B19 Transformation Research Articles

Shape memory characteristics of rapidly solidified Ti 50Ni 15Cu 35 alloy ribbons

The shape memory alloy ribbons of Ti 50Ni 15Cu 35 had been fabricated by two rapid solidification processes of melt spinner and melt overflow to achieve different rapidly solidified structures. The ribbon fabricated by the melt spinner was an amorphous and any martensitic transformations occurred in the crystallized ribbon due to the formation of an equilibrium intermetallic phase (Cu 4Ti 3). However, the microstructure of the ribbon prepared by the melt overflow exhibited columnar grains normal to the ribbon surface. X-ray diffraction analysis showed that one-step martensitic transformation of B2–B19 occurred in the ribbon. According to the DSC analysis, it was known that the martensitic transformation temperature of B2 → B19 was 71.2 °C. During thermal cyclic deformation with the applied stress of 120 MPa, transformation hysteresis and elongation associated with the B2–B19 transformation were observed to be 4.6 °C and 1.68%, respectively, in the Ti 50Ni 15Cu 35 alloy. It is considered that these shape memory characteristics were ascribed to solid–solution strengthening by the supersaturation of excess Cu content.

A study of TiNiCr ternary shape memory alloys

Effects of Cr on transformation sequence and wear characteristic in Ti 50− X Ni 50Cr X , Ti 50Ni 50− X Cr X and Ti 51Ni 49− X Cr X alloys with X = 0.5–1.5 at.% are investigated. The lattice parameters of Ti 51Ni 48.5Cr 0.5 martensite are calculated from the XRD spectra as a = 0.2894 nm, b = 0.4109 nm, c = 0.4639 nm and β = 97.49° and the parameters a, c, β are slightly smaller, but b nearly the same, than those in binary Ti 50.8Ni 49.2 alloy. Experimental results indicate that the Cr atoms replace Ni atoms, instead of Ti ones in these alloys. In addition, in Ti 50Ni 49.5Cr 0.5 and Ti 51Ni 48.5Cr 0.5 alloys show a two-stage B2 → R → B19′ transformation upon cooling and a one-stage B19′ → B2 transformation on heating. Both Ti 50Ni 48.5Cr 1.5 and Ti 51Ni 47.5Cr 1.5 alloys exhibit a two-stage B2 ↔ R ↔ B19′ transformation; Ti 49.5Ni 50Cr 0.5 alloy has a one-stage B2 ↔ B19′ transformation and Ti 48.5Ni 50Cr 1.5 alloy shows a B2 ↔ R transformation. The shape recovery of these alloys can be improved by Cr solid-solution hardening. With higher matrix hardness in these alloys, there is better shape recovery. The wear mechanisms of TiNiCr ternary alloys are found to be similar to those of binary TiNi alloys. The Ti 48.5Ni 50Cr 1.5, Ti 51Ni 47.5Cr 1.5, Ti 50Ni 48.5Cr 1.5 and Ti 49.5Ni 50Cr 0.5 alloys have a better wear resistance than the Ti 50Ni 49.5Cr 0.5 and Ti 51Ni 48.5Cr 0.5 alloys due to their higher hardness, high cyclic hardening and/or pseudoelastic behaviors of B2 structure.

Application of severe plastic deformation by torsion to form amorphous and nanocrystalline states in large-size TiNi alloy sample

Results of investigations of the initial structure of large-size Ti49.4Ni50.6 samples subjected to severe plastic deformation by torsion under a high pressure (HPT) are reported. The study was performed using transmission and scanning electron microscopy, energy dispersive X-ray analysis, X-ray diffraction, and measurements of mechanical properties. Under an applied pressure of 6 GPa, the alloy was found to undergo a martensitic B2 → B19′ transformation. Even after HPT using a single revolution of anvils, the granular structure of titanium nickelide is refined so that there is formed a nanocrystalline state of B2 austenite (i.e., the reverse martensitic B19′ → B2 transformation occurs) and amorphization of the alloy begins. The HPT with a high number of revolutions leads to the almost complete amorphization of the alloy, which is explained by a high degree of shear deformation. In this case, all nanocrystalline inclusions in the amorphous matrix have an ordered B2 structure.

