Two-stage Martensitic Transformation Research Articles

Accelerated improvement in tensile superelasticity of electron beam directed energy deposition manufactured NiTi alloys by artificial thermal cycling combined with low temperature aging treatment

ABSTRACT The low-temperature aging treatment at 250°C can significantly improve the tensile superelasticity of NiTi alloys fabricated by electron beam directed energy deposition (EB-DED). However, it requires a very long aging duration (up to 200 h) to achieve excellent tensile superelasticity due to inherent coarse grain size. To accelerate the aging process, the high-density dislocations are introduced by artificial thermal cycling treatment prior to the aging treatment (the original dislocation content in EB-DED processed NiTi alloys is very low), which will promote the subsequent uniform precipitation of nanoscale Ni4Ti3 particles during low-temperature aging treatment. The phase transformation behaviour always maintains a stable two-stage martensitic phase transformation. Under a cyclic tensile test at 6% strain, 24 h aged sample with thermal cycling maintains a recovery rate exceeding 90% even after 10 cycles, comparable to the performance of the sample aged for 200 h without thermal cycling, indicating a substantial improvement in aging efficiency.

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.

Influence of Wire Geometry on the Mechanical Behavior of the TiNi Design

The present article is aimed at studying the deformation behavior of TiNi wire and knitted metal TiNi mesh under uniaxial tension and revealing the role of wire geometry on their main mechanical characteristics and mechanisms of deformation behavior. The temperature dependence curve of the electrical resistance indicates that a two-stage martensitic transformation of B2→R→B19′ is occurring, and is responsible for the superelasticity effect. The TEM results showed that at room temperature, the TiNi wire has a nanocrystalline structure composed of B2 austenite grains. A change in the deformation mechanism was established under the uniaxial tension, where the TiNi wire exhibits the effect of superelasticity, while the knitted metal TiNi mesh made from this wire is characterized by hyperelastic behavior. Fracturing of the knitted metal TiNi mesh requires significant loads of up to 3500 MPa compared to the fracture load of the TiNi wire. With the uniaxial tension of the wire, which maximally repeats the geometry of the wire in knitted metal mesh, an increase in mechanical characteristics was observed.

In-situ neutron diffraction investigation of two-stage martensitic transformation in a 13%Mn steel with serrated deformation

Two stage austenite →ε-martensite →α-martensite phase transformation, phase-specific lattice strain evolution and load sharing during tensile deformation of a 13%Mn hot-rolled austenitic steel are revealed by in-situ neutron diffraction. Direct evidence is provided that serrated deformation behavior is attributed to discontinuous martensitic transformation.

Obtaining Ultrastrong and Ductile Steel with Hierarchical Lamellar Duplex Phase Microstructure by Two-Stage Martensitic Transformation Mechanism

Obtaining Ultrastrong and Ductile Steel with Hierarchical Lamellar Duplex Phase Microstructure by Two-Stage Martensitic Transformation Mechanism

Effect of ultra-fine martensite and retained austenite induced by slow quenching on the mechanical properties of 9Cr 1.5Mo1Co(FB2) steel

The effect of ultra-fine martensite (ultra-fine M) and retained austenite (RA) induced by slow quenching (furnace cooling) on the mechanical properties of FB2 steel was investigated using experimental observations and phase-field simulation. The results show that the partition of C from martensite into untransformed austensite (UA) during the slow quenching process gives rise to the split (two-stage martensitic transformation) and incompletion of martensitic transformation. And most C-riched UA with high internal stress transforms into ultra-fine M during the second stage martensitic transformation, while other UA remains at room temperature to form banded/blocky RA. After tempering at 700 °C, lathy M and ultra-fine M are decorated with large and fine M23C6 particles, respectively. Meanwhile, banded and blocky RA transforms into globular M23C6 and ferrite. The slow quenching treated steel has higher or better tensile strength and impact toughness of tempered state when compared to the tempered steel with fast and medium quenching (water and air cooling) treatments. This improved strength and impact toughness attributes to the combined effect of effective grain refinement, the carbides density increment, and ferrite. Further quantitative assessment of the effect of ultra-fine M and RA on the mechanical properties of FB2 steel indicates that the strength and impact toughness of 9-12Cr steels may be tailored by optimizing the ultra-fine M and RA.

