18R Martensite Research Articles

A Microstructural Study of Cu-10Al-7Ag Shape Memory Alloy in As-Cast and Quenched Conditions

Shape memory alloys (SMAs) represent an exceptional class of smart materials as they are able to recover their shape after mechanical deformation, making them suitable for use in actuators, sensors and smart devices. These unique properties are due to the thermoelastic martensitic transformation that can occur during both thermal and mechanical deformation. Cu-based SMAs, especially those incorporating Al and Ag, are attracting much attention due to their facile production and cost-effectiveness. Among them, Cu-Al-Ag SMAs stand out due to their notably high temperature range for martensitic transformation. In this study, a Cu-based SMA with a new ternary composition of Cu-10Al-7Ag wt.% was prepared by arc melting and the samples cut from this casting alloy were quenched in water. Subsequently, the phase composition and the development of the microstructure were investigated. In addition, the morphology of the martensite was studied using advanced techniques such as electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). The analyzes confirmed the presence of martensitic structures in both samples; mainly 18R (β1′) martensite was present but a small volume fraction of (γ1′) martensite also was noticed in the as-quenched sample. The observation of fine, twinned martensite plates in the SMA alloy with symmetrically occurring basal plane traces between the twin variants underlines the inherent correlation between microstructural symmetry and the properties of the material and provides valuable insights into its behavior. The hardness of the quenched sample was found to be lower than the as-cast counterpart, which can be linked to the solutioning of Ag particles during the heat treatment.

Outstanding shape memory effect and thermal stability of Cu–Al–Ni-based high temperature shape memory alloys achieved by Gd and Mn doping

In this paper, the effects of Mn element and rare earth element Gd on the microstructure, thermal stability, mechanical properties and shape memory effect of polycrystalline Cu-12.0Al-4.0Ni and Cu-12.0Al-4.0Ni-1.5Mn-xGd high-temperature shape memory alloy were investigated. The results show that the Cu-12.0Al-4.0Ni alloy exhibits millimeter-sized grain, dual-phase structure with coexistence of 18R and 2H martensite and poor thermal stability. When the Mn and Gd element was added to Cu-12.0Al-4.0Ni alloys, the grain size can be refined by an order of magnitude, and the 2H martensite disappeared completely, accompanied by the formation of the AlCu4Gd phase, meanwhile, thermal stability is significantly improved. The grain refinement improves the mechanical properties and shape memory effect of Cu-12.0Al-4.0Ni-1.5Mn-xGd alloys. When x = 0.3, the compressive fracture stress, compressive fracture strain and recoverable strain can reach 969 MPa, 15 % and 8 %, respectively.

Deformation mechanism of Cu-Al-Ni shape memory alloys fabricated via laser powder bed fusion: Tension-compression asymmetry

Introducing the unique advantage of additive manufacturing technology into copper-based shape memory alloys (SMAs) to fabricate high-performance alloys has garnered great attention in recent years, but the intrinsic relationships between microstructure and mechanical properties need to be further clarified. In this paper, the microstructural evolution of ternary CuAlNi SMAs fabricated by laser powder bed fusion (LPBF) under the tensile-compressive loading was investigated to determine the underlying mechanism of tension-compression asymmetry, that is, excellent compressive but poor tensile properties. Multiscale characterization of the different deformation stages revealed the numerous activated deformation mechanism on the 18R martensite matrix. A twin-related transformation dominated the main plastic deformation process due to lower stacking faults energy and high-density pre-existing planer defects in the CuAlNi SMAs. Deformation twinning nucleated at prior austenite boundaries and developed into parallel and network structures inside the parent grain of different sizes. In addition, the preferred orientation in different stages, the stress-induced γ phase transformation, and the interaction between dislocations and stacking faults are discussed. These results not only provide significant insights to understand the detwinning and deformation twinning process of SMAs but also establish the essential framework of microstructure and mechanical properties of Cu-based SMAs fabricated by LPBF.

