Al Ni Shape Memory Alloys Research Articles

Influence of Interfacial Stress on Microstructural Evolution in NiAl Alloys

A phase-field model for the phase transition between austenite and martensite and twinning between two martensitic variants is presented from our previous theory [1] with the main focus on the influence of interfacial stress that is consistent with the sharp interface limit. Each variant-variant transformation can be represented by only one order parameter. Thus, it allows us to get the analytical solution of interface energy and width. Coupled phase-field and elasticity equations are solved for cubic-to-tetragonal phase transformation in NiAl shape memory alloy. The effects of interfacial stress are studied for martensite-martensite interfaces in detail, which was absent in [1]. Additionally, stress and temperature-induced growth of the martensitic phase inside austenite and twining are simulated. Some of the nontrivial experimentally observed microstructures reproduced in the simulations [1] are analyzed in detail. It includes tip splitting and bending, and twins crossing. This theory can be extended for electric, reconstructive, and magnetic phase transformations.

Effect of Co additions on the damping properties of Cu–Al–Ni shape memory alloys

This study investigates the effect of Co additions on the damping properties of Cu-13.5Al–4Ni-xCo and Cu-14.0Al–4Ni-xCo (x = 0–2 wt%) shape memory alloys (SMAs). The Cu-14.0Al–4Ni SMA exhibits a higher inherent and intrinsic internal friction (IFPT + IFI) peak than the Cu-13.5Al–4Ni SMA, but its (IFPT + IFI) peak temperature is below 0 °C. Adding Co into the Cu-14.0Al–4Ni SMA can effectively increase the (IFPT + IFI) peak temperature and reduce the grain size of the alloys. The grain size of the Cu-14.0Al–4Ni-xCo SMAs decreases from approximately 300 μm to below 50 μm when the Co content increases from 0 to 2 wt%. The (IFPT + IFI) peak temperature for the Cu-14.0Al–4Ni-xCo SMAs increases from −0.1 °C to 77.7 °C when the Co content increases from 0 to 1 wt%, but their tan δ values decrease from 0.0402 to 0.0090 simultaneously. The Cu-14.0Al–4Ni–2Co SMA does not exhibit an observable (IFPT + IFI) peak. The tan δ values of the (IFPT + IFI) peaks decrease with increasing Co content because the movements of the parent/martensite phase interfaces and twin boundaries are impeded by the increased amounts of grain boundaries and γ2 phase precipitates. Among these Cu–Al–Ni–Co SMAs, the Cu-14.0Al–4Ni-0.5Co SMA is more suitable for high-damping applications as it possesses an (IFPT + IFI) peak with tan δ close to 0.02 at approximately 50 °C.

Effects of interfacial stress in phase field approach for martensitic phase transformation in NiAl shape memory alloys

A phase field approach for phase transition between austenite to martensitic variants and twinning between martensitic variants is presented with the main focus on the effects of interfacial stress on phase transformations. In this theory, each variant-variant phase transformations and twinnings within each martensitic variant can be represented by only one-order parameter. Thus, it allows us to get the analytical solution of the martensite-martensite interface profile, energy, and width. Moreover, this model allows us to include interface stress which is consistent with the sharp interface limit. The finite element method is utilized to solve the coupled phase field and elasticity equations for a cubic to tetragonal phase transformation in NiAl shape memory alloy. The stress fields are obtained and the effects of interfacial stress on the stress field at the interfaces are studied for both austenite–martensite and martensite–martensite interfaces in detail. Additionally, the temperature-induced growth of the martensitic phase inside austenite, martensitic phase transformation, and twining are solved. The evolution of the microstructure and stress fields are obtained and the effects of interfacial stress on the morphological evolution of martensitic nanostructures are examined. It is found that the interfacial stress is an important factor, influencing the stress distribution at interfaces and the phase field solution significantly. This theory can be extended for electric, reconstructive, and magnetic phase transformations.

