Phase Transformation In Ti Research Articles

Microstructural evolution and formation mechanism of FCC titanium during heat treatment processing

The phase transformation of Ti–20Zr–6.5Al–4V alloy was studied during different heat treatment processes. Due to the addition of zirconium possessing the high-stacking-fault energy, a small amount of Ti underwent allotropic transformation from hcp-Ti to face-centered cubic (fcc) Ti during heat treatment. The alloy, which was subjected to solid solution treatment at 950°C for 60min and subsequent water quenched, underwent the following phase transformations: β→α+β (residual)+α″+fcc phase. The fcc phase was also found in the subsequent aging process, and when aging temperature reached 700°C, the fcc-Ti completely decomposed into hexagonal close-packed α phase. The transus temperature of fcc-Ti was between 650°C and 700°C. It was found that the mechanical properties of the alloy are remarkably dependent on fcc phase content.

In situ neutron diffraction study of texture evolution and variant selection during the α → β → α phase transformation in Ti–6Al–4V

In the present study, in situ phase transformation experiments have been carried out using neutron diffraction to monitor the texture evolution during the α→β→α phase transformation in Ti–6Al–4V with and without 0.4% yttrium additions. The aim of adding yttrium was to control β grain growth above the β transus by Zener pinning. First, both alloys were thermomechanically processed to generate a similar starting α texture and grain morphology. Subsequently, both materials were heat treated above the β transus up to 1250°C followed by furnace cooling to 210°C to promote diffusional phase transformation starting from β grain boundaries. In situ texture measurements were taken during α→β→α phase transformation starting at room temperature, 800°C, 950°C, above β transus (1050 and 1250°C), and back to near room temperature. The degree of variant selection was determined by comparing the predicted transformation texture during heating and cooling based on the Burgers relationship and the assumption of no variant selection with the measured textures. It was found that during heating β grows from the pre-existing β and that the β texture evolved even before the β transus was exceeded. The β texture strengthened noticeably above the β transus in the case of conventional Ti–6Al–4V but not Ti–6Al–4V–0.4Y, which was related to β grain coarsening. The level of variant selection was clearly affected by grain coarsening and the formation of β texture components that contribute to the 〈111〉//normal direction (ND) γ fibre texture rotated about 10° away from ND.

The influence of rolling temperature on texture evolution and variant selection during α → β → α phase transformation in Ti–6Al–4V

The role of starting texture in variant selection has been studied during α→β→α transformation in Ti–6Al–4V. By hot rolling at different temperatures followed by recrystallization, material with either a strong basal texture or a strong transverse texture was generated. Subsequently, both conditions were heat-treated above the β transus followed by slow cooling. The degree of variant selection was assessed by comparing the strength of the measured and predicted α texture from high temperature β texture, assuming equal occurrence of all possible variants during β→α transformation. It was found that, even though the material rolled originally at 800°C displayed a stronger α texture after β heat treatment, it was the material rolled originally at 950°C that showed greater variant selection. The variant selection mechanism is discussed in terms of the generated β texture and common 〈110〉 poles in neighbouring β grains selecting a similar α variant on both sides of the prior β grain boundary. Predictions of possible 〈110〉 pole misorientation distributions for the two investigated β textures showed that the combination of texture components generated during rolling Ti–6Al–4V at 950°C increases the likelihood of having β grain pairs with closely aligned (110) planes compared to rolling at 800°C. Therefore, it can be proposed that avoiding the generation of certain combinations of β texture components during thermomechanical processing has the potential for reducing variant selection during subsequent β heat treatment.

