Lightweight Shape Memory Alloys Research Articles

Structural-functional integrated TiBw/Ti–V–Al lightweight shape memory alloy composites

Driven by the increasing demands in advanced aerospace engineering, the integrated structural and functional materials are explored. In present study, we fabricate the TiBw/Ti–V–Al lightweight shape memory alloy composites with large recoverable strain (>5 %), high specific strength (>200 MPa cm3/g) and good elongation (>20 %). The satisfied structural and functional performances are attributed to the unique gradient microstructure, including TiB whiskers, short-range martensitic nanodomains and long-range martensitic microdomains. TiBw with the optimized orientation exhibits high load-bearing capacity. The transition area between TiBw and matrix is composed of short-range martensitic nanodomains. Nanodomains are affected by the diffused interstitial B atoms and local internal stress field regulated by TiBw. The elastic interaction energy between nanodomains and TiBw are calculated according to the Eshelby method. Upon deformation, nanodomains grow to long-range martensitic laths. The long-range martensitic laths keep stable after unloading. The microstructure evolution ties well with the Landau free energy model. It achieves effective loading transfer from matrix to reinforcement phase, resulting in less irreversible defects and better shape memory effect. In addition, grain refinement strengthening, loading transfer strengthening and solution strengthening are utilized to achieve the improvement of the strength and plasticity. The finding offers a promising inspiration for the development of new type shape memory alloy composites with integrated structural and functional properties.

Room-temperature superelasticity in Mg–Sc shape memory alloys revealed by first-principles calculations

The discovery of the Mg–Sc shape memory alloy system has become a milestone in the development of lightweight shape memory alloys. However, for the application scenarios with the highest demand at room temperature, the Mg–Sc alloy faces problems such as excessively low phase transition temperatures and poor room-temperature superelasticity, directly hindering the practical application of this new type of lightweight memory alloy. In this work, Mg–Sc based shape memory alloys with exceptional room temperature superelasticity have been screened for the first time utilizing Tm, phase transformation strain, and stress-strain curves. Co and Ge are selected as the most promising elements for improving Mg–Sc based alloys at room temperature. Furthermore, the mechanism behind the superelasticity of Mg–Sc alloy has been systematically unveiled and the average rate of energy change per unit time of the alloys was calculated for the first time. Co doped Mg–Sc based alloys exhibited the lowest variations in the average rate of energy change per unit time, Helmholtz free energy, and M1, along with a higher charge density between Co and Sc atoms. Additionally, a coupling effect between the d orbital of Co and Sc electrons and a lowered Fermi level was observed. These findings demonstrate that Co doping in Mg–Sc based alloys significantly reduces the superelastic hysteresis area and transformation driving force, and increases Tm and strain, thereby enhancing superelasticity. This research guides further studies and the design of new Mg–Sc based lightweight SMAs with outstanding superelasticity at room temperature.

Effect of high temperature thermohydrogen treatment on the microstructure, martensitic transformation, and mechanical and corrosion behaviors of Ti-V-Al lightweight shape memory alloy

In the present study, hydrogenation treatment was adopted to tailor the phase constituents of the Ti-V-Al shape memory alloy, further optimizing its performances. It can be found that hydrogenation treatment induced the transition from the α″ martensite phase to the β parent phase. Moreover, large amounts of hydride precipitates can be observed in the hydrogenation treated Ti-V-Al shape memory alloy with longer time of 5h. Meanwhile, the grain size of the Ti-V-Al shape memory alloy was reduced as a result of hydrogenation treatment. The interstitial atom H serving as a β-stabilizing element led to the reduction of martensitic transformation temperature. In proportion, hydrogenation treatment caused the enhancement of yield strength and decrease of elastic modulus, which promoted its application in biomedical fields. Besides, by optimizing the time of hydrogenation treatment, the hydrogenation treated Ti-V-Al shape memory alloy with 1 h possessed the superior corrosion resistance.

