Thermally-induced Martensitic Transformation Research Articles

Effects of grain size and dislocation density on thermally-induced martensitic transformation of nanocrystalline NiTi alloys

Nanoscale grain and high dislocation density are the most common microstructures of nanocrystalline NiTi alloy. Although grain size and dislocation density influence the functional properties of NiTi alloy significantly, their effects on martensitic transformation have not been investigated quantitatively. In this work, the effects of grain size and dislocation density on thermally-induced martensitic transformation of nanocrystalline NiTi alloys are studied based on experiment, molecular dynamics simulation and theoretical analysis. Firstly, the drop of martensite-start-temperature and austenite-finish-temperature with decreasing grain size is determined, which is attributed to the increased fraction of grain boundary and the enhanced disordering of austenite near grain boundary via molecular dynamics simulation. Secondly, the positive correlation between dislocation density and transformation temperatures is verified by experiments and molecular dynamics simulations, which is ascribed to the enhanced stability of correspondence variant pair with habit plane of (−0.4138 0.8684 0.2688) and invariant shear direction of [−0.4432 −0.458 0.77706] under the dislocation-induced stress field. At last, a novel thermodynamic model of nanocrystalline NiTi considering grain size and dislocation density is proposed to predict martensite-start-temperature and austenite-finish-temperature. Prediction results accord to experimental data reasonably with the error generally less than 15 K. This study provides a basis for regulating the functional properties of nanocrystalline NiTi alloys through microstructural manipulation.

Ultrastrong interstitially-strengthened chemically complex martensite via tuning phase stability

Cantor alloy (Fe20Co20Cr20Ni20Mn20) and its derivatives, as the extensively studied chemically complex alloys (CCAs), exhibit remarkable ductility but relatively low strength due to face-centered cubic (fcc) matrix. By carefully tuning the phase stability of interstitial CCAs (iCCAs), we produced a lath-martensite matrix with a body-centered cubic (bcc) structure to achieve ultrahigh strengths. The iCCA with a martensite matrix (iCCM) displays an ultrahigh yield strength (YS) of ∼1.3 GPa, an ultimate tensile strength (UTS) of ∼ 1.9 GPa and a decent fracture elongation of ∼11 %. The thermally-induced martensitic transformation contributes to a remarkable YS increase, i.e., ∼ 1 GPa as compared with that of a fcc-iCCA, which provides an alternative strengthening route for CCAs without complex processing and profuse nanoprecipitates. This alloy design strategy combines the wide chemical composition space and potential advantages of CCAs and ultrahigh strength of bcc-martensite in steels, opening a new window to develop high-performance materials.

Large Elastocaloric Effect Driven by Low Stress Induced in [001]-Oriented Polycrystalline Co51.6V31.4Ga17 Alloy

In this work, we have studied the elastocaloric effect in directionally solidified Co51.6V31.4Ga17 alloys with a strong [001] preferred orientation. The entropy change for thermal-induced martensitic transformation is determined as 19.6 J kg−1 K−1. The sample exhibits stress-induced martensitic transformation with a hysteresis of 46 MPa, and the superelasticity is also verified by the in situ X-ray diffraction method. According to the elastocaloric effect tests, a noticeable change in adiabatic temperature up to 12.2 K has been achieved at the strain of 6%. The specific temperature change upon the critical stress loading can be attained as 132 K MPa−1. In addition, the difference in the loading–unloading temperature change can be ascribed to the imperfect adiabatic environment.

Thermal-induced martensitic transformation of NiTi shape memory alloy promoted by pulse current

The effect of transient and high-energy pulse current (THEPC) on the phase transformation behavior of the Ti-50.8 at.% Ni shape memory alloy was investigated in this study. The results show that the THEPC increases the start temperature of martensitic transformation, which indicates the facilitation of the thermal-induced martensitic transformation. Meanwhile, the transient and high-energy effect of the pulse current changes the phase transformation sequence of NiTi alloy during cooling, making it from one-stage phase transformation of A→M to two-stage of A→M1 and A→M2.

The Study of New NiTi Actuators to Reinforce the Wing Movement of Aircraft Systems.