Energy landscape for martensitic phase transformation in shape memory NiTi

First-principles calculations are presented for parent B2 phase and martensitic B19 and B19′ phases in NiTi. The results indicate that both B19 and B19′ are energetically more stable than the parent B2 phase. By means of ab initio density functional theory, the complete distortion–shuffle energy landscape associated with B2 → B19 transformation in NiTi is then determined. In addition to accounting for the Bain-type deformation through the Cauchy–Born rule, the study explicitly accounts for the shuffle displacements experienced by the internal ions in NiTi. The energy landscape allows the energy barrier associated with the B2 → B19 transformation pathway to be identified. The results indicate that a barrier of 0.48 mRyd atom −1 (relative to the B2 phase) must be overcome to transform the parent B2 NiTi to orthorhombic B19 martensite.

Hydrogen induced structural phenomena in amorphous and crystalline shape memory alloys

Hydrogen interacting with shape memory alloys can be accompanied by several types phenomena related to the structure, those are: atomic rearrangements, hydrogen induced amorphisation, change of structural parameters of crystallised materials, suppression of the martensite type of phase transformation B2 → B19, and hydrogen induced reversible and irreversible embrittlement.

SHAPE MEMORY PROPERTIES OF RAPIDLY SOLIDIFIED Ti50Ni50-XCuX (X = 20, 25) ALLOY STRIPS

The shape memory alloy strips of Ti 50 Ni 30 Cu 20 and Ti 50 Ni 25 Cu 25 have been fabricated by arc melt overflow technique. Their microstructures and shape memory characteristics were investigated by means of X-ray diffraction (XRD), optical microscopy and differential scanning calorimetry (DSC). The microstructure of the as-cast strips exhibited columnar grains normal to the strip surface. XRD analysis showed that the martensitic transformation of B2–B19 occurred in the alloy strips. During thermal cyclic deformation with the applied stress of 60 MPa, transformation hysteresis and elongation associated with the B2–B19 transformation were observed to be 4.9°C and 1.8% in Ti 50 Ni 30 Cu 20 alloy strip and 3.5°C and 1.7% in Ti 50 Ni 25 Cu 25 alloy strip. The as-cast strip of Ti 50 Ni 25 Cu 25 alloy also showed a perfect superelasticity and its stress hysteresis was as small as 14 MPa. These mechanical properties and shape memory characteristics of the alloy strips were ascribed to B2–B19 transformation and the controlled microstructures produced by rapid solidification of the arc melt overflow process.

Mechanical properties and martensitic transformations in nanocrystalline Ti 49.4Ni 50.6 alloy produced by high-pressure torsion

Nanocrystalline alloy Ti 49.4Ni 50.6 (numbers indicate at.%) in the shape of a disc 20 mm in diameter has been successfully produced by high-pressure torsion. High-pressure torsion and annealings at temperatures from 400 to 550 °C resulted in formation of nanocrystalline structures with the mean grain size ( D) in the range 20–300 nm. The alloys with D = 20 nm possess very high ultimate tensile strength (more than 2000 MPa) but limited ductility. The alloys with D = 300 nm possess high ductility and enhanced ultimate tensile strength. There are areas of strain-induced transformation B2–B19′ on the tensile curves, but their extent depends on the grain size. The relationship between mechanical behavior and martensitic transformation in nanocrystalline TiNi alloys is considered and discussed.

Transformation behavior and shape memory characteristics of Ti–40Ni–10Cu(at.%) alloy ribbons

Ti–40Ni–10Cu(at.%) alloy ribbons have been fabricated by melt spinning with a linear velocity of 31 m s− 1. Their microstructures, transformation behavior, shape memory characteristics and superelasticity were investigated. As-spun ribbons prepared at the melt spinning temperature of 1873 K were amorphous and crystalline mixtures, below which they were crystal. The B2–B19–B19′ transformation occurred in the ribbons and the maximum temperature gap between Ms′ (B2–B19′ transformation start temperature) and Ms (B19–B19′ transformation start temperature) was 46 K. Ti–40Ni–10Cu alloy ribbons showed perfect shape memory effects even at 160 MPa and perfect superelasticity over a temperature range of 13 K. The transformation hysteresis of Ti–40Ni–10Cu alloy ribbons associated with the B2–B19 transformation was 8 K. Rapid solidification was found to be effective in increasing the critical stress for slip of a Ti–40Ni–10Cu alloy without overlapping the B2–B19 and B19–B19′ transformations.