Grain refinement induced unusually large shape memory effect in lightweight titanium alloy

High-performance and lightweight shape memory alloys (SMAs) play a pivotal role in a variety of cutting-edge fields like aerospace, which however, are still quite difficult to be obtained until now. In the present work, we meticulously develop a novel lightweight Ti-12.5V-3.5Al-1Hf-0.8Fe-0.01B (at.%) SMA, which has a low mass density of ∼4.73 g/cm3. Prominently, distinct from the conventional wisdom in previous studies that is mainly based on eliminating grain boundaries or developing desired texture, we utilize the grain refinement strategy introduced by elaborate thermal-mechanical training, which effectively enables us to achieve exceptional shape memory response and strength-ductility combination simultaneously. It was revealed that a dominant β phase structure is achieved in this newly designed fine-grain SMA, accompanied by some α″ variants penetrating the grain boundaries. Compared to the coarse-grained counterpart, the deformation process becomes more intricate, involving stress-induced martensitic transformation and additional reorientations, which ultimately results in an unusually large recoverable strain up to 9.3 %. Furthermore, the fine-grain structure also contributes to an enhanced specific yield strength up to ∼159 MPa·m³/kg, while maintaining a large tensile ductility of ∼30 %. In light of its impressive structure-function integration and cost efficiency, this new-type SMA would emerge as a promising candidate for various advanced lightweight applications in a broad spectrum of fields.

Chemical ordering induced elastic hardening and thermoelastic properties of β phase Mg1-xScx lightweight shape memory alloys

The effect of chemical ordering on the thermoelastic properties of Mg-Sc light weight shape memory alloys are studied by the exact muffin-tinorbital method. Chemical ordering has significant hardening effect on elastic properties of Mg-Sc alloys, the C' is increased by 21% for Mg76Sc24 alloy, and the elastic hardening due to ordering can properly explain the increase in Vickers hardness observed in experiment. The quasi-harmonic Debye-Grüneisen model simulated the order–disorder transition temperature reasonably. The vibrational entropy increase due to elastic softening induced by disordering contributes considerably in driving the order–disorder transition. Mg-Sc alloys in ordered phase shows better thermoelastic stability over well-known commercial Mg alloys, therefore, ordering treatment of Mg-Sc are beneficial for structural application of this family of alloys. The Sc atoms (Mg atoms either) in bcc nearest neighboring lattice repel each other to form a B2-type ordered phase, and resultantly, the s-d and p-d hybridization between nearest neighboring Mg and Sc atoms is the main electronic reason for elastic hardening. The findings in current article not only give insight into understanding the strengthening mechanism during the ordering process of Mg-Sc alloys, but also provides a useful reference for the future development and modification of their shape memory effects.

Lattice dynamics of Mg-Sc lightweight shape memory alloys

Lattice vibrations play a significant role in the martensitic phase transformation of MgSc alloys, thus in the shape memory effects as well. In this article, the phonon spectra and thermodynamic properties of MgSc alloys with different structures were systemically studied by first-principles methods. The phonon spectra of Mg-18.75 at%Sc and Mg-20.83 at%Sc in austenite phase are fundamentally different, and the experimental martensitic transition behavior of MgSc alloys with different compositions can be well explained by the lattice dynamics. The occurred Fermi surface nesting at specific compositions leads to the appearance of phonon imaginary frequency, thus resulting in the lattice instability. The entropy change and latent heat obtained from harmonic phonon calculations suggests that the MgSc alloys are promising for the application in low-temperature elastocaloric cooling, and that the entropy changes are comparable to the well known cooling materials such as BaTiO3 and NiTi.

Microstructural design for achieving high performances in Ti-V-Al lightweight shape memory alloys by optimizing Zr content

In the present study, the quaternary Zr element was introduced to replace Ti in Ti-V-Al based shape memory alloys. The addition of Zr element resulted in the microstructural features of Ti-V-Al based shape memory alloys, such as the phase evolution of αˊˊ → β, the precipitation of C14-type Laves phase, reduction of grain size etc. Moreover, the two-stage martensitic transformation corresponding to ω → β and αˊˊ → β transformation gradually evolved into single ω → β martensitic transformation during heating process in Ti-V-Al based shape memory alloys, as Zr content was increased from 0.5 at.% to 5.0 at.%, which was firstly reported in Ti-V-Al based shape memory alloys. Meanwhile, the martensitic transformation temperatures of Ti-V-Al based shape memory alloys were decreased with Zr increasing. By optimizing Zr content, the highest yield strength and largest microhardness can be gained in Ti-V-Al based shape memory alloy with controlling 5.0 at.% Zr, as a result of solution strengthening, grain refinement and precipitation strengthening. Besides, the moderate Zr content caused the precipitation of C14-type Laves phase and formed the quasi-continuous network structure, which contributed to the superior elongation. And the perfect fracture ductility with an elongation of 40% in the present Ti-V-Al based shape memory alloys with 3.0 at.% Zr was significantly larger than that the other reported Ti-V-Al based shape memory alloys. Moreover, the superior strain recovery characteristics with a fully recoverable strain of 4% can be achieved in Ti-V-Al based shape memory alloys through controlling 3.0 at.% Zr.

