B2 Parent Phase Research Articles

Spherical indexing of overlap EBSD patterns for orientation-related phases – Application to titanium

We propose a new electron backscatter diffraction (EBSD) indexing approach that can handle overlapping patterns arising from fine-scale mixtures of two crystallographically distinct but related phases. The approach relies on a combination of spherical indexing (SI) with an overlap master pattern (OMP) in which the two phases have the proper orientation relation. We illustrate the approach using a Ti-10-2-3 sample and compare results from traditional Hough-based indexing of the α and β-phases with SI maps. We show that the SI approach produces superior orientation maps, and we present a symmetry-based algorithm for the determination of the α-phase variant when experimental patterns are indexed with respect to the OMP. We illustrate the use of the OMP to determine the parent β-phase volume fraction and obtain excellent agreement with Thermo-Calc phase fraction predictions.

Hot Rolling Effects on the Microstructure and Chemical Properties of NiTiTa Alloys

In the present paper, a Ni-37Ti-15Ta (wt.%) shape memory alloy was analyzed in as-cast and hot-rolled conditions, by differential scanning calorimetry, scanning electron microscopy coupled with energy-dispersive spectroscopy, x-ray diffraction and electrochemical tests into a simulated body fluid (SBF). Both processing states experienced reversible martensitic transformations, with lower temperatures of reverse transformation at hot-rolled specimen, where the amount of secondary phases suffered contrary variation tendencies: β-Ta solid solution increased and (Ti,Ta)2Ni intermetallic compound decreased. Hot rolling contributed to the increase in internal stresses which caused a slight decrease of 2θ angle and to the stress-induced formation of B19’ martensite from B2 parent phase. The electrochemical corrosion test, performed in an experimental SBF, emphasized the superior corrosion resistance of as-cast specimen, since it had a lower amount of internal stresses. The corrosion compounds, mostly oxides, were completely removed by ultrasonic cleaning.

Cold-pressing and vacuum arc melting of γ-TiAl based alloys

Beta (β) solidifying γ-TiAl intermetallic alloys of nominal composition Ti-48Al, Ti-48Al-2Nb, Ti-48Al-2Nb-0.7Cr alloys have been cold pressed and vacuum arc melted. The Al loss was due to compaction method used prior to the melting technique, since it was evident after compaction that Al particles migrated to the surface in contact with the die facets after cold pressing. Electron backscatter diffraction (EBSD)-orientation mapping demonstrated that the α-precipitation from the parent β-phase follows the Blackburn orientation relationship (BOR). Microstructural characterization of the alloys was studied by scanning electron microscopy (SEM) equipped with energy dispersion spectroscopy (EDS) for micro-analysis. X-ray diffraction (XRD) technique was used to detect phase compositions.

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.

Microstructure and Martensitic Transformation Behavior in Thermal Cycled Equiatomic CuZr Shape Memory Alloy

Equiatomic CuZr alloy undergoes a martensitic transformation from the B2 parent phase to martensitic phases (P21/m and Cm) below 150 °C. We clarified the effect of the thermal cycling on the morphology and crystallography of martensite in equiatomic CuZr alloy using a transmission electron microscopy. The 10th cycled specimens consisted of different multiple structures at the maximum temperature of differential scanning calorimetry (DSC) measurement −400 °C and 500 °C, respectively. At the maximum temperature 400 °C of DSC measurement, it is composed of the fine plate-like variants, and a lamellar eutectoid structure consisting of Cu10Zr7 and CuZr2 phases on the martensitic variant. Concerning the maximum temperature of 500 °C of DSC measurement, it is observed the martensitic structure and the lamellar structure in which the martensitic phase was completely eutectoid transformed. The formation of this lamellar eutectoid structure, due to thermal cycling leads to the shift of forward and reverse transformation peaks to low and high temperature side. In addition, new forward and reverse transformation peaks indicating a new transformation appeared by thermal cycling, and the peaks remained around −20 °C. This new martensitic transformation behavior is also discussed.