Mechanisms of two-stage martensitic transformation in Ti-Ni-Hf alloy powder

The Ti-Ni-Hf shape memory alloy powders were prepared by plasma rotating electrode process. And the powders were featured with the perfect sphericity and smooth surface. Moreover, the grain size was refined in the Ti-Ni-Hf alloy powders due to the rapid cooling rate during plasma rotating electrode process. It was found that the Ti-Ni-Hf alloy powders had the typical self-accommodation configuration including spear-like, mosaic-like and lath-like martensite. Various twins, such as (001) compound twin, (011) type I twin as well as {111} type I twin, were observed in Ti-Ni-Hf alloy powders. Differing from the bulk Ti-Ni-Hf alloy, two-stage transformation corresponding to B2⇌B19’ martensitic transformation was detected in the as-received Ti-Ni-Hf alloy powders. In addition, the martensitic transformation behaviors were closely related to the annealing temperatures. The Ti-Ni-Hf alloy powders with low annealing temperature of 500 ​°C still exhibited two-stage martensitic transformation, while two-stage martensitic transformation changed into single martensitic transformation in the Ti-Ni-Hf alloy powders with the higher annealing temperature of 900 ​°C. And finally the possible mechanisms for the evolution of martensitic transformation behaviors were discussed.

TU79. NEUROANATOMICAL AND GENETIC CORRELATES OF FACIAL SHAPE: POTENTIAL BIOMARKERS FOR SCHIZOPHRENIA

The Ti-Ni-Hf shape memory alloy powders were prepared by plasma rotating electrode process. And the powders were featured with the perfect sphericity and smooth surface. Moreover, the grain size was refined in the Ti-Ni-Hf alloy powders due to the rapid cooling rate during plasma rotating electrode process. It was found that the Ti-Ni-Hf alloy powders had the typical self-accommodation configuration including spear-like, mosaic-like and lath-like martensite. Various twins, such as (001) compound twin, (011) type I twin as well as {111} type I twin, were observed in Ti-Ni-Hf alloy powders. Differing from the bulk Ti-Ni-Hf alloy, two-stage transformation corresponding to B2⇌B19’ martensitic transformation was detected in the as-received Ti-Ni-Hf alloy powders. In addition, the martensitic transformation behaviors were closely related to the annealing temperatures. The Ti-Ni-Hf alloy powders with low annealing temperature of 500 ​°C still exhibited two-stage martensitic transformation, while two-stage martensitic transformation changed into single martensitic transformation in the Ti-Ni-Hf alloy powders with the higher annealing temperature of 900 ​°C. And finally the possible mechanisms for the evolution of martensitic transformation behaviors were discussed.

A novel two-stage martensitic transformation induced by nanoscale concentration modulation in a TiNb-based shape memory alloy

Martensitic transformation (MT) plays a critical role in determining both mechanical and functional properties of shape memory alloys (SMAs). The behavior of MT depends strongly on the microstructure of materials, and two- or multiple-stage MTs are typical examples of this. However, the physical origin of two- or multiple-stage MTs remains controversial although much effort has been made. In this study, a novel two-stage MT is observed in a TiNb-based SMA with nanoscale concentration modulation. The physical origin of this chemical-heterogeneity-induced two-stage MT is the difference in thermodynamic stability of the parent and martensitic phase between Nb-leaner and Nb-richer regions. The findings of this study not only expand the repertoire of mechanisms of two- or multiple-stage MT but also shed light on the long-standing controversy over the physical origin of multiple-stage MTs.