Induction Butt Welding Followed by Abnormal Grain Growth: A Promising Route for Joining of Fe–Mn–Al–Ni Tubes

The present study focuses on induction butt welding of Fe–Mn–Al–Ni tubes. By comparing different processing routes, characterized by different temperatures and forces during welding, it was possible to find adequate process parameters for realization of defect-free joints. Moreover, it was feasible to fully reset the microstructure prevailing in the heat-affected zone by a subsequent cyclic heat treatment promoting abnormal grain growth. Tensile testing up to a maximum strain of 6% revealed excellent pseudoelastic properties of the final microstructural condition. The present study shows for the first time that welding with superimposed pressure is well suited for joining of Fe–Mn–Al–Ni shape memory alloys. Furthermore, it is revealed that abnormal grain growth induced by a cyclic heat treatment can be applied independently of the geometry of the component.

Suppression of twinning mechanism on nanoscale: size effect in Cu–Ni–Al shape memory alloy

In Cu–Ni–Al shape memory alloy, we observed a significant size effect on the twinning stress, i.e. the dependency of compression stress needed for twin-variant reorientation on sample size using in situ loading of micro- and nanoscale pillars in scanning and transmission electron microscopes. With decreasing dimensions of pillars, the twinning stress sharply increases following scaling power law with an exponent approximately n = − 2/3. For very small nanopillars, the projected twinning stress is so high that the nanopillars are deformed by plastic deformation instead of twinning. Our results shed light on some of the fundamental aspects of nanoscale behaviour of shape memory alloys which is important for applications in microelectromechanical systems.

Designed polycrystalline ultra-high ductile boron doped Cu–Al–Ni based shape memory alloy

Cu–Al–Ni shape memory alloys have good thermal stability and shape memory properties, but mechanical working of these alloys is very difficult due to inter-granular fracture and brittleness. This paper deals with improvement of shape memory properties and ductility of these alloys with the addition of boron. It was observed that with addition of boron in a designed experiment, ductility improved by more than four times than that of the base alloy. In addition, shape memory properties also improved significantly. These results obtained are discussed in this paper systematically.

Influence of Thermomechanical Treatment on Structural-Phase Transformations and Mechanical Properties of the Cu–Al–Ni Shape-Memory Alloys

Using the methods of optical and electron microscopy and electron and X-ray diffraction analyses, the influence of thermomechanical treatment on the mechanical properties, average grain size, and structural and phase transformations is investigated in the Cu–Al–Ni triple alloys exhibiting a shape memory effect. In the alloys under study with a fixed content of 3wt% Ni the concentration of aluminum was varied from 9 to 14 wt%. It is shown that in the alloys subjected to thermal treatment, including forging and homogenizing annealing using controlled recrystallization in the austenitic state followed by quenching, the grain-boundary disintegration and segregation disappear. It is found out that the microstructure of the alloys in the hot-forged and hardened states with the content of aluminum 9–10 wt%. consists of the grains of the average dimensions within 60–80 μm, with the content of aluminum 10–12 wt% – 100–350 μm, while in the alloys with the content of aluminum up to 14 12 wt% the average grain size reaches 1 mm. According to the data of mechanical testing at room temperature, with a decrease in the content of aluminum the ultimate tensile strength (σUTS), the yield strength (σМ) and the relative elongation (δ) increase. An improvement of the mechanical properties of the alloys is attributed to the grain structure refinement of the β2-austenite and package substructure of the β'1-and γ'1-martensites as the content of aluminum in the alloys decreases. For instance, in the fine-grained alloys containing 9.2 and 9.5 wt% Al the value of relative elongation remains at a high level (>10%), while for the other alloys with 10–14 wt% Al it does not exceed 5%. As the content of aluminum in the alloys decreases, the character of the specimen fracture under uniaxial tension changes (from brittle to ductile).