Mechanical behavior and phase transformation of β-type Ti-35Nb-2Ta-3Zr alloy fabricated by 3D-Printing

Additive manufacturing (AM) has a substantial capability to produce superior and divergent properties of titanium alloys for biomedical implants, unlike the existing conventional technologies. This work investigated the mechanical properties and microstructure evolution of a β-type Ti-35Nb-2Ta-3Zr alloy prepared by selective laser sintering (SLS) process. The superelastic properties of the resultant specimen were characterized by cyclic loading-unloading tensile testing to evaluate the effect of SLS-process on the β-type Ti alloy specimen. The zigzag and V-shaped formation of {112}<111>β twins, coexisting with stress-induced ω-formation, were observed by the transmission electron microscopy (TEM). The formation of Type I twin martensite along with β-structure is attributed to superelastic recovery and elastic recovery of SLS-produced specimen. High resolution TEM (HRTEM) observation was used to investigate the transition between β and ω phases. Thin layers of ω-formation in weak interfacial stress regions along with the longitudinal twin boundaries were also analyzed. The orientation relationship between ω-structure and parent β-phase involves an overlapping of ω-phase, observed along with longitudinal β-matrix and β-twins. Moreover, dislocation tangles and dislocation pile-ups form along with twin martensite, stress-induced ω-phase, and β-phase.

Martensitic transformation and shape memory effects in Co-V-Al alloys at high temperatures

Martensitic transformation in Co-V-Al ternary systems was investigated by means of the structural and thermal analysis in single phase martensite alloys. The martensite phase presents a non-modulated L10 structure transformed from a B2 parent phase. With the increase of V content, martensitic transformation temperatures exceeding 100 °C and occurring at approximately 200–300 °C. A two-step shape memory effect occurring at different temperature ranges was found for the first time in Co60V30Al10 alloys, and considerable recoverable strains reaching up to 3.0% were observed. This study shows that Co-V-Al alloys have potential as new candidates for high-temperature shape memory alloys operating in a large temperature range.

Growth of B2 parent phase grain assisted by martensitic transformation in a nanocrystalline Ti-50.8Ni (at.%) shape memory alloy

Significant grain growth assisted by a martensitic transformation near or below room temperature was found in a nanocrystalline Ti-50.8Ni (at.%) alloy. Significant grain growth was observed when thermally- or stress-induced B2 ↔ R ↔ B19′ (B2: parent phase, R and B19′: two types of martensite phases) martensitic transformation is repeated while not when B2 ↔ R transformation is repeated. This implies that martensitic transformation to and from the B19′ martensite phase assist the grain growth. Transmission electron microscope observations indicate that migration of grain boundaries could be the main mechanism for the grain growth.

Crystallization process and microstructural evolution of as-spun Ti-Ni-Zr alloy ribbon

The rapid solidified Ti-Ni-Zr amorphous ribbon was crystallized to B2 phase and λ1 phase. The phase constitutions of Ti-Ni-Zr alloy ribbon were dependent on the annealing temperatures. The annealed ribbon with the annealing temperature of 773 K and 873 K consisted of B2 parent phase and λ1 phase. When the annealing temperature increased to 973 K, the B19′ martensite can be introduced except from the B2 phase and λ1 phase. With further increasing the annealing temperature to 1073 K, only B19′ martensite and λ1 phase was observed. The transformation temperatures increased successively with the increasing of the annealing temperature, which was closely related to the variation of the phase constituents. The annealed ribbons with the moderate annealing treatment showed the higher transformation temperature and the superior thermal cycling stability.

O Phase Precipitation and Variant Selection in Ti–22Al–25Nb Alloy during the Hot Shear Spinning

O phase precipitation and variant selection in Ti–22Al–25Nb alloy during the hot shear spinning are investigated using scanning electron microscope (SEM), electron backscattered second diffraction (EBSD), and transmission electron microscope (TEM). The results show that the random O phase precipitates from the B2 matrix during the hot shear spinning. With the increase of reduction ratio, the average thickness of O precipitates increases from ≈0.18 to ≈0.50 μm. Meanwhile, the O precipitates show various morphologies, including lamella, short rod, and sphere. Twelve O variants simultaneously form in the parent B2 phase and the orientation relationship between the B2–O phases is confirmed as [−111] B2//[1–10] O and (110) B2//(001) O. Different patterns of dislocation are also found in the B2 matrix, such as dislocation tangles, dislocation nets, and dislocation arrays. They play an important role in the variant selection of O precipitates, resulting in the different angled combinations of O variants. In addition, massive dislocation piles up along the O/B2 interface and some dislocation wall generates within the O phase, which are benefit for the globularization of O precipitates.

Role of hydrostatic pressure on the phase stability, the ground state, and the transformation pathways of NiTi alloy

Equiatomic NiTi shape memory alloys exhibit multiple martensitic transformations from a parent B2 phase. However, there is an apparent discrepancy between theoretical predictions and experimental observations of the transformation pathways and final products. We studied the phase stabilities of NiTi martensites with respect to the effect of hydrostatic pressure using a generalised solid-state nudge elastic band method implemented in density functional theory to simulate the minimum energy pathways of transformations. The analysis demonstrates that base-centred orthorhombic (BCO) phase is unstable, thus will undergo the BCO→B19′ phase transformation at above 9GPa pressure, as confirmed by phonon calculations.