Coupling effect of stretch-bending deformation and electric pulse treatment on phase transformation behavior and superelasticity of a Ti-50.8 at.% Ni alloy

In order to improve the functional stability of Ti-50.8 at.% Ni alloy, repeated stretch-bending deformation (SBD), electric pulse (EP), and age-hardening were combined. Phase transformation behavior and superelastic properties of NiTi alloy subjected to different treatments were compared and analyzed in terms of average grain size, dislocation morphology, and nanoscaled Ni4Ti3 precipitates. The results indicate that the average grain size was refined from the annealing state of 9.1 μm down to 5.4 μm, and that a certain amount of dislocations or dislocation networks were introduced via 7 SBD passes. As EP-processed, both local stress relaxation and the non-uniform temperature field facilitated the microstructure to be a more homogeneous and stable state. With the increase of pulse frequency from 150 Hz to 400 Hz, a two-stage martensitic transformation occurred to replace the one-stage phase transformation and the combined multi-process induced multi-stage martensitic transformation behavior. Nanoscaled Ni4Ti3 precipitates with sizes in the range of 20–40 nm were homogeneously distributed in the matrix of NiTi alloy after aging at 250 °C for 48 h. As a result, based on the coupling effect of “Annealing-SBD-150 Hz EP-aging”, excellent functional stability was obtained. The accumulative residual strain was reduced from the annealing state of 3.6%–1.1%.

A hidden single-stage martensitic transformation from B2 parent phase to B19′ martensite phase in an aged Ni51Ti49 alloy

The aged Ni-rich NiTi shape memory alloys (SMAs) exhibit the multi-stage martensitic transformation (MMT), which has important influences on functional properties and practical applications of the NiTi SMAs. A hidden single-stage martensitic transformation from B2 parent phase to B19′ martensite phase is found in an aged Ni51Ti49 alloy, which happens concurrently with a commonly observed two-stage martensitic transformation B2–R–B19′ (R: martensite phase) and actually composes one stage of a multi-stage martensitic transformation (MMT) together with the two-stage one. B2–B19′ martensitic transformation occurs in the NiTi matrix containing Ni4Ti3 precipitates with relatively large inter-particle space, while B2–R–B19′ transformation takes place in the NiTi matrix with Ni4Ti3 precipitates having relatively small inter-particle space.

Effect of Aging on Phase Transition Behavior of Ti<sub>50</sub>Ni<sub>15</sub>Pd<sub>25</sub>Cu<sub>10</sub> High Temperature Shape Memory Alloys

Formation of Ni4Ti3 precipitates during aging of Ni-rich binary NiTi shape memory alloys and its effect on transition behavior during transformation from austenite to martensite phase has been studied extensively. However for equi-atomic NiTi-based quaternary high temperature shape memory alloy, two-stage martensitic transformation was detected for the first time. The Ti50Ni15Pd25Cu10 high temperature shape memory alloys were investigated for the hardness and transformation temperatures at aging temperature of 550°C. Aging at 550°C for 6 h resulted in remarkable increase in the hardness, whereas the phase transformation temperatures decreased significantly. During forward transformation from austenite to martensite, two-stage martensitic transformation; B2 (cubic) → R-phase and R-phase → B19 (orthorhombic) was observed.

Effect of aging on martensitic transformation and superelasticity of TiNiCr shape memory alloy

The effect of aging on microstructure, martensitic transformation and superelasticity of a Ti48.4Ni51.1Cr0.5 alloy was investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), differential scanning calorimetry (DSC) analysis and tensile tests. When the alloy was aged at 400 °C for 30 min, Ti3Ni4 precipitate appears. When aged at a temperature between 400 °C and 500 °C, the samples show a two-stage martensitic transformation. The aged Ti48.4Ni51.1Cr0.5 alloy exhibits a well-defined superelasticity at room temperature. The recovery ratio increases with increasing aging temperature from 300 to 450 °C. Further increase of aging temperature leads to the decrease of recovery ratio. The stress hysteresis shows the opposite dependence on aging temperature. The relationship between aging treatment and martensitic transformation and superelasticity can be rationalized by the evolution of Ti3Ni4 precipitate.

