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 〈1 1 1〉//normal direction (ND) γ fibre texture rotated about 10° away from ND.     

Metastable phase formation in Ti–Al–Nb undercooled melts

Metastable phase formation processes in ternary Ti–Al–Nb alloys were studied by containerless electromagnetic levitation for melt undercooling up to 300 K below the liquidus temperature. Dendrite growth velocities of 15–25 m s⁻¹ for highly undercooled Ti–Al–Nb melts were consistent with primary β-phase formation, which is promoted by Nb addition. From double-recalescence events in Ti₄₀Al₅₀Nb₁₀ and Ti₄₅Al₅₀Nb₅ melts beyond a critical undercooling a subsequent β to α phase transformation in the semi-solid state was inferred. A second recalescence near 1300 °C, which was attributed to an α to γ solid state transformation, was observed in the pyrometer trace for the Ti₄₅Al₅₀Nb₅ and Ti₄₀Al₅₀Nb₁₀ alloys. The γ phase formation was suppressed in favour of a homogeneous α₂ phase in undercooled Ti₄₅Al₄₅Nb₁₀ samples quenched onto a chill substrate, whereas in Ti₄₀Al₅₀Nb₁₀ high undercooling enabled a direct γ phase solidification.     

In situ observation of texture evolution during α → β and β → α phase transformations in titanium alloys investigated by neutron diffraction

Texture changes during recrystallization and the α–β–α phase transformation in two titanium alloys were investigated in situ by time-of-flight neutron diffraction by heating in a vacuum furnace to 950 °C. In commercially pure titanium, a strong texture memory effect is observed. This effect is a direct consequence of an orientation-selective α → β transformation, favoring new orientations produced during nucleation and grain growth. The β–α transformation favors β orientations with minimal misorientations, resulting in a strong final α texture that emphasizes the grain growth component. In Ti–6Al–4V, the texture memory effect is less pronounced. The high-temperature β texture is obtained by growth of pre-existing β nuclei. In a similar way, during cooling, the growth of α domains is controlled by high-temperature α orientations inherited from the β grains with Burgers orientation relation.     

Deformation-induced γ ↔ α2 phase transformation in TiAl alloy compressed at room temperature

Deformation-induced γ ↔ α₂ phase transformation in a Ti–47Al–2Cr–2Nb–0.2Y (at.%) alloy compressed at room temperature was investigated by high-resolution transmission electron microscopy (HREM) and energy dispersive spectrum (EDS). The deformation-induced (DI) γ → α₂ phase transformation occurred in a twin intersection region. On the other hand, the deformation-induced α₂ → γ transformation nucleated at the stacking faults of α₂ phase. The composition analysis of the γ and α₂ laths by EDS suggested that composition of some laths deviated much from their equilibrium values. These composition deviations promoted the deformation-induced γ ↔ α₂ phase transformation to occur; EDS results also suggested that there was no composition difference between the DI-γ plate and the primary γ phase. Based on the HREM and EDS experimental results, the mechanism of the deformation-induced γ ↔ α₂ phase transformation has been discussed.     

Phase equilibria in the Ni-rich portion of the Ni–Ga binary system

The phase equilibria in the composition range from 0 to 60 at% Ga of the Ni–Ga system were determined by electron probe microanalysis (EPMA) using diffusion couples, differential scanning calorimetry (DSC) and X-ray diffraction (XRD). It was found that while the phase equilibria between the α′ (L1₂: Ni₃Ga) and α (Ni-solid solution) or β (B2: NiGa) phases are basically in agreement with the diagram evaluated by Lee and Nash, those between γ (B8₁: Ni₁₃Ga₇), δ (Cmmm: Ni₅Ga₃) and ɛ (C2/m: Ni₁₃Ga₉) are topologically different from that diagram. Three eutectoid reactions (γ → δ + ɛ, β → γ + ɛ, β → α′ + γ) and one peritectoid reaction (α′ + γ → δ) were confirmed and the temperatures and concentrations of those invariant reactions were determined.     

A novel ternary eutectic in the Nb–Al–Ni system

A novel ternary eutectic reaction L → Nb₂Al + Al₃Nb + AlNbNi occurs at the composition of Al–40.4Nb–2.42Ni and the temperature of 1553.6 °C. The third phase is an Al-rich AlNbNi phase with the composition Al–33.3Nb–12.3Ni. Ternary Nb–Al–Ni eutectic exhibits regular microstructure with the predominantly fibrous morphology.     

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 〈1 1 0〉 poles in neighbouring β grains selecting a similar α variant on both sides of the prior β grain boundary. Predictions of possible 〈1 1 0〉 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 (1 1 0) 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.     