Actuators using Shape Memory Alloy (SMA) springs could operate in different mechanical systems requiring geometric flexibility and high performance. The aim of the present study is to highlight the potential of these actuators, using their dimensional variations resulting from the phase transformations of NiTi springs (SMA) to make the movements of the system’s mobile components reversible. This reversibility is due to thermal-induced martensitic transformation of NiTi springs. The transformation promotes the extended and retracted of the springs as the phase changing (martensite–austenite) creates movement in part of the system. Therefore, the phase transition temperatures of NiTi, evaluated by differential scanning calorimetry (DSC), are required to control the dimensional variation of the spring. The influence of the number of springs in the system, as well as how impacts on the reaction time were evaluated. The different numbers of springs (two, four, and six) and the interspaces between them made it possible to control the time and the final angle attained in the mobile part of the system. Mechanical resistance, maximum angle, and the system’s reaction time using different NiTi springs highlight the role of the actuators. Fused Deposition Modelling (FDM)/Material Extrusion (MEX) or Selective Laser Sintering (SLS) was selected for shaping the composite matrix system. A new prototype was designed and developed to conduct tests that established the relationship between the recoverable deformation of the matrix suitable for the application as well as the number and distribution of the actuators.

The nucleation mechanism of martensite and its interaction with dislocation dipoles in dual-phase high-entropy alloys

The recent progresses in experiments have proven that the recently synthesized dual-phase high-entropy alloys (DP-HEAs) exhibit ultrahigh ductility without sacrificing the high strengthen. However, the original atomic mechanisms of the martensitic transformation process and its interaction with dislocations remain mysterious. In this work, the nucleation mechanisms of martensite are thoroughly investigated with various arrangement of dislocation dipoles via molecular dynamics (MD) simulations. Both stress-induced and thermal-induced martensitic transformation are reproduced in our simulations, which obeys the Burgers orientation relationship. Under the thermal fluctuation, the nucleation of martensite is controlled by the localized stress levels and strain energies. The thermal-induced martensite is found to nucleate near the dislocation dipoles once the strain energy reaches the required value. Upon loading, the fast-moving stress-induced martensite bends and wraps the dislocation dipoles in propagation path. These results provide a fundamental understanding of the martensitic nucleation in DP-HEA and its strengthening and toughening mechanisms, contributing benefits to design new alloys.

Exploring thermomechanical functionality of CuAlMn as an extreme low temperature shape memory alloy

Solid state actuation characteristics of CuAlMn shape memory alloys (SMAs) are reported at extreme low temperatures (<-100 °C). The effects of composition on the martensitic transformation (MT) characteristics are demonstrated at low temperatures (below −200 °C). The thermomechanical experiments revealed that the alloys exhibited 2.9% actuation strain under 250 MPa at subzero temperatures. CuAlMn SMAs with different Mn concentrations displayed 2% superelasticity at temperatures lower than −100 °C. The results illustrate that CuAlMn SMAs are promising candidates for cryogenic actuation and superelastic applications. The results are significant because none of the well-known NiTi SMAs demonstrate reversible thermally-induced MT below −60 °C. These findings are important for enabling solid state actuation in space and subsea environments.

Atomic scale modeling of the coherent strain field surrounding Ni4Ti3 precipitate and its effects on thermally-induced martensitic transformation in a NiTi alloy

Precipitation of Ni4Ti3 in NiTi alloy profoundly affects material properties, while the coherency strain fields and their effects on the thermally-induced martensitic transformation are not known in detail, especially for the B19’ variants and morphologies. Therefore, molecular dynamics simulations, as well as an Eshelby solution and phase-field microelasticity theory were applied to investigate the strain fields caused by the Ni4Ti3 precipitates with different aspect ratios. The maximum strain (along the central axis of the precipitate) in the matrix and its relative position are formulated as function relationships. Ms (martensitic transformation start temperature) and Af (austenitic transformation finish temperature) during martensitic transformation were determined and analyzed. Two previously published self-accommodation B19’ structures, the triangular and “herring-bone’’ morphologies are investigated in detail. An intermediate state between the triangular and “herring-bone’’ morphologies, termed mixed self-accommodation, is observed for the first time and proved to be the most unstable structure, providing a potential design route for low hysteresis microactuators. The formation processes of these self-accommodation morphologies are analyzed and discussed. Our simulations are the first time to reveal a variety of twinned B19’ morphologies selected by precipitates and their effects on thermally-induced martensitic transformation at the atomic scale.

Effects of grain size on body-centered-cubic martensitic transformation in metastable Fe46Co30Cr10Mn5Si7V2 high-entropy alloy

Systematic investigation of microstructure characterization and mechanical property was performed on metastable Fe46Co30Cr10Mn5Si7V2 high-entropy alloy (HEA) with varying the grain size. The grain coarsening led to the decrease in stability of face-centered-cubic (FCC) phase, and consequently generated the formation of laminate-morphology hexagonal-close-packed (HCP) and butterfly-morphology body-centered-cubic (BCC) martensite. During the tensile test, a transformation-induced plasticity (TRIP) from the FCC to BCC via the intermediate HCP occurred, and the accelerated transformation rate of BCC increased the strain-hardening rate. Our results suggest that microstructural evolutions related to thermally-induced martensitic transformation and strain-hardening mechanisms can be well interpreted by understanding the FCC to HCP to BCC martensitic transformation.