Full shape memory effect of Cu-13.5Al-4Ni-6Fe shape memory martensite single crystal

The Cu−13.5Al−4Ni−6Fe centimeter-leveled martensite single crystals with full shape memory effect were prepared through simply annealing the cast alloy. The microstructure, reversible martensitic transformation, shape memory effect of this single crystal were investigated using optical microscopy, transmission electron microscope, differential scanning calorimeter, thermal mechanical analysis and compression test. The results show that Cu−13.5Al−4Ni−6Fe single crystal consists of 18R and 2H martensite, as well as rich-Fe bcc nanoprecipitate. The starting temperatures and the finishing temperatures of the forward and reverse martensitic transformations are 387, 397, 365 and 432 K, respectively. When the pre-strain is 6%, 7%, and 8%, this single crystal exhibits full shape memory effect of 4.4%, 4.7% and 5.2%, respectively. The obtained Cu−13.5Al−4Ni−6Fe martensite single crystal possesses shape memory properties that can be comparable to those of traditional shape memory alloys.

The ultrahigh functional response of CuAlMnNb shape memory alloy by selective laser melting

In this study, the Cu-11.85Al-3.88Mn-1.6Nb (wt.%) shape memory alloy with low cost and a highly functional response was manufactured by selective laser melting(SLM). The effects of energy inputs (123–185J/mm3) on their microstructure, mechanical, and shape memory properties were investigated. It was observed that all of the samples were composed of 18R and 2H martensite, and the nanosized (Cu, Mn)3Al3Nd4 phase was formed during selective laser melting(SLM). In addition, the good compressive fracture strength and strain were 1652Mpa, and 15.1% respectively. An excellent shape memory effect of 99.26% recovery ratio and 5.32% shape memory strain was obtained by SLM.

Effect of Y on microstructure, damping properties and mechanical properties of Cu-Al-Ni-Mn alloy

The effects of Y content on the microstructure, mechanical and damping properties of Cu-11Al-3Ni-2.5Mn-χY (χ = 0, 0.5, 1, 2, wt.%) alloys were investigated. The results of X-ray diffraction (XRD) analysis showed that the alloy was mainly composed of 18R martensite, α-Cu and γ2-Cu9Al4. The Y2O3 particles appear after the addition of Y elements. The grain size is reduced to 36.5 μm when 1 wt.% Y element was added, and the combined effect of the grain size reduction and the Y2O3 particles increases the tensile strength to 601.75 MPa. Meanwhile, the alloy at this content has high damping properties, the damping peaks at room temperature and temperature 600 °C were 0.038 and 0.24, respectively.

Effect of Nd addition on the microstructure, mechanical properties, shape memory effect and corrosion behaviour of Cu–Al–Ni high-temperature shape memory alloys

The effects of rare earth element Nd on the microstructure, mechanical, shape memory properties and corrosion behaviour of Cu-13.0Al-4.0Ni-xNd (wt.%) high-temperature shape memory alloys were investigated in this paper. It was observed that the grains in Cu-13.0Al-4.0Ni-xNd alloys can be refined by adding Nd, and the grain size decreased from the millimeter scale to several hundred microns. The matrix of the Cu-13.0Al-4.0Ni-xNd alloys, composed of 18R and 2H martensite, was not changed by Nd doping. However, the hexagonal (Al,Ni)Cu4Nd phase was formed as the Nd content increased. The grain refinement led to an improvement in the mechanical properties of the Cu-13.0Al-4.0Ni-xNd alloys. When x = 0.5, the compressive fracture stress and strain increased from 580 MPa to 10.5% to 940 MPa and 18.3%, respectively. The Nd addition also enhanced the shape memory effect of the Cu-13.0Al-4.0Ni-xNd alloys, and a reversible strain of 6.8% was obtained for x = 0.2 under a prestrain of 10% after heating. In addition, the results of electrochemical impedance measurement and polarization measurement showed that the corrosion resistance of Cu–Al–Ni alloy were slightly reduced by Nd addition, and the corrosion behaviour of Cu–Al–Ni alloy was mainly controlled by Cu element.

