Ga High-temperature Shape Memory Alloys Research Articles

Effect of Ni doping on mechanical properties and phase transformation in Co–V–Ga high-temperature shape memory alloys

In the present work, the effects of Ni doping on the microstructure and mechanical properties of Co–V–Ga high-temperature shape memory alloy have been studied. It has been found that γ phase occurs in the form of precipitation or even dendrite microstructure when Ni content continuously increases in the alloy. The composition distribution showed the elements Co and Ni segregated in the γ phase, while elements V and Ga concentrated in the martensite phase. Moreover, the phase transition temperature increased by Ni-doping in Co–V–Ga alloys due to the increase in e/a of the alloy. However, the abundant presence of γ phase hindered the shear phase transformation and created a large number of grain boundaries to reduce Ms temperature when Ni content continuously increased. In addition, electron backscattered diffraction (EBSD) results verified that the presence of γ phase could hinder the expansion of cracks, and improve the strength and plasticity of the alloy. Furthermore, it could be found the shape memory effect could maintain a relatively high recovery rate when Ni content was within a certain extent. However, the shape memory effect of the alloy significantly decreased in the presence of γ phase with dendrite microstructure. The current research results not only clarify the influence of Ni doping on the microstructure and mechanical behavior of Co–V–Ga alloys, but also provide guidance for element doping to prepare high-temperature shape memory alloys (HTSMAs) with excellent performance.

Suppression of γ phase and its effect on mechanical behavior of melt-spun and annealed Ni–Mn–Ga high-temperature shape memory alloys

The suppression of the γ phase on the mechanical behavior of Ni58Mn25Ga17 and Ni60Mn25Ga15 melt-spun ribbons is investigated. The Ni60Mn25Ga15 ribbon, which contains the γ phase, generally exhibits a much higher fracture strain (average, 3.83 %) than the Ni58Mn25Ga17 ribbon, in which the γ phase is suppressed (average, 0.76 %). However, the reappearance of the γ phase after annealing under various conditions does not clearly increase the tensile fracture strain of the Ni58Mn25Ga17 ribbons. These results demonstrate that the proportion of precipitates associated with the original composition significantly affects the tensile strength. In particular, the distribution of the γ phase plays an important role in optimizing the tensile behavior. In addition, seven-layered modulated martensite and intermediate martensite structures appear in the ribbons owing to the fast cooling rates, inner stresses, and occurrence of the inter-martensitic transformation.

Tailoring the Microstructure in Polycrystalline Co–Ni–Ga High-Temperature Shape Memory Alloys by Hot Extrusion

Co–Ni–Ga alloys represent a new class of promising high-temperature shape memory alloys allowing realization of functional components for applications at elevated temperatures. Single crystals show a fully reversible pseudoelastic response at temperatures up to 500 °C. However, for most industrial applications, the application of polycrystalline material is needed. Polycrystalline Co–Ni–Ga alloys suffer from the anisotropic properties inherent to shape memory alloys, i.e., a strong orientation dependence of transformation strains, and therefore, are prone to intergranular fracture. This drawback can be curtailed by using appropriately textured material with a favorable grain-boundary orientation distribution. The current study discusses the impact of a hot-extrusion process on microstructural evolution and functional properties of polycrystalline Co–Ni–Ga high-temperature shape memory alloys paving the way to their robust application.

Pathways Towards Grain Boundary Engineering for Improved Structural Performance in Polycrystalline Co–Ni–Ga Shape Memory Alloys

In recent years, Co–Ni–Ga high-temperature shape memory alloys (HT-SMAs) attracted a lot of scientific attention due to their superior functional material properties. In the single-crystalline state, Co–Ni–Ga HT-SMAs feature a good pseudoelastic response up to 500 °C. However, in the polycrystalline condition Co–Ni–Ga suffers significant grain constraints and premature fracture at grain boundaries. In this regard, crystallographic orientations of the grains being involved as well as morphology and geometrical orientation of the grain boundaries with respect to the loading direction under pseudoelastic deformation are expected to be of crucial importance. Therefore, this study addresses the structural integrity of engineered grain boundaries, i.e., specifically selected grain boundaries in terms of orientation, grain boundary morphology, and crystallographic grain orientations of adjacent grains. Mechanical tests combined with in situ methods and post-mortem scanning electron microscopy investigations are used to shed light on the prevailing microstructural features resulting in any kind of structural degradation.