First-principles study of phase stability of Ti2N under pressure

A first-principles study of phase stability of various phases of Ti${}_{2}$N under normal conditions and as a function of pressure was carried out. Among the \ensuremath{\epsilon} and \ensuremath{\delta}\ensuremath{'}phases of Ti${}_{2}$N that are observed experimentally, \ensuremath{\epsilon}-Ti${}_{2}$N is the most stable. The \ensuremath{\delta}\ensuremath{'} phase can only exist at high temperature due to the soft acoustic modes at the $X$ point. The origin of the tetragonal structure of both the \ensuremath{\epsilon} and \ensuremath{\delta}\ensuremath{'} phases is supposed to be caused by the tetragonal local lattice distortion around a nitrogen vacancy. Based on the results of the total-energy and phonon-spectrum calculations at zero temperature, the following sequence of phase transformation in Ti${}_{2}$N under pressure is predicted: \ensuremath{\epsilon}-Ti${}_{2}$N (space group P4/mnm), $P$ $=$ 77.5 GPa \ensuremath{\rightarrow} Au${}_{2}$Te type (space group C2/m), $P$ $=$ 86.7 GPa \ensuremath{\rightarrow} Al${}_{2}$Cu type (space group I4/mcm). The present study shows that, to correctly predict relative phase stability, the peculiarities of the phonon spectra of the materials under investigation have to be properly accounted for.

Analysis of effects of reactant particle size on phase transformations in the Ti–Si–Cu system using differential thermal analysis and x-ray diffraction

Abstract

Effect of β grain growth on variant selection and texture memory effect during α → β → α phase transformation in Ti–6 Al–4 V

In the present study, the texture evolution and the role of β grain growth on variant selection during β→α phase transformation have been investigated in Ti–6 Al–4 V with and without 0.4wt.% yttrium addition. The aim of adding yttrium was to control β grain growth above the β transus by pinning grain boundaries with yttria. Both materials were first thermomechanically processed to generate similar starting microstructures and crystallographic textures. Subsequently, both materials were solution-heat-treated above the β transus followed by slow cooling to promote growth of the α lath structure from grain boundary α. Additional interrupted slow cooling experiments were carried out to identify the α lamellae that nucleate first from β grain boundaries. Detailed electron backscatter diffraction analysis was carried out and it was found that the β heat treatment did not generate new texture components although the intensities of the individual components changed dramatically depending on the alloy/β grain size. Variant selection was assessed by comparing measured α texture components with predicted α texture components based on the high-temperature β texture assuming equal variant selection. It was found that with increasing β grain size variant selection intensified favouring the {φ1, Φ, φ2} {90°, 30°, 0°} texture component. Interrupted cooling experiments revealed that α nucleates first on β grain boundaries that are formed by two β grains having a common (110) normal and that these α lamellae display almost exclusively a {φ1, Φ, φ2} {90°, 30°, 0°} orientation. Consequently, the dominance of this variant with increasing β grain size can be related to the relative free growth of this particular α texture component into an “empty” β grain.

Electronic structures and transformations of intermediate phases and intermetallic compounds of binary Ti–Al alloys

Effects of interstitial impurities on valence electron structures and phase transformation of Ti–Al alloys are analyzed using the average lattice and atom models of the empirical electron theory of solids and molecules (EET). The descendant degree of bond energy, melting point, and liquidus temperatures affected by interstitial impurities are calculated based on the bond energy formula of the EET. It is demonstrated that because of the effects of interstitial impurities, the atom states of the intermediate phases increase, and bond structures of intermetallic compounds become seriously anisotropic, which results in very complex phase transformations in intermediate Al content. It is also shown that the melting point and liquidus temperatures decrease due to interstitial impurities, and the average decreased degree can be estimated very well using the EET.

An empirical theoretical investigation into phase and structural behaviour in Ti–Al alloys

According to the average lattice and atom models of the empirical electron theory of solids and molecules (EET), effects of interstitial impurities on valence electron structures and phase transformation of Ti–Al alloys are analysed. Furthermore, the descendant degree of bond energy, melting point, and liquidus temperatures affected by interstitial impurities are calculated based on the bond energy formula of the EET. Moreover, the main controversial experimental results on the phase transformation in Ti–Al alloys are explained well. It is demonstrated that because of the effects of interstitial impurities, atom states increase, bond structures are seriously anisotropic, which results in very complex phase transformations in intermediate Al content. It is also shown that the melting point and liquidus temperatures decrease due to interstitial impurities, and the average decreased degree can be estimated very well using the EET.