Evaluation of Cu-26Zn-5Al shape memory alloy fabricated by accumulative roll bonding process

Cu/Zn/Al multilayered composites consisting of pure copper, zinc, and aluminum were produced by accumulative roll bonding. Afterwards, heat treatment processes at various temperatures (750, 825, and 900 °C) were followed on the prepared composites to fabricate Cu-Zn-Al shape memory alloys. The prepared samples were characterized by scanning electron microscopy equipped with energy dispersive X-ray spectroscopy and X-ray diffraction. Mechanical properties of the composites and alloys were also investigated by hardness and tensile tests. The shape memory effect of the prepared alloys was evaluated by bending tests and subsequent heating. The results presented that the fabricated composite has a homogeneous distribution of Zn and Al layers in the Cu matrix. It was also found that the alloys treated at 750 and 825 °C consist of α and martensite phases, whereas only 18R martensite is formed in the alloy fabricated at 900 °C. Additionally, the alloy annealed at 900 °C exhibited a good strength (about 500 MPa), ductility (about 22%) and shape memory effect. It was concluded that Cu-Zn-Al shape memory alloys can be successfully fabricated by accumulative roll bonding and subsequent heating treatment processes.

Microstructure, mechanical properties and shape memory effect of Cu–Hf–Al–Ni alloys

ABSTRACTThe influence of hafnium element’s incorporation on a Cu–xHf–13.0Al–4.0Ni (wt-%) (x = 0.5, 1.0 and 2.0) high-temperature shape memory alloy was investigated systematically. The results show that the matrix of Cu–xHf–13.0Al–4.0Ni (x = 0.5, 1.0 and 2.0) alloys is 18R martensite, and an orthorhombic-structured Cu8Hf3 phase is formed and distributed at the grain boundaries. The grain size is significantly reduced with increasing Hf content. The mechanical properties of Cu–xHf–13.0Al–4.0Ni (x = 0.5, 1.0 and 2.0) alloys are improved by Hf doping due to the combination of refinement strengthening, solid solution strengthening and second phase strengthening. After heating under pre-strain of 10%, the shape memory effect of the Cu–1.0Hf–13.0Al–4.0Ni alloy reaches 5.6%, which is obviously higher than that of the Cu–13.0Al–4.0Ni alloy.

Electrically activated reactive synthesis (EARS) and electro-annealing of 3Ni–Al powder compacts

The present study investigates the electrically activated reactive synthesis (EARS) and post-reaction electro-annealing of 3Ni–Al powder compacts. The effect of initial relative green density on EARS processing and on the microstructure and phases produced was investigated. Post-reaction electro-annealing was also conducted to investigate its effects on phase transformations and consolidation. The initial reaction resulted in the formation of Ni3Al as the major phase, with 3R martensite also being formed. The investigated variation in relative green density was found to have limited effect on the product porosity and microstructure. Post-reaction electro-annealing, however, resulted in harder products with unique microstructures, particularly the formation of Ni4.22Al0.9 in very high amounts.

Study of Cu–Al–Ni–Ga as high-temperature shape memory alloys

The effect of Ga element on the microstructure, mechanical properties and shape memory effect of Cu–13.0Al–4.0Ni–xGa (wt%) high-temperature shape memory alloy was investigated by optical microscopy, SEM, XRD and compression test. The microstructure observation results showed that the Cu–13.0Al–4.0Ni–xGa (x = 0.5 and 1.0) alloys displayed dual-phase morphology which consisted of 18R martensite and (Al, Ga)Cu phase, and their grain size was about several hundred microns, smaller than that of Cu–13.0Al–4.0Ni alloy. The compression test results proved that the mechanical properties of Cu–13.0Al–4.0Ni–xGa alloys were improved by addition of Ga element owing to the grain refinement and solid solution strengthening, and the compressive fracture strains were 11.5% for x = 0.5 and 14.9% for x = 1.0, respectively. When the pre-strain was 8%, the shape memory effect of 4.2 and 4.6% were obtained for Cu–13.0Al–4.0Ni–0.5 Ga and Cu–13.0Al–4.0Ni–1.0 Ga alloys after being heated to 400 °C for 1 min.