Martensite aging in 〈001〉 oriented Co49Ni21Ga30 single crystals in tension

Co–Ni–Ga high-temperature shape memory alloys (HT-SMAs) are well-known candidate materials for damping applications at elevated temperatures. Recent studies showed that upon heat treatment in stress-induced martensite under compressive loads transformation temperatures can be increased significantly, qualifying Co–Ni–Ga for HT-actuation. The increase in transformation temperatures is related to a change in chemical order recently validated via neutron diffraction experiments. Since SMAs show distinct tension–compression asymmetry in terms of theoretical transformation strains and bearable stresses, understanding the impact of martensite aging in tension is crucial for future applications. The current results indicate that martensite aging in tension provides for a further improvement in functional properties.

Correlation between microstructures and mechanical properties in Ni-rich Ni–Mn–Ga high-temperature shape-memory alloys

ABSTRACTMicrostructures and mechanical properties of polycrystalline and single-crystalline Ni55+xMn25Ga20−x (x = 1, 2, 3) alloys are investigated. The alloys exhibit a single martensite phase for x = 1, and two phases containing tetragonal martensite phase and face-centered γ phase for x = 2 and 3. The appearance of γ phase can apparently raise the strength and plasticity of the alloys. Moreover, single crystals show better mechanical properties compared with the polycrystals with the same composition. Furthermore, a kind of net-like γ phase is observed in directionally solidified Ni58Mn25Ga17 single crystal. This alloy exhibits excellent plasticity due to that the crack propagation is seriously hindered by the plastic net-like γ phase.

In Situ Neutron Diffraction Analyzing Stress-Induced Phase Transformation and Martensite Elasticity in [001]-Oriented Co49Ni21Ga30 Shape Memory Alloy Single Crystals

Recent studies demonstrated excellent pseudoelastic behavior and cyclic stability under compressive loads in [001]-oriented Co–Ni–Ga high-temperature shape memory alloys (HT-SMAs). A narrow stress hysteresis was related to suppression of detwinning at RT and low defect formation during phase transformation due to the absence of a favorable slip system. Eventually, this behavior makes Co–Ni–Ga HT-SMAs promising candidates for several industrial applications. However, deformation behavior of Co–Ni–Ga has only been studied in the range of theoretical transformation strain in depth so far. Thus, the current study focuses not only on the activity of elementary deformation mechanisms in the pseudoelastic regime up to maximum theoretical transformation strains but far beyond. It is shown that the martensite phase is able to withstand about 5% elastic strain, which significantly increases the overall deformation capability of this alloy system. In situ neutron diffraction experiments were carried out using a newly installed testing setup on Co–Ni–Ga single crystals in order to reveal the nature of the stress–strain response seen in the deformation curves up to 10% macroscopic strain.

Microstructure, thermomechanical and shape memory properties of two-phase Ni–Mn–Fe–Ga high-temperature shape memory alloys with B addition

Microstructure, martensitic transformation, thermomechanical stability and shape memory effect of two-phase Ni–Mn–Fe–Ga high-temperature shape memory alloys with small B addition, were investigated by thermomechanical cycling. It is found that the high degree of martensite stabilization is observed in all studied alloys due to the compressive pre-strain. The results show that B addition and thermomechanical cycling facilitate the precipitation of the ordered γ' phase. The precipitation alters the chemical compositions of martensite and results in the decrease of martensitic transformation temperature of the alloys. As a result, the formation of the ordered γ' precipitate has negative effect on the thermomechanical stability of the alloys. Results further demonstrate that the martensite reorientation proceed becomes easier with small B addition due to the decrease of the yield strength, which can effectively improve shape memory effect of the alloys. However, with increasing B concentration and number of cycles, the shape memory effect gradually decreases due to the combined effects of the roles of γ phase and γ' precipitate, and thermomechanical cycling process.