Transformation behaviors and superelasticity of Ti50Ni48Fe2 shape memory alloy subjected to cold-rolling and subsequent annealing

The effects of annealing on the phase transformation behavior and superelasticity of cold-rolled Ti50Ni48Fe2 shape memory alloy were extensively investigated. Curves of temperature dependence of electrical resistivity reveal that both the cold-rolled and annealed specimens exhibit a B2 → R→B19′ two-stage martensitic transformation upon cooling and a B19′ → B2 one-stage transformation upon heating, although the austenitic transformation temperature decreases with the increase of the annealing temperature. Tensile stress–strain curves show the critical stress for stress-induced martensite (σ SIM) of Ti50Ni48Fe2 alloys decreases with the increase of annealing temperature due to the decrement of dislocation density caused by the recrystallization. As a result, the σ SIM decreases. Upon a cold-rolling and annealing at 623 K for 30 min, the Ti50Ni48Fe2 alloy exhibits excellent superelasticity with the maximum recoverable strain of 5.8 % at a loading strain of 7 %. In such a case, a complete superelasticity of 5 % can be obtained in the Ti50Ni48Fe2 alloy after deformation increasing to 15 cycles.

Elastic and Irreversible Energies of a Two-Stage Martensitic Transformation in NiTi Utilizing Calorimetric Measurements

Elastic energy and irreversible energy are quantified based on calorimetric measurements. We analyze energetics for each stage of the stress-free, thermally induced two-stage phase transformation A ⇔ R ⇔ B19′ in an aged Ni-rich NiTi shape memory alloy. Heating/cooling rates are imposed from 1 K/min up to 100 K/min. We compare energetic analysis after multiple thermal cycles to virgin (i.e., first-cycle) material. Fundamental thermodynamic formulations are applied from two perspectives: the free energy change dG, and the rate of change of free energy expressed as dG/dfm. Two measures of irreversible contributions are defined: the difference between the forward and reverse transformation heats, and the product of the entropy and the thermal hysteresis. Higher values are determined for the former. For scan rates of 10 K/min and greater, the energetic values become relatively stable. Substantial variations are evident at 1, 5, and 10 K/min. The scan rate impacts the elastic strain energy and irreversible energy of the B19′ markedly compared with the R-phase transition. The findings are rationalized considering morphologic changes at the lower scan rates and the impacts on elastic and irreversible energies.

The Microstructure and Properties of Ti<sub>50</sub>Ni<sub>47</sub>Fe<sub>3</sub> and Ti<sub>50</sub>Ni<sub>46.75</sub>Fe<sub>3</sub>Cr<sub>0.25</sub> Shape Memory Alloy

Comparing with Ti50Ni47Fe3alloys, the influences of Cr on the mechanical and shape memory properties of Ti50Ni47Fe3alloys are investigated by study on phase transformation and microstructure analysis. The results show that Ti50Ni47Fe3and Ti50Ni46.75Fe3Cr0.25shape memory alloys exhibit two-stage martensitic transformation. The transformation temperatures decrease with the addition of Cr. The microstructure of the Ti50Ni47Fe3and Ti50Ni46.75Fe3Cr0.25alloys consists of TiNi matrix, Ti2Ni phase. Fe element prefers to substitute for Ni in the matrix than black particles. Cr all substitute for Ni in the matrix and not be analyzed in the Ti2Ni phase. The mechanical property of Ti50Ni46.75Fe3Cr0.25alloy is better than Ti50Ni47Fe3alloy.

Size effects in the superelastic response of Ni54Fe19Ga27 shape memory alloy pillars with a two stage martensitic transformation