Transformation entropy change and precursor phenomena in Ni-rich Ti–Ni shape memory alloys

Abstract

Ternary Ti-Zr-Nb and quaternary Ti-Zr-Nb-Ta shape memory alloys for biomedical applications: Structural features and cyclic mechanical properties

Ti-18Zr-14Nb, Ti-18Zr-13Nb-1Ta, Ti-18Zr-12Nb-2Ta and Ti-18Zr-11Nb-3Ta (at%) shape memory alloys were obtained by vacuum arc remelting and subjected to different thermomechanical treatment (TMT) schedules. The transmission electron microscopy analysis revealed that a TMT, which consisted of cold rolling with a true logarithmic strain of e = 0.3 and post-deformation annealing at 600°C (30min), led to a subgrained structure formation with a size of structural elements of 200–300nm and to a room temperature superelastic behavior. The X-Ray diffraction (XRD) analysis showed that a gradual replacement of Nb content by its Ta equivalent decreases the impact of alloying elements on the β-phase stabilization phenomenon and suppresses the ω-phase formation: at room temperature, Ti-18Zr-14Nb and Ti-18Zr-13Nb-1Ta alloys contain mainly parent β-phase, while Ti-18Zr-12Nb-2Ta and Ti-18Zr-11Nb-3Ta alloys contain considerable amount of α”-martensite. The crystallographic resource of recovery strain for all the alloys exceeds 5% and reaches 6%, for the Ti-18Zr-11Nb-3Ta alloy. The fatigue life of the Ti-18Zr-13Nb-1Ta alloy is almost twice as high as that of the Ti-18Zr-14Nb alloy. During multi-cycle mechanical testing, the following regularity appeared for all the alloys tested: during 10–15 first loading-unloading cycles, the mechanisms of plastic deformation and stress-induced martensite formation and stabilization prevail, while after that, the β⇄α” martensitic transformation mechanisms become dominant, and the alloys’ behavior turns from elasto-plastic into superelastic.

Martensite Transformations in the TiNi(Fe,Mo)Ag Alloys After Thermal Cycling

This paper presents the results on influence of thermal cycling on martensite transformations in (TiNiFeMo)Ag alloys with silver additive up to 1.5 at.%. The analysis of temperature dependences of the electrical resistivity allows to determine the characteristic temperatures and construct the diagram of martensite transformations. The thermal cycling leads to the reduce in Ms temperature stabilizing the parent B2 phase in all considered cases of Ag additive. The strongest influence of thermal cycling on the Ms temperature was founded in alloy with 1 at.% Ag. Authors describe the factors leading to a change of the characteristic temperatures of martensite transformations during thermal cycling.

Band Jahn Teller Like Distortion In TiPd Shape Memory Alloy On Addition Of Magnetic Impurity Co

In this paper, we have reported the electronic structure and magnetic properties of TiPd alloyed with two different concentrations of magnetic impurity Co in the framework of density functional theory using the full potential linearized augmented plane wave method. The prominent high temperature shape memory alloy (HTSMA) - TiPd, undergoes martensitic transformation from the parent B2 (cubic) phase to the product B19 (orthorhombic) phase. Among the two configurations [Ti(Pd,Co) and (Ti,Co)Pd] considered, Co is found to prefer Ti sublattice more than the Pd sublattice from the formation energy calculations for both B2 and B19 phases. The addition of Co induced magnetic behaviour in the compounds investigated. The densities of states (total, site and angular momentum decomposed) are studied in detail. Band Jahn Teller (BJT) effects are used to understand structural instability in solids. In BJT, the peak in the vicinity of Fermi level, in the spin down channel of the high symmetry phase splits or broadens on deformation. A band Jahn Teller like broadening is observed in the density of states curve of Ti25Pd50Co25. This broadening is accompanied by a decrease in electronic energy and number of states at Fermi level. BJT effect is an accompanying feature of martensitic transformation.

