NiTi Strips Research Articles

Rate-dependent domain spacing in a stretched NiTi strip

Superelastic fine-grained Nickel–Titanium (NiTi) polycrystalline shape memory alloys under tensile loading deform collectively via the nucleation and growth of macroscopic martensite domains. Recent experiments on a stretched NiTi strip showed that the number of nucleated domains (or the domain spacing) increased (decreased) with increasing applied stretching rate. It is also shown that the rate dependence of the domain formation is due to the coupling between the transfer of the locally released heat and the temperature dependence of the transformation stress. In this paper, a simple one-dimensional model is developed to quantify this effect of thermo-mechanical coupling on the observed domain spacing. Analytical relationship between the domain number, thermo-mechanical properties of the material, heat transfer boundary conditions and the externally applied strain rate is established. It is found that for the case of strong heat convection the domain spacing is inversely proportional to the applied stretching rate, while for the case of weak convection, the domain spacing is dictated by a power-law scaling with exponent −0.5. The latter theoretical prediction agrees well quantitatively with the experimental data in stagnant air.

Role of time scale in the deformation patterns and hysteresis of NiTi SMA

In this paper we present our recent modelling and experiment on the roles of loading time scale and thermo-mechanical two-field coupling in the deformation domain patterns and stress hysteresis in NiTi SMA. We first report the effects of loading time or stretching rate on the domain patterns and stress hysteresis observed in NiTi strips in the strain rate range of 10-5/s ~ 10-1/s. It was found that the nominal stress-strain curve of the specimen changed from near-isothermal plateau-type with distinct stress-drops in the low strain rate region to the near-adiabatic smooth hardening-type in the high strain rate region. The pseudoelastic hysteresis varies non-monotonically with the external loading rate and the maximum hysteresis occurs at certain intermediate loading rate. The corresponding deformation mode changed from the localized-propagation mode with a few parallelepiped martensite domain patterns to the near-homogeneous multiple nucleation mode with fine alternating austenite-martensite stripes. The analysis shows that the number of the macroscopic domains (domain spacing) of the specimen increased (decreased) with the applied stretching rate in a power-law form, i.e., the higher the loading rate, the finer the deformation pattern. Such observed power-law dependency is originated from the coupling between the material’s nonlinear property (formation and growth of domains) and the transfer of heat in the phase transition process. The important roles of heat transfer dynamics and loading time in imposing the emerging length scale in the deformation pattern of the material is demonstrated. For the hysteresis, a simple two-scale thermo-mechanical coupling model is developed to explain the observed rate-dependent hysteresis phenomenon. We incorporate the effects of the released/absorbed heat of the phase transition and the heat transfer on the pseudoelastic hysteresis. An analytical expression for the rate-dependent hysteresis is derived and the theoretical predictions agree quantitatively well with the experiments.

A finite element study on localized deformation and functional fatigue in pseudoelastic NiTi strips

Thin tensile specimens of pseudoelastic NiTi shape memory alloys deform by the formation and propagation of distinct martensitic bands. During cyclic loading, functional fatigue (i.e., degradation of functional properties due to the repeated martensitic transformation) is limited to the regions that participate in the transformation. In this study, we present finite element calculations based on a simple elasto-plastic approach that utilizes strain softening to simulate localized transformation/deformation. We investigate the effect of typical experimental factors (clamping conditions and small superimposed bending moments during tensile testing) on the macroscopic mechanical behavior and on the propagation mode of the macroscopic phase interfaces. Moreover, we study how localized functional fatigue influences the macroscopic stress–strain data. The simulations demonstrate that a detailed understanding of experimental conditions and of cyclic experiments can be obtained by finite element modeling of localized deformation.

Texture and structure of grain boundary in Ni-Ti strip produced by twin roll casting technique

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Multi-dimensional constitutive modeling of SMA during unstable pseudoelastic behavior

A multi-dimensional continuum-level constitutive model of shape memory alloys exhibiting pseudoelastic behavior has been developed in this paper. The current model is an extension of the one-dimensional model previously developed by the authors, which consists of a constitutive relation and a transformation evolution rule (kinetic relation). The constitutive relation is constructed based on the gradient of a transformation potential function (effective stress), and the kinetics of transformation are expressed based on a set of transformation surfaces in stress-temperature space. The constitutive relation allows for the reorientation of the transformation strain tensor according to the current state of stress. The model is used to simulate the localized deformation and transformation front propagation in NiTi strip under quasi-static extension. Special attention has been paid to the multi-axiality of the stress state at the transformation front during the forward and reverse transformations. It is shown that the ability of the model to consider the reorientation of the transformation strain tensor results in full recovery of transformation strain upon unloading, and a uniform distribution of stress in the transformed areas in spite of the localization of transformation. It also produces a stress gradient that dictates the direction of the transformation front propagation during both the forward and reverse transformations.

Effect of Cyclic Loading on Apparent Young&amp;rsquo;s Modulus and Critical Stress in Nano-Subgrained Superelastic NiTi Shape Memory Alloys

A series of uni-axial tensile cycling tests were conducted at room temperature in superelastic NiTi strip specimens with nano-grain size. The NiTi superelastic strip specimen’s Apparent Young’s Modulus (AYM) and the critical stress decrease when the specimen is subjected to an external uni-axial stress and the strain being higher than 1.5%. Both of the AYM and the critical stress become steady after 10-time cycling. The number of the (111)½1�� oriented grains increases with extending the strain value. The sub-grain size grows with increasing mechanical cycling number due to the annihilation of the small angle boundaries. The AYM-softening is related to the grain re-orientation (texture evolution) and the formation of irreversible-stabilized B19 0 martensitic variants. The softness of the critical stress is principally attributed to the aspect that the grains re-orient to align along the two textural components (111)½1� 1� and (111)½1�� when the external stress being applied. The rotation of grains towards the observed orientation gives higher Schmid factor for the transformation and is one of the reasons for the decrease in AYM and critical stress. The orientation relationships between B2 parent phase and the strain-induced B190 martensite are observed to be: ½111� B2 == ½10� 1� M, ð1� 1ÞB2 == ð010ÞM and ½111� B2 == ½� 1� M and ð1� 1ÞB2 == ð001ÞM.