The B2–B19–B19′ transformation behaviour in Ti–(45–x)Ni–5Cu–xV alloys

Transformation behaviour, shape memory characteristics and superelasticity of Ti–(45 − x)Ni–5Cu–xV(at.%) (x= 0.5–2.0) alloys were investigated using different techniques. The B2–B19′ transformation occurs when V content is less than 1.0 at.%, above which the B2–B19–B19′ transformation occurs. The temperature gap between (the B2–B19 transformation start temperature) and Ms (the B19–B19′ transformation start temperature) increased from 8 to 22 K with increasing V content from 1.0 to 2.0 at.%. V content dependence of (the B2–B19 transformation start temperature) was 20 K at.% −1. One-stage elongation occurred in Ti–44.5Ni–5.0Cu–0.5 V and Ti–44.0Ni–5.0Cu–1.0 V alloys, while two-stage elongation occurred in Ti–43.5Ni–5.0Cu–1.5 V and Ti–43.0Ni–5.0Cu–2.0 V alloys. Transformation hysteresis decreased from 16 to 12 K with increasing V content from 0.5 to 2.0%. The superelastic recovery ratio increased from 65.0 to 86.1% with increasing V content from 0.5 to 2.0%.

Microstructure modifications of Ti–Ni–Cu shape memory alloy strips fabricated by arc melt overflow process

Six batches of Ti50Ni50-xCux alloy (x=0–25) were prepared by a melt overflow process and strips of about 550 μm thickness were fabricated by rapid solidification. Microstructures and shape memory characteristics were investigated by means of x-ray diffraction (XRD), optical microscopy, differential scanning calorimetry and thermal cycling tests under a constant load. The microstructures of the as-cast strips exhibited columnar grains normal to the strip surface. XRD showed B2–B19′ martensitic transformation in the Ti50Ni50 binary alloy strip, while B2–B19 martensitic transformation in all the ternary alloy strips. The transformation temperature (Ms) increased with Cu-content. During cycle deformation with an applied stress of 60 MPa, the smallest transformation hysteresis and the largest elongation associated with the B2–B19 transformation were observed to be 3.5% in Ti50Ni25Cu25 alloy strip and 3.02% in Ti50Ni40Cu10 alloy strip.

Combinatorial study of phase transformation characteristics of a Ti–Ni–Pd shape memory thin film composition spread in view of microactuator applications

Phase transformation characteristics of a Ti–Ni–Pd shape memory thin film composition spread have been investigated. The thin film composition spread was fabricated from elemental targets using an ultra-high vacuum combinatorial magnetron sputter-deposition system and subsequent annealing at 500 °C for 1 h in situ. Automated temperature-dependent resistance measurements ( R( T)), energy dispersive X-ray analysis (EDX) and X-ray diffraction measurements (XRD) have been applied for the high-throughput characterization of the composition spread. Reversible phase transformations within the measurement range of −40 to 250 °C within the Ti–Ni–Pd system were observed for compositions with Ti content between 50 and ∼59 at.%. For Ti-richer films, Ti 2Ni and Ti 2Pd precipitates are inhibiting reversible phase transformations. The transformation temperatures and the thermal hysteresis were determined from R( T) measurements. Rising transformation temperatures with increasing Pd content and significantly lower thermal hysteresis for the B2–B19, compared to the B2–R–B19′ transformations were found in good agreement with published data. For low Pd contents (<7–12 at.%, depending on the Ti content) two-stage B2–R–B19′ transformations were observed. Compositions with higher Pd contents showed a single-stage B2–B19 transformation. Increasing Ti content within the B2–B19 transformation region results in a linear increase of the thermal hysteresis and decreasing transformation temperatures.