Cu-Al-Ni-V high-temperature shape memory alloys

The effects of vanadium element on the microstructure, mechanical properties and shape memory effect of Cu-13.0Al-4.0Ni-xV (x = 0.5, 1.0 and 2.0) (wt.%) high-temperature shape memory alloy were investigated systematically. The results show that the Cu-13.0Al-4.0Ni-xV alloys display dual-phase morphology which consists primarily 18R martensite and (Al,Cu,Ni)V3 phase. The grain size is significantly reduced by the addition of vanadium. The mechanical properties of Cu-13.0Al-4.0Ni-xV alloys are also improved due to the grain refinement and solid solution strengthening. When the pre-strain is 10%, the shape memory effect of 5.8% can be obtained for Cu-13.0Al-4.0Ni-0.5 V alloy.

Design of High Temperature Ti-Pd-Cr Shape Memory Alloys with Small Thermal Hysteresis

The large thermal hysteresis (ΔT) during the temperature induced martensitic transformation is a major obstacle to the functional stability of shape memory alloys (SMAs), especially for high temperature applications. We propose a design strategy for finding SMAs with small thermal hysteresis. That is, a small ΔT can be achieved in the compositional crossover region between two different martensitic transformations with opposite positive and negative changes in electrical resistance at the transformation temperature. We demonstrate this for a high temperature ternary Ti-Pd-Cr SMA by achieving both a small ΔT and high transformation temperature. We propose two possible underlying physics governing the reduction in ΔT. One is that the interfacial strain is accommodated at the austenite/martensite interface via coexistence of B19 and 9R martensites. The other is that one of transformation eigenvalues equal to 1, i.e., λ2 = 1, indicating a perfect coherent interface between austenite and martensite. Our results are not limited to Ti-Pd-Cr SMAs but potentially provide a strategy for searching for SMAs with small thermal hysteresis.

Effects of Gd addition on the microstructure, mechanical properties and shape memory effect of polycrystalline Cu-Al-Ni shape memory alloy

The effects of Gd addition on the microstructure, mechanical properties and shape memory effect of (Cu-13.0Al-4.0Ni)-xGd (x=0, 0.3, 0.9, 1.5 and 6) (wt%) alloys were investigated. The results showed that the grain of (Cu-13.0Al-4.0Ni)-xGd alloys was refined by the Gd addition. The 18R martensite and a small amount of 2h martensite were present for x<0.9wt%, and dual phases containing 18R martensite and hexagonal structured AlCu4Gd phase were observed for x≥0.9wt%. In addition, Gd addition significantly enhanced the mechanical and shape memory properties. The compressive fracture strain of 15.3% and reversible strain of 6.6% were obtained in (Cu-13.0Al-4.0Ni)-0.3Gd alloy.

Crystallography and morphology of antiphase boundary-like structure induced by martensitic transformation in Ti–Pd–Fe alloy

The antiphase boundary (APB)-like structure of both 9R and B19 martensites in the Ti–Pd–Fe alloy was investigated by means of transmission electron microscopy. Some APB-like structures with curved and wide contrasts along the (001)9R basal plane are observed in 9R martensitic plates. The atomic displacement on the APB-like structure reflects the atomic movement stemming from the microdomains formed as a pre-martensitic transformation. The displacement vector of the APB-like structure in the B19 martensite can be expressed as R=〈1/30−1/2〉B19. The density of APB-like contrasts increases by the substitution of Fe for Pd in Ti–Pd–Fe alloy.