Investigation of Fe content in Cu–Al–Ni Shape Memory Alloys

Polycrystalline Cu–Al–Ni–Fe-based shape memory alloys with different chemical composition were produced in an arc-melting furnace under an argon atmosphere. Homogenized and aged specimens were prepared for multiple analyses. The temperatures of reversible martensitic transformations, namely As, Af, Ms, Mf, Amax and ΔH enthalpy values were determined by a DSC device. The phase transition analysis from the room temperature to 850°C was undertaken by DTA. To characterize the lattice structure, an XRD analysis was conducted, the results of which were confirmed by microstructure images obtained from optical microscope observations.

Selective Leaching and Surface Properties of Cu–Al–Ni Shape Memory Alloys

Selective Leaching and Surface Properties of Cu–Al–Ni Shape Memory Alloys

Stress- and Magnetic Field-Induced Martensitic Transformation at Cryogenic Temperatures in Fe–Mn–Al–Ni Shape Memory Alloys

Stress-induced and magnetic-field-induced martensitic transformation behaviors at low temperatures were investigated for Fe–Mn–Al–Ni alloys. The magnetic-field-induced reverse martensitic transformation was directly observed by in situ optical microscopy. Magnetization measurements under pulsed magnetic fields up to 50 T were carried out at temperatures between 4.2 and 125 K on a single-crystal sample; full magnetic-field-induced reverse martensitic transformation was confirmed at all tested temperatures. Compression tests from 10 to 100 K were conducted on a single-crystal sample; full shape recovery was obtained at all tested temperatures. It was found that the temperature dependence of both the critical stress and critical magnetic field is small and that the transformation hysteresis is less sensitive to temperature even at cryogenic temperatures. The temperature dependence of entropy change during martensitic transformation up to 100 K was then derived using the Clausius–Clapeyron relation with critical stresses and magnetic fields.

High-pressure torsion driven phase transformations in Cu–Al–Ni shape memory alloys

Severe plastic deformation (SPD) frequently induces phase transformations like decomposition of supersaturated solid solution, dissolution of precipitates, amorphization, nanocrystallization etc. Such diffusive phase transitions are combined with SPD-driven accelerated mass transfer. Displacive (or martensitic) phase transitions can also take place and in combination with diffusive ones have not been investigated in depth in severely deformed materials. The goal of this work is to investigate the combination of displacive (austenite↔martensite) and diffusive (decomposition of supersaturated solid solution) phase transitions in two different Cu–Al–Ni shape memory alloys under the influence of high-pressure torsion (HPT). After homogenization in the one-phase (austenitic) β-area of Cu–Al–Ni phase diagram and quenching, the first alloy was in martensitic state (mainly β′3 martensite with a small amount of γ′3 martensite), and the second one remained austenitic (β3 phase). The HPT of these alloys led to the precipitation of α1-phase in the first case and γ1-phase in the second one (as if they were annealed at an effective temperature Teff = 620 ± 20 °C). As a result of precipitation, the matrix in the first alloy was enriched and in the second one depleted in Al. After HPT, both alloys contained mainly β′3 martensite with a certain amount of γ′3 martensite. Thus, the HPT-driven diffusive transformations (precipitation of α1-and γ1-phase) influence the followed displacive (martensitic) transformation. Simultaneously, a dramatic grain refinement is obtained and the reported results open new possibilities to investigate the superelastic and shape memory effects in nanostructured Cu–Al–Ni alloys.

Effect of chromium element on transformation, mechanical and corrosion behavior of thermomechanically induced Cu–Al–Ni shape-memory alloys

Cu–Al–Ni shape-memory alloys are considered as high potential materials for high-temperature applications. The aim of this research was to evaluate the increasing strain value and Cr addition on martensite morphology, transformation temperatures, mechanical, and corrosion properties of Cu–Al–Ni alloy. To this purpose, thermomechanical treatment which includes successive hot rolling, annealing, and hydraulic pressing passes was applied. In addition, tensile test, differential scanning calorimetry, and potentiodynamic polarization were carried out to compare the properties of prepared samples. The results showed that by increasing the applied strain, morphological transition from wide laths to acicular martensite with monoclinic structure was occurred. The chromium element acts as a grain refiner in this alloy by restricting the grain growth. This element leads to microstructural embrittlement, diminishing the mechanical properties. Besides, the influence of applied strain and Cr content on corrosion resistance of Cu–Al–Ni alloy was reciprocal. Despite suitable effect of Cr on corrosion behavior, increasing the applied strain facilitated the corrosion rate. Another subtle point is that both Cr addition and higher strain value reduce austenite to martensite transformation temperatures and hysteresis temperature interval.