Microstructure and shape memory behavior of annealed Ti–36.8 at.% Ni–11.6 at.% Cu thin film

Amorphous films of Ti–36.8 at.% Ni–11.6 at.% Cu, formed by sputtering, were annealed at 773, 873 and 973 K for 1 h and their structure and shape memory behavior were investigated. Transmission electron microscopy and electron diffraction showed that the structure of such a film evolves in the following sequence: (1) Guinier–Preston zones form at 773 K, (2) Ti 2Cu plate-shaped precipitates and Ti 2Ni spherical precipitates appear at 873 K and (3) Ti 2Ni spherical precipitates form at 973 K. The strain–temperature curves under various constant stresses of the annealed films were measured. They showed a two-step deformation associated with the two-step martensitic transformation of B2 → B19 → B19′. The transformation temperature increased with increasing annealing temperature. The temperature hystereses of the films were largely reduced compared with those of the binary alloy thin films. The residual strain decreased with decreasing annealing temperature. This improvement in shape memory effect was ascribed to the presence of fine precipitates such as Guinier–Preston zones and Ti 2Cu precipitates.

New (3 3) long-period microtwin variant in the martensitic phase of the PtTi alloy

The martensitic phase in the PtTi near-equiatomic alloy was extensively studied by electron microscopy techniques. A new 3 3 long-period stacking based on the expected B19 structure was observed as well as twinning on (1 1 1) planes. Based on electron diffraction and high-resolution imaging and the B2 to B19 transformation mechanism a new model was proposed and the agreement with experiments was checked by simulation of electron diffraction patterns and high-resolution images.

Effect of annealing conditions on microstructures and shape memory characteristics of Ti 50–Ni 30–Cu 20 alloy ribbons

Ti 50–Ni 30–Cu 20 alloy ribbons without any crystal phases have been fabricated at the wheel velocities of 55 m/s and the melt spinning temperature of 1500 °C by melt spinning process. Their microstructures and shape memory characteristics were investigated by means of optical microscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). The crystallization temperature was measured to be 440 °C. The crystallized ribbon exhibited fine microstructure after annealed at 440 °C for 10 min and was deformed up to about 6.8%. During thermal cycle with the applied stress of 220 MPa, transformation hysteresis and elongation associated with a B2–B19 transformation were observed to be 4 and 3.6%. The ribbons showed a perfect superelasticity and their stress hysteresis was as small as 20 MPa which was attributed to the single pair martensite variants in small grains. This controlled microstructure was achieved by a proper heat treatment of amorphous ribbons in Ti 50–Ni 30–Cu 20 alloy.

Annealing effect on martensitic transformation of severely cold-rolled Ti 50Ni 40Cu 10 shape memory alloy

The annealing effect on the recovery of B2 → B19 and B19 → B19′ transformations of 40% cold-rolled Ti50Ni40Cu10 is studied by the differential scanning calorimetry (DSC) and dynamic mechanical analyzer (DMA) tests. The DSC test is only sensitive for measuring the recovery of B2 → B19 transformation, while the DMA test is suitable for both transformations. The change of internal friction values of these two transformations affected by annealing is due mainly to the difference in the rate of change of the transformation volume between them, which is related to their different recovery behaviors and microstructures.

Superelasticity and Corrosion Behavior of 50Ti–(45-X)Ni–5Cu–XMo (at.%) Alloys

In this study superelasticity and corrosion characteristics of 50Ti–(45-X)Ni–5Cu–XMo alloys have been investigated by means of electrical resistivity measurements, X-ray diffraction, tensile tests, polarization tests, and optical microscopy. Substitution of Mo for Ni in a Ti–45Ni–5Cu alloy induces the B2–B19 transformation and the stability of the B19 martensite increases with an increase in Mo content. Superelastic recovery of 50Ti–(45-X)Ni– 5Cu–XMo alloys increases from 33 to 91% with an increase in Mo-content from 0 to 0.5%. The 50Ti–(45-X)Ni–5Cu–XMo alloys show small stress hysteresis below 100 MPa. The pitting potential of 50Ti–(45-X)Ni–5Cu–XMo alloys also increases with the increase in Mo content.