Effect of oxygen content on deformation mode and corrosion behavior in β-type Ti-Mo alloy

This study examined microstructural characteristics and mechanical properties in a β-type Ti-15Mo alloy (mass%) with different oxygen contents, and their corrosion behavior in simulated physiological media. With increasing oxygen content from 0.1–0.5%, lattice parameter of parent β-phase increased from X-ray diffraction profiles, and spots of athermal ω-phase became weak and diffuse through transmission electron microscopy observations. {332}<113> twin density decreased with an increase in oxygen content from 0.1–0.3% based on electron backscattered diffraction analyses, and it became almost zero when further increased oxygen content up to 0.5%. The solute oxygen atoms led to both a transition of {332}<113> twinning to dislocation slip and a suppression of β-phase to ω-phase transformation. Room-temperature tensile testing of this alloy with oxygen content ranging from 0.1–0.5%, revealed that yield strength ranged from 420MPa to 1180MPa and that uniform elongation ranged from 47–0.2%. The oxygen-added alloys kept a low elastic modulus obtained from stress-strain curves, and exhibited good corrosion resistance in Ringer's solution from open-circuit potential and potentiodynamic polarization measurements. A desirable balance between mechanical properties and corrosion resistance is obtainable in this alloy as biomaterials through utilizing oxygen to control the deformation mode.

Phase Constitution and Martensitic Transformation Behavior of Au-51Ti-18Co Biomedical Shape Memory Alloy Heat-Treated at 1173K to 1373K

Phase constitution and martensitic transformation behavior were investigated for a Au–51Ti–18Co alloy heat-treated at 1173 K to 1373 K for 3.6 ks. The Au–51Ti–18Co alloy was fabricated by Ar arc-melting technique and subsequently by hot-forging at 1423 K for 10.8 ks. X-ray diffraction analysis revealed that B2 parent phase, B19 martensite phase and AuTi3 simultaneously appeared regardless of the heat-treatment temperatures. By increasing the heat-treatment temperature, the volume fraction of AuTi3 was slightly reduced. Besides, the lattice transformation strain which was calculated from the precisely-determined lattice parameters was evaluated to be 7 % in the Au–51Ti–18Co alloy in all the heat-treated conditions. This value is comparable to that of NiTi practical alloys. From differential scanning calorimetry (DSC) analysis, reverse martensitic transformation temperature was slightly increased with the heat-treatment temperature. From the lattice transformation strain point of views, the Au–51Ti–18Co has a large potential for novel biomedical shape memory alloy.

Effect of microstructural parameters on the mechanical behavior of TiAlNb(Cr,Mo) alloys with γ+σ microstructure at ambient temperature

The mechanical properties of novel alloys with two-phase γ-TiAl + σ-Nb2Al microstructures were evaluated under compression at room temperature. Microstructures of varying scales were developed through solutionizing and aging heat treatments and the volume fraction of phases were varied with changes in composition. Ultra-fine, aged γ+σ microstructures were achieved for the alloys which effectively retained high volume fractions of the parent β-phase upon quenching from the solutionizing temperature. The yield strength and compressive strain to failure of these alloys show a strong dependence on the relative scale and volume fraction of phases. Surprisingly, the hard brittle σ-phase particles were not found to control fracture in the refined microstructures.

Martensite Reorientation after Thermal Cycling in NiTiCu Shape Memory Alloys Studied by EBSD Technique

The Ni25Ti50Cu25 shape memory alloy exhibits one-step martensitic transformation. Transformation occurs between the B2 parent phase and the B19 orthorhombic martensite. The course of the martensitic transformation was in situ studied in the alloy with use of electron backscatter diffraction. During heating, reverse martensitic transformation occurs from the B19 orthorhombic martensite to the B2 parent phase. On cooling, from the parent phase the B19 martensite is formed. Obtained results proved that the B19 martensitic plates forms the variants indexed as 1, 3, and 5. Thermal cycling does not change the crystallographic correlation between the parent phase and the martensite. However, after cycling, different rearrangement of the martensitic plates can be received. In consequence, characteristic temperatures of the martensitic transformation are shifted.

Microstructure evolution of Ti–30Nb–(4Sn) alloys during classical and step-quench aging heat treatments

Recent studies involving step-quench (SQ) heat treatments have shown that, unlike classical quench-and-aging (QA) heat treatments, SQ route can favour precipitation mechanisms that are strongly dependent on decomposition of the parent β-phase. In this work, Ti–30Nb and Ti–30Nb–4Sn (wt-%) alloys were subjected to cold-working, recrystallisation and then to either classical or SQ heat treatments at 400°C. Among QA samples, ω-phase is formed in both alloys during the heating ramp. By the SQ route, we found extensive isothermal ω-phase precipitation in Ti–30Nb during aging. On the other hand, a complete suppression of ω-phase in Ti–30Nb–4Sn was observed, thus, α-phase precipitation could take place improving hardness while still preserving a low elastic modulus.