The superelastic behavior of Ni54Fe19Ga27 shape memory alloy (SMA) single crystalline pillars was studied under compression as a function of pillar diameter. Multiple pillars with diameters between 10μm and 200nm were cut on a single crystalline bulk sample oriented along the [110] direction as the compression axis and that had undergone fully reversible two stage martensitic transformation, i.e. L21 austenite to 10M/14M modulated martensite and then to L1o martensite. The results revealed an increase in the critical stress for stress-induced martensitic transformation and the yield strength of martensite with decreasing pillar size. The stress hysteresis also increased with the reduction in pillar size and the superelastic response started to diminish below 500nm pillar diameter. Two-stage martensitic transformation was suppressed for pillar sizes of 1μm and below, which were shown to exhibit a direct austenite to L1o transformation. Such a change in the transformation pathway, i.e. from a two stage to one stage transformation, was also observed in bulk single crystals with increasing temperature. We demonstrated the absence of two stage transformation in bulk at high temperatures. This finding suggests that decreasing the sample size and increasing the temperature have similar effects on the superelastic response of NiFeGa SMAs that had undergone two-stage transformation and indicates that a reduction in pillar diameter decreases the transformation temperature due to the difficulty of martensite nucleation on small scales. The damping coefficients of the pillars were also calculated and the results highlighted that damping capacities higher than those of bulk metallic alloys can be achieved using submicron sized pillars.

The microstructure and thermomechanical behavior of Ti 50Ni 47Fe 2.5Nd 0.5 shape memory alloys

This work presents a study of a novel Ti 50Ni 47Fe 2.5Nd 0.5 shape memory alloy by EPMA, E-R analysis and tensile tests. Structural characterization shows the alloy consists of TiNiFe matrix, Nd 3Ni and Ti 2Ni intermetallic compounds with Fe solute. The latter two phases are well distributed in the matrix. Compared with Ti 50Ni 47.5Fe 2.5 alloys without rare earth Nd, the novel alloy does not change the two-stage martensitic transformation behavior, however, characteristic temperatures increase rapidly. The critical stress needed for inducing martensitic transformation is near linear relationship with the increasing of the testing temperature above M s, which is in good agreement with Clausius–Clapeyron equation. The novel alloy exhibits an improved shape memory effect as a consequence of precipitation strengthening and the maximum recoverable strain attains to 7.8%.

Effect of low-temperature precipitation on the transformation characteristics of Ni-rich NiTi shape memory alloys during thermal cycling

Thermal cycling of NiTi shape memory alloys is associated with functional fatigue: the characteristic phase transformation temperatures decrease with increasing number of cycles, and the transformation behavior changes from a single- to a two-stage martensitic transformation involving the intermediate R-phase. These effects are usually attributed to a gradual increase of dislocation density associated with micro-plasticity during repeated cycling through the transformation range. Here, these changes are shown to increase at a higher maximum temperature (in the fully austenitic state) during differential scanning calorimetric cycling of a Ni-rich alloy. Additional thermal cycling experiments without repeated phase transformations, and post-mortem microstructural observations by transmission electron microscopy, demonstrate that a relevant portion of functional fatigue is due to the formation of nano-scale Ni-rich precipitates of type Ni 4Ti 3 even at temperatures relatively close to the austenite finish temperature. These results show that both dislocation generation during the diffusion-less phase transformation, and diffusion-controlled nucleation and growth of Ni 4Ti 3 precipitates, can interact and contribute to the evolution of functional properties during thermal cycling of Ni-rich NiTi.

Shape memory characteristics of as-spun and annealed Ti 51Ni 49 crystalline ribbons

The as-spun Ti 51Ni 49 ribbon is essentially crystalline and shows a two-stage B2 ⇒ R and R ⇒ B19′ martensitic transformation with a significant transformation enthalpy. The as-spun ribbon already has plate-like Guinier-Preston (GP) zones which are proposed to induce the R-phase in martensitic transformation. After being annealed at 200 °C for 1 h, the size of the GP zones increases but their number decreases simultaneously. The maximal recoverable strain of the as-spun and 200 °C annealed Ti 51Ni 49 ribbons reaches 6.2% and 6.4%, respectively. With the increase of the annealing temperature up to 500 °C and 600 °C, only Ti 2Ni precipitates can be observed in the ribbons and the two-stage martensitic transformation tends to coalesce. Experimental results reveal that the ribbons with GP zones show a higher recoverable strain, a lower permanent strain and a lower Ms temperature than those with Ti 2Ni precipitates. The reason why the GP zones are formed in as-spun Ti 51Ni 49 ribbon during melt-spinning procedure is also discussed.