled cyclic tension-unloading tests at various strain rates (3.3 × 10 −4 –3.3 × 10 −2 /s), the effect of strain rate on the uniaxial cyclic deformation of a super-elastic NiTi shape memory alloy (SMA) was investigated. It is concluded that apparent degeneration of super-elasticity occurs during the cyclic loading, i.e., the residual strain and transformation hardening increase, but the start stress of forward transformation and the maximum responding stress decrease with the increasing number of cycles and strain rate, and finally reach to their saturated states after certain cycles. Although the dissipation energy per cycle decreases progressively during the cyclic loading and is saturated after certain cycles, it does not change monotonically with the increasing strain rate. Moreover, the temperature oscillation is observed due to the internal heat production from the inelastic dissipation and transformation latent heat, and the extent of temperature variation increases monotonically with the increasing strain rate, which is the physical nature of rate-dependent cyclic deformation of the NiTi SMA. [ABSTRACT FROM AUTHOR]

Abstract: Nanofretting refers to cyclic movements of contact interfaces with the relative displacement amplitude at the nanometer scale, where the contact area and normal load are usually much smaller than those in fretting. Nanofretting widely exists in microelctromechanical systems (MEMS) and may become a key tribological concern besides microwear and adhesion. With a triboindenter, the nanofretting behaviors of a nickel titanium (NiTi) shape memory alloy are studied under various normal loads (1–10mN) and tangential displacement amplitudes (2–500nm) by using a spherical diamond tip. Similar to fretting, the nanofretting of NiTi/diamond pair can also be divided into different regimes upon various shapes of tangential force–displacement curves. The dependence of nanofretting regime on the normal load and the displacement amplitude can be summarized in a running condition nanofretting map. However, due to the surface and size effects, nanofretting operates at some different conditions, such as improved mechanical properties of materials at the nanometer scale, small apparent contact area and single-asperity contact behavior. Consequently, different from fretting, nanofretting was found to exhibit several unique behaviors: (i) the maximum tangential force in one cycle is almost unchanged during a nanofretting test, which is different from a fretting test where the maximum tangential force increases rapidly in the first dozens of cycles; (ii) the tangential stiffness in nanofretting is three orders magnitude smaller than that in fretting; (iii) the friction coefficient in nanofretting is much lower than that in fretting in slip regime; (iv) no obvious damage was observed after 50 cycles of nanofretting under a normal load of 10mN. [Copyright &y& Elsevier]