Stability of thermal cycling and high temperature mechanical properties for Ti–V–Al–B lightweight shape memory alloy

The microstructure of the Ti–V–Al shape memory alloy with refined grain and in-situ TiB phase was modified by doping minor Boron (B), which contributes to the superior mechanical performances and strain recovery characteristics. Compared with other quaternary Ti–V–Al-X alloys, the Ti–V–Al–B alloy showed the largest ultimate tensile stress due to the solution strengthening, grain refinement and precipitation strengthening of in-situ TiB phase. Moreover, the Ti–V–Al alloy added 0.1 ​at.%B possessed the maximum yield stress of 701 ​MPa and the largest tensile fracture strain of 27.6% at the temperature of 150 ​°C. Meanwhile, the excellent strain recovery characteristics with fully recoverable strain of 4% could be obtained due to B addition. Besides, B addition suppressed the precipitation of ω phase during thermal cycling and further improved the thermal cycling stability of the Ti–V–Al alloy.

The microstructure and martensitic transformation of Ti-13 V-3Al light weight shape memory alloy deformed by high-pressure torsion

Ti-13 V-3Al (at%) high temperature shape memory alloy was considered attractive material due to its low density. The demand for the better shape memory performance is proposed to widen the scope of application. In the present study, Ti-13 V-3Al is processed by high-pressure torsion and annealing. The microstructure, martensitic transformation and mechanical properties were investigated. The severe plastic deformation introduced high density defects and decreased the stability of the α″ phase. The phase constitution is the sole β phase after high-pressure torsion, which was related to the Gibbs-Thomson effect. After the annealing treatment at 700 °C, numerous small parallel α″ martensite variants were observed, which was affected by the residual defects. More nucleation sites were provided, resulting that the reverse martensitic transformation was accomplished in a narrow temperature range. Moreover, the formation of (111) type I twinning become easier due to the assistance of the (111) stacking faults. The α″ martensite variants became larger and evolved to the V-shaped and the triangular self-accommodated morphologies as the annealing temperature increased to 800 °C. The twinning relationship in the different annealing temperature both were the (111) type I twinning. The properties were characterized by the tensile loading-unloading tests and microhardness tests. The shape memory effect was optimized and 4% full recoverable strain was obtained in the alloy annealed at 700 °C. The apparent hardness decreased from 323HV and 263HV as the annealing temperature raised from 700 °C to 800 °C.

Martensitic transformation mechanism of Mg-Sc lightweight shape memory alloys

Mg-Sc alloys are known as novel and promising lightweight shape memory alloys (LWSMAs), which have outstanding performance. Yet, a precise understanding of the microscopic picture and interactions governing the martensitic transformation (MT) remains elusive. We systematically investigate the MT of Mg-Sc alloys using first-principles methods. The result of generalized solid-state nudged elastic band methods confirms that no energy barrier inhibits the MT. We show that the bcc structure of Mg26Sc6 is dynamical instability at 0 K caused by electron-phonon coupling and Fermi surface nesting. Particularly, the high-temperature stability of Mg26Sc6 is revealed for the first time using the temperature-dependent effective potential method. The softening of the acoustic mode at Γ-R corresponds to two neighboring (1 0 1) planes moving towards each other, and forms martensite phase. Our calculations provide the complete and atomic-level mechanism for the MT of Mg-Sc alloys and shed some light on the design of new LWSMAs.

Martensite phase structure of Mg-Sc lightweight shape memory alloy and the effect of rare earth elements doping

With the continual increase of application requirement of lightweight shape memory alloys (SMAs), MgSc alloys have drawn great attention as a new SMA. However, the ambiguous martensitic structure, unclear transformation path, and low martensitic transformation temperature (Tm) of Mg-Sc alloys are still issues. Herein, the martensitic structure and transformation path of Mg-Sc alloys were investigated systematically. The martensite phase was identified finally to be an orthorhombic structure by formation energy. Moreover, to improve the Tm, the effect of rare earth elements doping on Tm was studied, and Tm was increased by 28.2 K with 1.25 at.% Gd doping. The total density of states indicated that the phase stability determined the increase of Tm. Furthermore, the partial density of states indicated that the d orbital electrons coupling between Sc and Gd atom led to the changing of Tm. Our results provide a firm base for designing the new Mg-Sc-based lightweight SMAs.