Martensitic phase transformations in shape memory alloy: phase field modeling with surface tension effect

The coupled phase-field and elasticity equations are presented for multivariant martensitic phase transformations (PTs) to investigate the effect of surface tension on PTs. The finite element method is utilized to solve the coupled phase-field and elasticity equations for two dimensional problems. Several numerical examples for cubic-to-tetragonal PTs in NiAl shape memory alloy are studied including surface-induced pretransformation and transformation, the stress- and temperature-induced growth of a martensitic nucleus inside austenite, and temperature- and surface-induced martensitic PTs for single and two martensitic variants. The phase and stress fields are obtained and the crucial effect of surface tension on stress and nucleation and propagation of martensitic nanostructures is revealed and discussed. It is found that the surface tension can suppress nucleation so that a larger driving force is needed for PTs to happen. Also, it can significantly change the stress and phase field solution. Moreover, it can suppress the propagation of transformed region.

Grain boundary engineering of Co–Ni–Al, Cu–Zn–Al, and Cu–Al–Ni shape memory alloys by intergranular precipitation of a ductile solid solution phase

Many polycrystalline shape memory alloys, e.g., Co–Ni–Al, Cu–Zn–Al, and Cu–Al–Ni, undergo intergranular fracture. To improve their transformation ductility, we perform Grain Boundary Engineering and stimulate the precipitation of a ductile second phase, which is a face-centered-cubic solid solution, along grain boundaries, by tailoring composition and thermal processing. Orientation imaging confirms precipitation along grain boundaries and unimpeded martensite growth toward grain boundary precipitates. Differential Scanning Calorimetry confirms reversible martensitic transformations in these dual-phase samples. These precipitates can accommodate transformation strain, relieve constraint in adjacent austenite grains, and arrest cracks by extensive plastic deformation, thereby improving transformation ductility and shape memory effects.

Cyclic degradation in bamboo-like Fe–Mn–Al–Ni shape memory alloys — The role of grain orientation

In the present study the cyclic deformation behavior within differently oriented grains in Fe–34.8Mn–13.5Al–7.4Ni (at.%) shape memory polycrystals featuring a bamboo-like structure was investigated. In cyclic tensile tests up to 50cycles, the degree of degradation in pseudoelasticity was evaluated and contributing elementary mechanisms are discussed. The results reveal rapid cyclic degradation in the bamboo-like samples. The unexpected stabilization of parent phase in reverse transformed areas and the proceeding activation of new martensite variants in subsequent cycles were found to be the prevailing degradation mechanisms. Dislocation activity is found to be the most detrimental factor.

Effect of a fourth alloying element on the microstructure and mechanical properties of Cu–Al–Ni shape memory alloys

Abstract

On the effect of gamma phase formation on the pseudoelastic performance of polycrystalline Fe–Mn–Al–Ni shape memory alloys

In recent years, iron-based shape-memory-alloys such as Fe–Mn–Al–Ni came into focus, due to superior pseudoelastic performance and concomitantly low costs of the alloying elements. The formation of the ductile γ-phase in polycrystalline Fe–Mn–Al–Ni and its impact on the pseudoelastic performance have not been addressed, yet. Based on modification of quenching conditions this study shows that controlled γ-phase precipitation at grain boundaries can suppress intergranular cracking without affecting pseudoelasticity. In situ incremental strain tests revealed good recoverability up to about 8% strain.