Evolution of microstructure and texture in Ti–46Al–9Nb sheet material during tensile flow at elevated temperatures

Microstructure and texture evolution as well as phase transformations in Ti–46Al–9Nb sheet material tensile tested between 700 and 1000 °C have been analyzed. Up to ∼850 °C the microstructure shows characteristic deformation features such as mechanical twins and deformation bands. The texture, presented as inverse pole figures, contains pure deformation components, i.e. <001>, <111> and ∼<301>. Above 900 °C dynamic recrystallization becomes the dominant process. Bulged grain boundaries can clearly be distinguished in the microstructure of the tensile specimens, and the pure deformation orientation maxima <001> and <301> disappears from the texture map in favor of a new recrystallization component around <100>. Under constant temperature conditions the size of dynamically recrystallized grains is influenced by the applied strain rate. Under certain test conditions a α 2 → ω/B8 2 phase transformation can be observed. This process is enhanced by stress and depends strongly on temperature and time or strain rate.

In situsynchrotron high-energy X-ray diffraction analysis on phase transformations in Ti–Al alloys processed by equal-channel angular pressing

Mixtures of 47-Al and 53-Ti powders (atomic %) have been consolidated using back pressure equal-channel angular pressing starting with both raw and ball-milled powders. In situ synchrotron high-energy X-ray diffraction studies are presented with continuous Rietveld analysis obtained upon a heating ramp from 300 K to 1075 K performed after the consolidation process. Initial phase distributions contain all intermetallic compounds of this system except Al, with distribution maxima in the outer regions of the concentrations (alpha-Ti, TiAl(3)). Upon annealing, the phase evolution and lattice parameter changes owing to chemical segregation, which is in favour for the more equilibrated phases such as gamma-TiAl, alpha(2)-Ti(3)Al and TiAl(2), were followed unprecedentedly in detail. An initial delta-TiH(2) content with a phase transition at about 625 K upon heating created an intermediate beta-Ti phase which played an important role in the reaction chain and gradually transformed into the final products.

First-principles study of deformation-induced phase transformations in Ti–Al intermetallics

The structural phase stability and electronic properties of the Ti–Al intermetallic compounds were investigated by means of density-functional theory (DFT) calculations in a generalized gradient approximation. Through comparison of the calculated formation energies of the parent and product phases, an in-depth theoretical understanding of the deformation-induced γ ↔ α2 phase transitions observed previously in TiAl alloys was achieved. The formation energy plays an important role in evaluating the feasibility of these phase transformations during plastic deformation of TiAl alloys. In addition, the density of states (DOS) was also calculated and used to analyze the stability of Ti–Al intermetallic compounds. The reasons for the absence of the deformation-induced (DI)-α2 and DI-γ (L12) phases in underformed TiAl alloys were analyzed.

Calculations for displacive ω-phase transformations in Ti–Al alloys with Nb additions at finite temperature

We examine by means of first-principles calculations the bcc-like (bcc: body centered cubic) toω-like phase transformations in Ti–Al alloys with Nb additions at finite temperature. To simulatethe alloy we use different discrete atomic configurations in a six atom unit cell of the stoichiometryTi3Al2Nb. Calculated ground state energies show an instability in the ternaryTi3Al2Nb alloyagainst the ω structure type atomic displacement. To better understand the role of entropyin the stability/instability of these systems, the first-principles calculationsare extended to finite temperature by including various contributions to thefree energy. In particular, the vibrational free energy is calculated within aquasiharmonic approximation. It is shown that the bcc structure is stabilizedby the contribution of the low energy modes to the lattice entropy againstω type atomic displacements. We find that configurational entropy plays a major role in theω toB82 transformation. Calculated lattice parameters and transition temperatures are found to bein excellent agreement with experiment.