Martensitic Transformation Mechanism of Mg-Sc Lightweight Shape Memory Alloys

Mg-Sc alloys are known as novel and promising lightweight shape memory alloys, which have outstanding performance. Yet, a precise understanding of the microscopic picture and interactions governing the martensitic transformation remains elusive. In this study, we systematically investigate the martensitic transformation of Mg-Sc alloys using first-principles methods. The result of G-SSNEB confirms that no energy barrier inhibits the martensitic transformation. We show that the bcc structure of Mg26Sc6 is dynamical instability at 0 K caused by electron-phonon coupling and Fermi surface nesting. Particularly, high-temperature stability of Mg26Sc6 was revealed for the first time using the TDEP method. Mg26Sc6 becomes dynamically stable above 175 K. The softening of the acoustic mode at Γ-R corresponds to two neighboring (1 0 1) planes moving towards each other, and the martensite phase forms. Our calculations provide the complete and atomic-level mechanism for Mg-Sc alloys and shed some light on the design of new LWSMAs.

Simultaneous enhancement of strength and ductility in quaternary Ti–V–Al–B light weight shape memory alloys with high transformation temperature

In the present paper, the mechanical and functional properties of Ti–V–Al alloy were tailored by adjusting the boron (B) content. In addition, the effect of B content on the microstructure and martensitic transformation behaviors of Ti–V–Al alloys was also explored. The results indicated that the microstructure of Ti–V–Al alloys was sole α'' martensite phase, irrespective of the B content. B was completely soluble in Ti–V–Al alloy, as B content was not more than 0.01 at.%. Further increased B content not only promoted to the formation of TiB phase, but also resulted in the remarkable grain refinement. During heating process, the orthorhombic structure α'' martensite transformed into β phase with a body-centered cubic structure in one stage manner for all Ti–V–Al alloys containing different B contents. With the increased B content, the transformation temperature firstly decreased and then increased. Moderate B addition can improve significantly the mechanical properties and strain recovery characteristics of Ti–V–Al alloys owing to the perfect combination of solution strengthening, grain refinement, and precipitation strengthening of in-situ TiB phase. The maximum yield strength of 711 MPa can be obtained in 0.1 at.%B doped Ti–V–Al alloy with superior elongation of 18.5%. Moreover, it showed the largest recoverable strain of 5.1% under the condition of 6% pre-strain.

A theoretical approach to the structural, elastic and electronic properties of Ti8−xV4−yMox+y+zAl4−z lightweight shape memory alloys for biomaterial implant applications

The physical properties of the off-stoichiometric Ti8−xV4−yMox+y+zAl4−z alloys to reduce toxic effects and increase biomaterial efficiency have been studied systematically with Density Functional Theory (DFT) based ab-initio calculation methods. The calculated formation and cohesive energies show that thermodynamic and structural stability of the Ti8−xV4−yMox+y+zAl4−z alloys increases due to the increase in Mo concentration. The results of Pugh's ratio (G/B) show that the ductility of all phases decreases with increasing Mo concentration (0.15 → 0.35). The plasticity of the Ti8−xV4−yMox+y+zAl4−z alloys is downgraded with the increase of Mo concentration by the Poisson's ratio υ. The Young's modulus E of Ti8V4Mo3Al1 and Ti8V3Mo4Al1 phases were calculated as 49.84 GPa and 48.71 GPa, respectively. These phases are very suitable candidates for real biomaterial applications. Further, the electronic structure shows that as Mo concentration increases the Ti-3d-Mo-4d bonds become stronger and phases become more stable by the effect of Ti–Mo bonds.

Achieving fine-grained Ti-V-Al light weight shape memory alloys with higher transformation temperature, superior performances by doping Gd

Effect of Gd addition on microstructure, martensitic transformation and strain recovery characteristics was investigated. Gd addition resulted in the significant reduction of grain size and formation of Gd-rich phase. The Gd-rich phase was identified as Gd3(Ti,V) phase with an orthorhombic structure. The solution treated (Ti-13V-3Al)100−xGdx alloys underwent a α″ → β transformation during heating, irrespective of Gd content. But Gd addition led to a slight drop of martensitic transformation temperatures. In addition, the moderate Gd addition can promote the achievement of outstanding mechanical and functional properties due to the combination of the grain refinement and precipitation strengthening of Gd-rich phase in grain interior. Ti-V-Al alloy with an optimal Gd content, which was Ti-V-Al alloy containing 0.3 at.% Gd, showed the largest recoverable strain of 4.9% at a pre-strain of 6% and the ultimate tensile stress of 925 MPa. Meanwhile, Ti-V-Al alloy had a superior elongation of 20.7%, as 0.3 at.% Gd was added.