Effect of directional solidification and porosity upon the superelasticity of Cu–Al–Ni shape-memory alloys

Abstract Strain incompatibilities between grains in polycrystalline Cu–Al–Ni shape-memory alloys undergoing stress-induced reversible transformation reduce their ductility and their recoverable superelastic strains on unloading. These strain incompatibilities can be mitigated by creation of large, textured grains through directional solidification or large, bamboo grains intersecting the free surfaces of pores. To study these two approaches to improve superelasticity in Cu–Al–Ni alloys, polycrystalline Cu–13.5Al–4Ni (wt.%) samples were cast in porous and dense form, with and without directional solidification. When tested in compression, directionally-solidified, oligocrystalline bulk (non-porous) Cu–Al–Ni exhibits recoverable unloading strains as high as 6.6% at 210 °C, as compared to 3.1% for their conventionally-solidified counterparts. Similarly, when comparing conventionally-solidified Cu–Al–Ni SMA with 58% open porosity shows 1.4% recoverable unloading strain at 260 °C, whereas a value of 2.6% is achieved in directionally-solidified porous samples with bamboo grains straddling pores. This improvement in superelasticity remains present after 30 mechanical load–unload cycles at 260 °C. Thus, both directional solidification and addition of porosity can reduce strain incompatibilities between neighboring grains in polycrystalline Cu–Al–Ni alloys, allowing them to approach the intrinsic high superelasticity of single crystals.

Correlation of microstructural and corrosion characteristics of quaternary shape memory alloys Cu–Al–Ni–X (X=Mn or Ti)

The effects of different contents (0.4%, 0.7%, and 1.0%, mass fraction) of Mn or Ti additions on the micro structure, shape memory effect and the corrosion behaviour of Cu–Al–Ni shape memory alloys were studied by field emission scanning electron microscopy, transmission electron microscopy, X-ray diffraction, differential scanning calorimetry and electrochemical and immersion tests in NaCl solution. It was observed that the microstructure, shape memory effect and corrosion characteristics are highly sensitive to the composition variations. It was found that the highest strain recovery was with 0.7% addition of Mn or Ti. This may be attributed to the presence of precipitation with a high volume fraction and the grain refinement. The electrochemical test showed that the formation of oxide layers in both Cu–Al–Ni–Mn and Cu–Al–Ni–Ti shape memory alloys (SMAs) provided good passivation which enhanced the corrosion resistance of the alloys. Immersion test showed that in Cu–Al–Ni–Mn– SMAs, pitting corrosion occurred through feebleness in the oxide layer. A corrosion product adjacent to the pits was rich in Al/Mn oxide and depleted in Cu while inside of the pit it was rich in Cu. In Cu–Al–Ni–Ti– SMAs, localized corrosion occurred on the surface of the specimens and dealuminization attack was also observed in the matrix.

Nanoindentation studies of small-scale martensitic transformations and ductile precipitate effects in dual-phase polycrystalline shape memory alloys

A ductile solid solution phase γ is introduced into austenite β of a polycrystalline Co–Ni–Al Shape Memory Alloy (SMA) using thermal treatments. Thermally-induced martensitic transformation in this dual-phase SMA is detected by Differential Scanning Calorimetry and X-ray Diffraction. We perform nanoindentation tests using a Berkovich tip to study mechanically-induced martensitic transformations and transformation–precipitate interactions. Deviation of load–depth curve of austenite β from Hertz elastic prediction indicates initiation of plastic deformation and possibly also martensitic transformation, the occurrence of which is supported by stress analysis. Compared to non-transforming γ, strain recovery is significantly higher and percent energy dissipation is much lower in β. Indents in β but at β/γ interfaces exhibited enhanced strain recovery, higher nanohardness, and lower energy dissipation in comparison to austenite β. There is local strengthening at the β/γ interface. Additionally, γ accommodates transformation strain in nearby β by extensive plastic deformation, alleviating stress concentration beneath the indenter. The plastic accommodation by γ also relieves the constraint imposed on transforming β and decreases the energy barrier for transformation. As a result, less material deforms plastically and more transforms martensitically, improving superelastic properties in β adjacent to γ. Our results suggest that incorporation of a ductile second phase is promising for enhancing ductility and superelasticity of polycrystalline SMAs.