Role of shape-memory alloy reinforcements on strain evolution in lead-free solder joints

Microelectronic solder joints are exposed to aggressive thermomechanical cycling (TMC) during service, resulting in strain localization near solder/bond-pad interfaces, which eventually leads to low-cycle fatigue (LCF) failure of the joint. In order to mitigate these strain concentrations, a “smart solder” reinforced with a martensitic NiTi-based shape-memory alloy (SMA) has been proposed before. In the present work, the role of NiTi particles on strain evolution in composite solders was studied using a combination of experimental and numerical means. Finite element modeling showed that NiTi pariculate reinforcements can reduce inelastic strain levels in the solder via shape recovery associated with the B19′ → B2 transformation. In situ TMC studies in the scanning electron microscope (SEM), in conjunction with strain analysis via digital image correlation (DIC), showed evidence of reverse deformation in the solder commensurate with the NiTi phase transformation, demonstrating the conceptual viability of the smart solder approach. The SEM-DIC experiments also suggested that the presence of particulates mitigates shear localization, which is commonly observed in monolithic solder joints close to joint/bond-pad interfaces. Finally, TMC experiments on monolithic solder and NiTi/solder single-fiber composite joints highlighted the beneficial effect of shape-memory transformation in reducing inelastic strain range of solders.

Spontaneous formation of the B2 phase from a decagonal quasicrystal under reduced constraint

Since the discovery that vacancies can quasiperiodically order in a basic B2 structure to yield a one-dimensional quasiperiodicity, these phases were subjected to intense investigations for possible links with quasicrystal. The B2 phase was found to coexist extensively with the icosahedral quasicrystalline phase, particularly in Al–Cu–TM (TM = transition metal) systems where the two phase fields are often adjacent to each other. The two phase exhibit a well-defined orientation relationship. There exist several studies dealing with quasicrystalline to B2 transformation, particularly in decagonal quasicrystals Doblinger et al. has carefully explored different metastable states in Al–Co–Ni decagonal quasicrystals including nanodomained 1D quasicrystals and multiple twinned approximant phase. The decagonal to a cubic B2 ordered phase transformation can often be observed. The nanocrystals of the B2 phase could be observed on the surface of the decagonal quasicrystal. Zhang and Urban have studied the stability of the QC phase by employing electron irradiation in an electron microscope and observed the transformation of the decagonal quasicrystal (DQC) phase, in an Al–Cu–Co–Si alloy, first to a disordered phase [body centered cubic (BCC phase)] and then to an ordered B2 phase. Zurkirch et al. observed a decagonal to crystalline structural transformation occurring on a surface of Al–Co–Ni upon sputtering the surface. The annealing treatment was found to restores the decagonal structure. Shimoda et al. confirmed the above observation. A similar result has been reported recently by Fluckinger et al. who have reported more than one variant of the cubic crystal at the surface. These authors attributed the structural change to the change in the surface composition during sputtering. The annealing treatment allows the A1 atoms from the subsurface to diffuse back and restore the composition resulting in a reappearance of the decagonal quasicrystal. The formation of a BCC phase during ion milling has also been reported in Al–Cu–Co decagonal quasicrystals. Recently Muthy et al. has reported a quasicrystalline to $_\beta$ phase transformation in as-cast $A1_{65}Cu_{20}Fe_{15}, A1_{65}Cu_{20}Co_{15}$ and $A1_{72}Pd_{19:5}Mn_{8:5}$ alloys during high energy ball milling. They have correlated the phase change with a critical grain size of 20 nm.

Lattice deformation and shape memory characteristics of Ti–30Ni–20Cu(at.%) alloy ribbons

Lattice deformations along the principal axes (η1, η2 and η3) associated with the B2–B19 transformation in Ti–30Ni–20Cu(at.%) alloy ribbons were found to be 0.9556, 0.9921 and 1.0482, respectively. The maximum recoverable elongation and transformation hysteresis associated with the B2–B19 transformation were 3.4% and 4 K, respectively. Superelastic recovery occurred in the temperature range 338–403 K.

Stability of the B19 martensite in rapidly solidified Ti–Ni–Cu alloys

Stabilization of the B19 martensite by rapid solidification in a Ti–45Ni–5Cu (at.%) alloy has been investigated by means of differential scanning calorimetry and transmission electron microscopy. Rapid solidification process induced the B2–B19 transformation prior to formation of the B19′ martensite, and thus the two-stage B2–B19–B19′ transformation occurred in as-spun Ti–45Ni–5Cu alloy ribbons. Inducement of the B19 martensite was ascribed to the formation of Ti 2Ni particles coherent with the B2 matrix. With increasing Ti content from 49 to 51 at.%, the temperature range where the B19 martensite exists was expanded from 7 to 25 K, which was attributed to an increase in volume fraction of Ti 2Ni particles from 10 to 55% and a decrease in the size from 16 to 5 nm.