Microstructures and shape memory characteristics of dual-phase Co–Ni–Ga high-temperature shape memory alloys

The influence of microstructure on mechanical properties and shape memory characteristics of Co–Ni–Ga high-temperature shape memory alloys were investigated in this study. X-ray diffraction, scanning electron microscopy and transmission electron microscopy were employed to detect the microstructures. We found that these alloys were composed of dual phases, a non-modulated tetragonal L1 0 martensite and a face-centered cubic (fcc) γ phase. The martensite was twinned and well self-accommodated. The γ phase was a Co-based solid solution with 30% lower hardness than martensite. Although the fracture mode was intergranular, the strength and plasticity of the alloys increased markedly with the increasing volume fraction of the γ phase. The presence of the γ phase in grain boundaries rather than in the martensite is favorable to shape memory recovery. This was revealed by the maximum shape recovery strain over 5.0% that was obtained in the Co 46Ni 25Ga 29 alloy, with the γ phase formed mainly in grain boundaries.

The ductility and shape-memory properties of Ni–Mn–Co–Ga high-temperature shape-memory alloys

Ni–Mn–Co–Ga alloys with Ni/Mn or Ni and Mn substituted by Co were investigated as candidates for high-temperature shape-memory alloys. Ni 56− x Co x Mn 25Ga 19 alloys with x < 8 consist of single phase martensite, whereas Ni 56− x Co x Mn 25Ga 19 ( x ⩾ 8), Ni 56Mn 25− y Co y Ga 19 ( y = 4, 8) and Ni 56− z /2Mn 25− z /2Co z Ga 19 ( z = 4, 6) alloys consist of a two-phase mixture of martensite and γ phase. The mechanical and shape-memory properties of Ni 56Mn 25− y Co y Ga 19 and Ni 56− z /2Mn 25− z /2Co z Ga 19 alloys, which were hot-rolled into 0.5 mm thin plates by conventional hot rolling process, were investigated. The ductility and hot-workability of Ni–Mn–Co–Ga alloys were greatly improved by increasing the amount of ductile γ phase. Dynamic tensile tests and scanning electron microscopy observations of fracture surfaces confirm that the existence of γ phase plays a key role in improving the ductility of Ni–Mn–Co–Ga alloys.

Ni–Mn–Ga high-temperature shape memory alloys

The relationship among the composition, structure and martensitic transformation temperatures have been studied in detail for Ni 50− x Mn 25Ga 25− x (at.%) alloys by optical and transmission election microscopy, X-ray diffraction and differential scanning calorimeter measurements. The martensitic transformation temperatures almost linearly increase with increasing Ni content up to 670 K showing their potentials as high-temperature shape memory alloys. The results show that the tetragonality ratio ( c/ a) increases with the Ni substitution for Ga, but the unit-cell volume reduces simultaneously. The martensitic transformation characteristics of Ni 54Mn 25Ga 21 alloy with single martensite phase show excellent stability during thermal cycles or aging. The polycrystalline Ni 54Mn 25Ga 21 alloy exhibited a shape-memory effect of 4.2% and compressive strain larger than 10% by using grain refining technique. Addition of Fe as a fourth element is an effective method to improve the mechanical properties, especially plasticity of polycrystalline Ni–Mn–Ga alloy due to the precipitation of γ-phase. A compressive strain of 30% and a shape-memory effect of 2.8% are obtained for Ni 56Mn 17Fe 8Ga 19 alloy.

The effect of aging on the Ni–Mn–Ga high-temperature shape memory alloys

Abstract Martensite aging results in martensite stabilization in single-phase Ni54Mn25Ga21 alloy; however, it has no effect on the transformation temperatures of dual-phase Ni58Mn25Ga17 alloy. The reverse martensitic transformation temperatures of both alloys decrease sharply with the increase of aging time and then graduate to a constant level after austenite aging.