Tension-Compression asymmetry of single-crystalline and nanocrystalline NiTi shape memory alloy: An atomic scale study

In this study, the tension-compression asymmetry of single-crystalline and nanocrystalline NiTi shape memory alloys (SMAs) was investigated by molecular dynamics (MD) simulations. Compound twinning martensite variants were simulated via thermally-induced martensitic transformation. The characteristics of the forward and reverse martensitic transformations were derived using atomic structural evolution. The tension-compression asymmetry of single-crystalline NiTi was attributed to different stress-induced martensitic variants and different deformation modes, which led to stress asymmetry and strain asymmetry, respectively. However, the phenomenological tension-compression asymmetry in nanocrystalline NiTi was mainly attributed to stress-induced martensitic variants, and no clear martensitic reorientation occurred under tension and compression loads. The corresponding atomic-scale structural evolution also explained the shorter tensile stress plateau in the nanocrystalline NiTi when compared to that in the single crystal samples. Moreover, the nanocrystalline tension-compression asymmetric behaviour was simulated under a broad temperature range to obtain the critical stress-temperature relation. Finally, the tension-compression asymmetry mechanisms of single-crystalline and nanocrystalline NiTi SMAs were numerically derived at the atomic scale.

Neutron diffraction study on martensitic transformation under compressive stress in an ordered Fe3Pt

We have studied the structure change of an ordered Fe3Pt (degree of order ∼0.75) under a compressive stress applied in the [001] direction by neutron diffraction. In the absence of the stress, the alloy exhibits a weak first order martensitic transformation at 90 K from the L12-type cubic structure to the L60-type tetragonal structure. Under the compressive stress of 100 MPa, the first order nature of the thermally-induced martensitic transformation was undetectable in the temperature range of between 70 K and 270 K. The first order nature of the stress-induced martensitic transformation was also undetectable in the stress range of between 6 MPa and 300 MPa when tested at 120 K and higher temperatures. Under these conditions, the lattice parameters change continuously both in the cooling process and in the stress-applying process. Despite the disappearance of the first order nature of martensitic transformation, a significant stress-induced softening of lattice, which is regarded as a precursor phenomenon of martensitic transformation, was observed between 120 K and 265 K but not at 93 K and 295 K.

Achieving high oligocrystalline degree via strut architecture tailoring to increase the damping and mechanical properties of spherical porous CuAlMn SMAs

Spherical and uniform CuAlMn shape memory foams (SMFs) with quasi-oligocrystalline microstructure and various foam structures were manufactured by the silica-gel beads infiltration method. The strut architecture was quantitatively characterized by strut node size N, strut length L and L/N, and the oligocrystalline degree of foams was characterized by grain size d over N (d/N) for the first time. d/N is found to increase linearly with L/N due to the constricted effect of complex strut architecture on two-dimensional grain growth. The coupling effect of strut sizes and oligocrystalline degree on properties of the SMFs was explored. Too thin struts (smaller N) result in higher quenching rate and consequently more quenched-in vacancies, hindering the thermal-induced martensitic transformation and low-amplitude martensite damping. The peak damping becomes more dependent on d/N and tends to increase with increasing d/N, while the high-amplitude martensite damping improves linearly with d/N. The compression recovery strain also increases linearly with d/N under certain porosity, although higher porosity tends to compromise the favorable effect of higher d/N. The results demonstrate that higher d/N, which corresponds to higher oligocrystalline degree and lower grain constraints, favors the boundary mobility and martensite accommodation and thus improves the damping and compression recovery properties of Cu-based SMFs.

In-situ studies on martensitic transformation and high-temperature shape memory in small volume zirconia

Recently, the shape memory effect with significant recoverable shape deformation has been discovered in small volume single-crystal zirconia. The application potential for such shape memory ceramics has spurred in-depth exploration of the martensitic transformation crystallography and high temperature shape memory effect. In this work, the martensitic transformation of micron-sized zirconia has been studied in both a pristine as-produced condition and after micromechanical compression, using synchrotron scanning micro X-ray diffraction (μXRD). The pristine zirconia was found to transform into the monoclinic phase via a different crystallographic path than the compressed zirconia, resulting in distinct monoclinic variant preferences. The characteristic martensitic transformation temperatures were also found to be higher after uniaxial compression than in the pristine condition. Such observations provide insight on the differences between stress-induced and thermally-induced martensitic transformation. Moreover, we have observed a full cycle of the shape memory effect with large recoverable strain (7%) in micron-sized zirconia at a temperature of 400 °C. Our findings provide an important guideline to tailor the high-temperature shape memory properties of zirconia for further scientific research and engineering applications.