Tuning martensitic transformations via coherent second phases in nanolaminates using free energy landscape engineering

We explore the possibilities and limitations of using a coherent second phase to engineer the thermo-mechanical properties of a martensitic alloy by modifying the underlying free energy landscape that controls the transformation. We use molecular dynamics simulations of a model atomistic system where the properties of a coherent, nanoscale second phase can be varied systematically. With a base martensitic material that undergoes a temperature-induced transformation from a cubic austenite to a monoclinic martensite, simulations show significant ability to engineer the transformation temperatures, from a ∼50% reduction to a ∼200% increase, with 50 at. % of the cubic second phase. We establish correlations between the properties of the second phase, the transformation characteristics, and the microstructure via the free energy landscape of the two-phase systems. Coherency stresses have a strong influence on the martensitic variants observed and can even cause the non-martensitic second phase to undergo a transformation. Reducing the stiffness of the second phase increases the transformation strain and modifies the martensitic microstructure, increasing the volume fraction of the transformed material. This increase in transformation strain is accompanied by a significant increase in Af and thermal hysteresis, while Ms remains unaltered. Our findings on the tunability of martensitic transformations can be used for informed searches of second phases to achieve desired material properties, such as achieving room temperature, lightweight shape memory alloys.

Microstructure and mechanical properties of Ti–V–Al–Cu shape memory alloy by tailoring Cu content

The microstructure, martensitic transformation behavior, mechanical and shape memory properties of the Ti–V–Al–Cu light weight shape memory alloys were investigated systematically. The results showed that the phase constitution gradually evolved from single α″ martensite phase to the complete β phase with the Cu content increasing in the solution treated Ti–13V–3Al-xCu alloys at room temperature. The spear-like martensite variant constituted the typical self-accommodation configuration, and {111} type I twins and <211> type II twins coexisted in the Ti–V–Al–Cu alloys. Moreover, Ti2Cu precipitate can be observed in the Ti–V–Al–Cu alloys with the higher Cu contents. The martensitic transformation temperature of Ti–V–Al–Cu alloys decreased continuously with the increased Cu content. The Ti–V–Al–Cu alloys with the higher transformation temperature and higher strength as well as the superior shape recovery characteristics can be obtained by tailoring Cu content. It was the perfect combination of solid solution strengthening, grain refinement and precipitation strengthening to promote the enhancement of matrix strength. The maximum recoverable strain was 4.4%, for 6% pre-strain and the reverse martensite transformation temperature was 334 °C in the optimal Ti–13V–3Al-0.5Cu alloy.

Shape Memory Alloy Enables Folding Wings in Flight

Abstract The Spanwise Adaptive Wing project intends to obtain a wide spectrum of aerodynamic benefits in flight by folding wings through the use of an innovative and lightweight shape memory alloy.

First-principles investigation of phase stability in the Mg-Sc binary alloy

The recent discovery of shape memory behavior in Mg-Sc alloys has opened the door to the possibility of lightweight shape memory alloys. Very little is known, however, about martensitic phase transformations or about equilibrium phase stability in this alloy system. Here we report on a first-principles statistical mechanics study of zero Kelvin and finite temperature phase stability of hcp, bcc, and fcc based phases in the Mg-Sc binary. Our calculations reveal a rich array of phase transitions among the different low-temperature ordered and high-temperature disordered phases. Ground state orderings on hcp, bcc, and fcc belong to families of hierarchical structures containing rods of scandium atoms assembled in layers that repeat periodically. Both fcc and bcc are found to undergo a series of second-order phase transformations with increasing temperature until they completely disorder. A high degree of degeneracy is predicted at low and high temperatures among hcp, bcc, and fcc, a property that is likely to play an important role in the shape memory effects observed in this alloy.

Martensitic transformation and shape memory behavior of Ti-V-Al-Fe lightweight shape memory alloys

Effects of Fe addition on martensitic transformation and shape memory behavior of Ti-V-Al-Fe alloys were investigated. A little amount of Fe addition reduces martensitic transformation temperature rapidly and enhances shape memory effect of Ti-V-Al alloy. Alloy with an optimal Fe content, which is Ti-13V-3Al-1Fe, shows the largest recoverable strain of 5.8% and highest recovery stress of 250 MPa when pre-strain is 6%. Meanwhile, Ti-13V-3Al-1Fe alloy has a large elongation of 30%.