Corrosion behavior of AISI 316L stainless steel surface-modified with NiTi

AISI 316L stainless steel was surface-modified with NiTi using three different methods: (1) laser surface alloying using NiTi powder (LSA-NiTi-powder), (2) laser cladding using NiTi strips (LC-NiTi-strip), and (3) microwave-assisted brazing using NiTi plates (MB-NiTi-plate). These methods are capable of bringing significant improvement in the cavitation erosion resistance as reported elsewhere. The present work aims at studying the corrosion behaviors of these surface-modified stainless steel samples in 3.5% NaCl solution. Cyclic polarization tests reveal that the pitting potentials of the laser-treated samples are comparable to that of the AISI 316L substrate, but the protection potentials and the corrosion potentials are somewhat lower. On the other hand, the corrosion behavior of the brazed sample is similar to that of as-received NiTi, which does not show pitting up to potentials exceeding 1500 mV. Galvanic tests of couples with AISI 316L as one member and the surface-modified sample as the other member show only a small galvanic current density in all the three cases. The ratio of the average galvanic current density to the corrosion current density of the uncoupled anodic member is small (not exceeding 4 in all cases), indicating good compatibility when these techniques are used in local surface modification of AISI 316L substrate.

Laser cladding of austenitic stainless steel using NiTi strips for resisting cavitation erosion

Being part of a larger project on using different forms of nickel titanium (NiTi) in the surface modification of stainless steel for enhancing cavitation erosion resistance, the present study employs NiTi strips as the cladding material. Our previous study shows that laser surfacing using NiTi powder can significantly increase the cavitation erosion resistance of AISI 316 L stainless steel [K.Y. Chiu, F.T. Cheng, H.C. Man, Mater. Sci. Eng. A 392 (2005) 348–358]. However, from an engineering point of view, NiTi strips are more attractive than powder because NiTi powder is very expensive due to high production cost. In the present study, NiTi strips were preplaced on AISI 316 L samples and remelted using a high-power CW Nd:YAG laser to form a clad layer. To lower the dilution due to the substrate material, samples doubly clad with NiTi were prepared. The volume dilution ratio in the singly clad sample was high, being in the range of 13–30% depending on the processing parameters, while that of the doubly clad sample was reduced to below 10%. Analysis by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS) and X-ray diffractometry (XRD) reveals that the clad layer is composed of a NiTi B2 based matrix together with fine precipitates of a tetragonal structure. Vickers indentation shows a tough cladding/substrate interface. The microhardness of the clad layer is increased from 200 HV of the substrate to about 750 HV due to the dissolution of elements like Fe, Cr and N in the matrix. Nanoindentation tests record a recovery ratio near to that of bulk NiTi, a result attributable to a relatively low dilution. The cavitation erosion resistance of the doubly clad samples is higher than that of 316-NiTi-powder (samples laser-surfaced with NiTi powder) and approaches that of NiTi plate. The high erosion resistance is attributed to a high hardness, high indentation recovery ratio and the absence of cracks or pores.

Simulations of localized thermo-mechanical behavior in a NiTi shape memory alloy

Previous experiments have shown that stress-induced martensitic transformation in certain polycrystalline NiTi shape memory alloys can lead to strain localization and propagation phenomena when loaded in uniaxial tension. The number of nucleation events and kinetics of transformation fronts were found to be sensitive to the nature of the ambient media and imposed loading rate due to the release/absorption of latent heat and the material's inherent temperature sensitivity of the transformation stress. A special plasticity-based constitutive model used within a 3-D finite element framework has previously been shown to capture the isothermal, purely mechanical front features seen in experiments of thin uniaxial NiTi strips. This paper extends the approach to include the thermo-mechanical coupling of the material with its environment. The simulations successfully capture the nucleation and evolution of fronts and the corresponding temperature fields seen during the experiments.

Initiation and propagation of localized deformation in elasto-plastic strips under uniaxial tension

This paper deals with the evolution of inhomogeneous deformation in shape memory alloy strips and mild steel strips under uniaxial tension. New experiments on NiTi strips, which initially are in an austenitic phase, show that at a critical stress level martensite nucleates in sharp bands inclined at 55° to the axis of loading. Under prescribed end displacement martensite subsequently spreads either by steady-state propagation of inclined transition fronts or via a criss-cross pattern of finger-like features. Similar events have been reported in the literature regarding the evolution of Luders bands in fine grained steel strips and wires. The similarity of macroscopic events, despite the different mechanisms of instability at the micro-level, prompted us to approximate the material behavior as a finitely deforming elasto-plastic solid with a trilinear up-down-up nominal stress-strain response. Two such stress-strain responses were used in finite element simulations of strip tension tests. In the first the true stress-strain response maintains its stability and in the second the intermediate branch has a negative slope. While both material models produced inhomogeneous deformations with features similar to those of the experiments, the larger initiation peak associated with the second gave results which closely resembled specific experiments. The numerical simulations confirmed that the evolution of events seen in experiments on SMAs and mild steels is strongly influenced by overall geometric (structural) effects. Furthermore, the success of this simple continuum constitutive model strongly suggests that continuum level events remain dominant players in such fine grained materials.