Self‐Propagating High‐Temperature Synthesis of Ti3SiC2: II. Kinetics of Ultra‐High‐Speed Reactions from In Situ Neutron Diffraction

In situ neutron diffraction data at 0.9‐s time resolution during the self‐propagating high‐temperature synthesis of Ti3SiC2 from stoichiometric 3Ti+SiC+C mixtures have been further analyzed to give the phase quantities during all five stages of the identified reaction [(i) preheating, (ii) α→β phase transformation in Ti, (iii) preignition reactions, (iv) intermediate phase formation, and (v) rapid nucleation and growth of Ti3SiC2]. The phase quantities thus determined have been used, in conjunction with temperatures estimated from lattice parameters and thermal expansion data, to determine the kinetic parameters for the nucleation and growth stage using a nonisothermal form of the Avrami kinetic equation. The analysis gave an Avrami exponent, n, close to 3 in agreement with nucleation and the observed two‐dimensional growth of Ti3SiC2 crystals. An activation energy of ∼45 kJ/mol was also estimated.

Classical potential describes martensitic phase transformations between theα,β, andωtitanium phases

A description of the martensitic transformations between the $\ensuremath{\alpha}$, $\ensuremath{\beta}$, and $\ensuremath{\omega}$ phases of titanium that includes nucleation and growth requires an accurate classical potential. Optimization of the parameters of a modified embedded atom potential to a database of density-functional calculations yields an accurate and transferable potential as verified by comparison to experimental and density-functional data for phonons, surface and stacking fault energies, and energy barriers for homogeneous martensitic transformations. Molecular-dynamics simulations map out the pressure-temperature phase diagram of titanium. For this potential the martensitic phase transformation between $\ensuremath{\alpha}$ and $\ensuremath{\beta}$ appears at ambient pressure and 1200 K, between $\ensuremath{\alpha}$ and $\ensuremath{\omega}$ at ambient conditions, between $\ensuremath{\beta}$ and $\ensuremath{\omega}$ at 1200 K and pressures above 8 GPa, and the triple point occurs at 8 GPa and 1200 K. Molecular-dynamics explorations of the kinetics of the martensitic $\ensuremath{\alpha}\text{\ensuremath{-}}\ensuremath{\omega}$ transformation show a fast moving interface with a low interfacial energy of $30\text{ }\text{meV}/{\text{\AA{}}}^{2}$. The potential is applicable to the study of defects and phase transformations of Ti.

Calculation of valence electron structures and melting points in Ti–Al alloys with interstitial impurities

According to the average lattice and atom model of the empirical electron theory (EET) of solids and molecules, effects of interstitial impurities on valence electron structures (VESs) and phase transformation of Ti–Al alloys are analysed. In particular, melting points or allotropic transformation in Ti–Al phase diagram affected by interstitial impurities are calculated using the bond energy formula of the EET. It is demonstrated that because of the effects of interstitial impurities, the atom states of VESs increase, and the chemical bonds are seriously anisotropic. Such changes in VESs and chemical bonds make the melting points decrease. The decrease in the melting points can be further estimated by the EET, which is basically consistent with the available experimental results.

Temperature effect on the elastic modulus, internal friction and related phase transformations in Ti–Nb–2%Al quenched alloys

Quenched alloys from the Ti–Nb system are of considerable scientific and applied practical interest due to properties such as superconductivity, shape memory effect and high damping capacity. In this work, Ti–Nb quenched alloys with 2 wt.% Al, and Nb content varying from 15 to 40% were investigated with respect to the effects on phase transformations, internal friction and elastic modulus caused by the temperature up to 800 °C. The results showed that the internal friction presented peaks and the elastic modulus displayed minimum values associated with the increasing temperature. These results were also correlated to phase transformations occurring with increasing temperature.