Relationship between martensitic plate size and austenitic grain size in martensitic transformations

A systematic experimental analysis based on an assessment of the mean martensite plate size (hplate) in sub-grain domains was implemented to characterize the martensite morphology in polycrystalline Cu-based shape memory alloys. In the grain size range below 100 μm, a linear relationship between the average width of the martensite plates and the mean grain size was obtained for a thermal-induced martensitic transformation. This evaluation allows us to perform an analysis of how microstructural length scales affect the martensitic transformation.

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.

Grain Size Effect on the Thermal-induced Martensitic Transformation in Polycrystalline Cu-based Shape Memory Alloys

In Cu-based SMA alloys, the grain size (d) effect on the martensitic transformation temperature was investigated for a wide range of d. Specimens were prepared by different heat treatments in order to create a range of grain sizes, from about 500nm (ribbons and tapes obtained by rapid solidification techniques) up to 6mm diameter single-crystals (grown by the Bridgman method). Information obtained from the literature was also included in the set of analyzed experimental data. The reduction of grain size shifts the forward transformation temperature downwards. These grain-size effects are observed in specimens with d below ∼ 100μm, and become more pronounced for d below ∼ 20μm. An empirical expression was obtained that describes the grain-size effect over the whole temperature range. The obtained curve differs considerably from the Hall-Petch behaviour reported in the literature by some other investigators.

Superelasticity at Low Temperatures in Cu-17Al-15Mn (at%) Shape Memory Alloy

Superelasticity at temperatures between 77 K and 273 K in Cu-17Al-15Mn alloy was investigated. In this alloy, where no thermally-induced martensitic transformation appears, an excellent superelasticity with recovery strain of 8.5% was confirmed at 77 K. The critical stresses for stress-induced martensitic transformation decrease with decreasing temperature and its gradient becomes smaller at low temperatures due to the decrease of entropy change. The entropy change estimated at 77 K is −0.46 J/(mol·K), which is almost one-third of that at 221 K.

A method of discrimination between stress-assisted and strain-induced martensitic transformation using atomic force microscopy

A method of discrimination between stress-assisted and strain-induced martensitic transformation (MT) using atomic force microscopy is established. The shear angles of the three kinds of face-centered cubic to body-centered tetragonal MT in an Fe–0.1C–1.7Mn–1.2Si transformation-induced plasticity steel are calculated based on our developed method. Comparing the theoretical shear angle with the practical shear angle, the features of three kinds of MT, including stress-assisted, strain-induced and thermal-induced MTs, are revealed.

Effect of heat treatment on martensitic transformation in Fe-12.5%Mn-5.5%Si-9%Cr-3.5% Ni alloy

In this study, thermally-induced martensitic transformation (γ(fcc) → ε(hcp)) in Fe-12.5%Mn-5.5% Si-9%Cr-3.5% Ni (weight) alloy was studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The effect of cooling rate was investigated. It was observed that fast cooled sample exhibited regular overlapping of stacking faults and ε martensite plates were formed parallel to each other. TEM investigations showed that the orientation relationship between γ-ε phases corresponds to Shoji-Nishiyama type orientation relationship.

Stability of thermally-induced martensitic transformations in Bi-atomic lattices

A well-known feature of some metallic alloys, such as NiTi, is their thermally induced, stress-dependent shape memory behavior. These alloys' remarkable properties are due to one or more martensitic transformations near room temperature, in which the crystalline configuration changes from a higher symmetry austenite (cubic lattice), to a lower symmetry martensite (rhombohedral, orthorhombic, tetragonal or monoclinic lattice) with decreasing temperature. In contrast to existing phenomenological approaches, the present work constructs a continuum energy density function W(F;θ) (as a function of a uniform deformation gradient and temperature) of a perfect periodic bi-atomic lattice from temperature dependent atomic potentials. Of interest in this work are the equilibrium solutions and their stability as functions of temperature for crystals under an applied pressure. Although the full problem is solved numerically, a post-bifurcation asymptotic analysis is necessary to guide the numerical solution near multiple bifurcation points. For the particular choice of a Morse-type pair potential, two stable cubic phases are predicted, one that corresponds to austentite (CsCl structure), which is stable at higher temperatures, and one that has a denser packing (NaCI structure), which is stable at lower temperatures. Theses stable portions overlap at intermediate temperatures, which is suggestive of a hysteretic temperature-induced martensitic transformation. Lower symmetry crystals, such as orthorhombic, monoclinic, and rhombohedral structures, are also predicted.