Oxidation parameters determination of Cu–Al–Ni–Fe shape-memory alloy at high temperatures

In this study, isothermal oxidation behavior of a Cu–Al–Ni–Fe shape-memory alloy between 500 and 900 °C was investigated. Alloy samples were exposed to oxygen by TG/DTA for 1 h at a constant temperature, allowing for calculation of the oxidation constant and activation energy values of the oxidation process. The oxidation constant value increased with temperature, reaching saturation at 800 °C. The effect of oxidation on crystal structure, surface morphology and chemical composition of the Cu–Al–Ni–Fe alloy was determined by X-ray diffractometer (XRD) and scanning electron microscope (SEM)–energy-dispersive X-ray (EDX) analyses. With increasing oxidation temperature, number and intensity of the characteristic 18R martensite phase peaks were reduced while Al2O3 phase peaks were increased. In parallel to the XRD results, the same variations were also detected by SEM–EDX measurements.

Effects of Heat Treatment and Deformation on 2H and 18R Martensites in Cu–9.97 % Al–4.62 % Mn Alloy

In this study, some physical and mechanical properties of martensitic transformation in Cu–9.97 % Al–4.62 % Mn (wt%) alloy were investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and compression deformation test techniques. In SEM and XRD observations of Cu–9.97 % Al–4.62 % Mn alloy, two kinds of martensitic phases: thermally and deformation effect, \({\beta_{1}^{\prime}}\) and \({\gamma_{1}^{\prime}}\) from DO3 parent phase, and the type of the produced martensite structures were noted.

Evolution of the phase stability of Ni–Al under low energy ball milling

Low energy mechanical alloying of Ni–35at.%Al and Ni–40at.%Al material was performed and the resulting structures were investigated by XRD and TEM. The final intermetallics observed consist of two phases, NiAl(B2) and Ni3Al while 7R and 3R martensite was observed in post-annealed samples. Different integrated milling times were associated to the intermetallic consolidation and initial blend dissociation.

Comparative topographic study of surface micro‐relief of primary martensite plates in shape memory alloys with different crystalline structures

AbstractThe surface micro‐reliefs of primary martensite plates, representative for two shape memory alloys (SMAs) with different crystalline structures were compared from qualitative and quantitative point of view by scanning electron microscopy and atomic force microscopy, respectively. Qualitative evaluations revealed larger widths and heights of the primary plates of ε hexagonal close packed (hcp) martensite, in an Fe‐Mn‐Si‐Cr‐Ni SMA than those of β2′ orthorhombic (9R) martensite, in a Cu‐Zn‐Al SMA. Quantitative evaluations were based on systematic dimensional measurements of the width and height of primary plate profiles. The measurements were performed on one hundred and twenty five profiles, five on each martensite plate belonging to five typical groups of primary plates, with length above 50 micrometers, of both ε hcp and β2′ 9R martensites. In order to compare the topographies of the two types of plates a statistical evaluation of the dimensional intervals of width and height of measured plates was performed.

Characterization of 3R martensite in a Ni–40Al–15Pt bulk alloy

As Pt promotes the reversible martensite transformation, the aim of this study was to assess the influence of Pt on the crystallography of (β′) martensite, in terms of ordering adapted, twinning and stacking faults. The focused ion beam (FIB) milling process was used to prepare transmission electron microscopy (TEM) samples of the NiAlPt bulk alloys, whereby 3R, L10 ordering of the martensite was confirmed. The addition of Pt, coupled with high temperature oxidation resulted in more stacking faults, due to greater Pt diffusion and subsequent enrichment in martensite. For Pt diffusion the temperature must be in excess of 900°C, both for isothermal and cyclic oxidation. When the favorable temperature environment is achieved Pt can diffuse preferentially allowing for promotion of the transformation to martensite.