Pressure–composition isotherms and reversible hydrogen-induced phase transformations in Ti–6Al–4V

Reversible stepwise phase transformations induced by the addition and removal of hydrogen in pure Ti and Ti–6Al–4V (Ti64) were examined by means of pressure–composition (P–C) isotherm measurement from 550 to 700°C. Both Ti–H and Ti64–H systems exhibit two pressure plateaus in the P–C isotherms which indicate phase transformations. Phase transformations take place at lower hydrogen content and higher hydrogen pressure in Ti64 than in Ti due to the effect of substitutional elements Al and V. Upon hydrogenation, the Ti–H system exhibits the sequence of phase transformation α↔αH↔αH+βH↔βH↔βH+δ↔δ, whereas the Ti64–H system shows a sequence of α+β↔αH+βH↔βH↔βH+δ↔δ because of the presence of the original β phase. Partial phase diagrams for Ti–H and Ti64–H were established based on their P–C isotherms. Modification of microstructure of Ti64 occurs after cyclic hydrogenation. This can be ascribed to hydrogen-induced atomic migration during the sequential phase transformations.

Effects of boron on phase transformation of high Nb containing TiAl-based alloy

The effect of 0.7 at% boron on the phase transformation of Ti–46.5Al–8Nb during a range of different continuous cooling rates from the alpha single phase region has been investigated. In addition the microstructure of Ti–46.5Al–8Nb and Ti–46.5Al–8Nb–0.7B during water quenching at a range of different temperatures near the alpha transus temperature ( T α) has been examined. It has been found that the trend of the variation of microstructures with different cooling rates is the same in the two alloys varying from massive gamma (γ m) to lamellar, but boron addition increases the cooling rate to obtain these microstructures. During water quenching at different temperatures ( T 1 ≅ T α, T 2 < T α, and T 3 ≪ T α) boron addition has a very strong effect on the trend of the variation of microstructures.

Transformation behaviour of Ti–Ni and Ti–Ni–Cu alloy ribbons with nano Ti 2Ni particles

Ti–Ni and Ti–Ni–Cu alloy ribbons have been prepared by single roll melt spinning, and then martensitic transformation behaviours were investigated by means of transmission electron microscopy, differential scanning calorimetry and X-ray diffraction. In 51Ti–49Ni(at.%) alloy ribbons, Ti 2Ni particles of a size less than 50 nm with the coherent interface with the matrix were found, while they were not found in 50Ti–50Ni alloy ribbons. In 49Ti–46Ni–5Cu and 50Ti–45Ni–5Cu alloy ribbons, Ti 2Ni particles of size less than 30 nm with coherent interface with the matrix were found. Coherent Ti 2Ni particles induced the R phase transformation in Ti–Ni alloys, and consequently transformation occurred in two-stages, i.e., B2–R–B19. Coherent Ti 2Ni particles induced the B19 martensite in a Ti–45Ni–5Cu alloy, and consequently transformation occurred in two-stages, i.e., B2–B19–B19′. The change in transformation behaviour was ascribed to strain fields developed around coherent Ti 2Ni particles.

Formation of titanium hydride in Ti–6Al–4V alloy

Use of hydrogen as a temporary element in titanium alloys is an attractive approach for enhancing processability, and also for modification of microstructures and thereby improving final mechanical properties. In this article, microstructures, phases and phase transformations in Ti–6Al–4V alloy specimens containing 0.1, 0.3 and 0.5 wt.% hydrogen has been investigated using optical microscopy, XRD and TEM. It was shown that there are fcc δ titanium hydride plates precipitated in the specimens containing hydrogen 0.302 and 0.490 wt.%. Simultaneously, mass orthorhombic martensite α″ has been found in the specimens as well as δ hydride. One eutectoid transformation mechanism from β H to δ hydride and α based on diffusion transformation theory was presented.