Nucleation and variant selection of secondary α plates in a β Ti alloy

The microtexture of secondary α plates in Ti–4.5Fe–6.8Mo–1.5Al has been investigated by electron backscatter diffraction (EBSD) to obtain more insight in the nucleation and variant selection of these α plates. A statistical analysis of the EBSD data shows that for most β grain boundaries the variant selection of the α plates is in agreement with a commonly used variant selection criterion yielding that the α-{0 0 0 1} pole is nearly parallel to the closest β-{1 1 0} poles of the two adjacent β grains. For a small angle between the β-{1 1 0} poles nucleation is predominantly observed at both sides of the grain boundary, while with increasing angle some β grain boundaries exhibit nucleation of α plates at only one side. In the β grain interior many so-called Type 2 α–α grain boundaries are observed which are thought to originate from autocatalytic nucleation when a new α plate is formed at an existing α–β interface.     

The effect of β phase on microstructure and texture evolution during thermomechanical processing of α + β Ti alloy

Microstructure and texture evolution have been investigated in both α and β phases during the hot rolling of β-quenched Ti–6Al–4V at 800 and 950 °C, followed by annealing at 950 °C and air cooling using detailed electron backscattered diffraction mapping. The textures of primary and secondary α in the bi-modal microstructure were analysed separately, and the high-temperature β orientations were calculated by a variant based reconstruction from the inherited α_s orientations. Crystal plasticity finite element modelling has been employed to predict the rolling texture based on common α phase slip systems and compare with the measured α texture. It was found that despite the severe deformation during rolling, a large proportion of the primary α grains retain a Burgers relationship with the β phase. Consequently, the β phase in combination with a variant selection mechanism seems to control the α texture, which explains the discrepancy between predicted and measured rolling textures. The consequence of this mechanism for macrozone formation is also discussed.     

β Phase decomposition in Ti–5Al–5Mo–5V–3Cr

The decomposition of the β phase during rapid cooling of the near β titanium alloy Ti–5Al–5Mo–5V–3Cr has been studied using in situ X-ray synchrotron diffraction combined with ex situ conventional laboratory X-ray diffraction and transmission electron microscopy (TEM). Evidence is found supporting the suggestion by De Fontaine et al. (Acta Mater. 1971;19) that embryonic ω structures form by the correlation of linear (1 1 1)β defects at high temperatures. Further cooling causes increased correlation of these defects and the formation of athermal ω structures within the β matrix at temperatures ～500 °C. Post-quench aging at 570 °C resulted in the nucleation of α laths after ～90 s at temperature, with the laths all initially belonging to a single variant type. Aging for 30 min produced an even distribution of α precipitates with a lath morphology ～1.5 μm × 0.2 μm in size composed of both the expected Burgers variants. Mechanical property data suggests that the ω structures alone have no real effect; however, hardness increases were observed as the α phase developed. The utilization of thermal regimes similar to those presented in this paper could offer a method to engineer the α phase in near β titanium alloys and hence control mechanical properties.     

Study of the role of Ta and Cr additions in the microstructure of Nb–Ti–Si–Al in situ composites

Niobium silicide-based in situ composites are Nb-base alloys with high Si content that have the potential for higher temperature capability than the Ni-base superalloys. Microstructure-property studies of these alloys have been the subject of many research programmes, where the differentiation between the αNb₅Si₃ and βNb₅Si₃ is usually not clear, even though it is essential to understanding the solidification of the alloys and the stability of their microstructures at high temperatures. In this work, the effects of Cr (5 or 8 at.%) and Ta (6 at.%) in the microstructures of as-cast and heat-treated Nb–24Ti–18Si–5Al in situ composites have been studied. The main phases observed in the as-cast and heat-treated (100 h at 1400 or 1500 °C) alloys were the niobium solid solution, (Nb,Ti)_ss, the niobium 5–3 silicides, αNb₅Si₃ and βNb₅Si₃, and a Cr-rich C14 silicide Laves phase. During solidification, Al additions promoted the formation of βNb₅Si₃, while the Cr additions caused the appearance of the C14 silicide Laves phase that was probably formed congruently from the remaining liquid. During heat treatment, the βNb₅Si₃ phase transformed to αNb₅Si₃ according to the reaction βNb₅Si₃→αNb₅Si₃+(Nb,Ti)_ss. The Cr addition lowered the melting temperature of the alloys as liquation was observed after 100 h at 1500 °C in the two Cr-rich alloys. Ta and Cr retard the βNb₅Si₃→αNb₅Si₃+(Nb,Ti)_ss transformation. Solid state diffusion was sluggish in the presence of Ta, but the Ta addition did not destabilize the three-phase equilibrium among (Nb,Ti)_ss, αNb₅Si₃ and the C14 silicide Laves phase, in the Nb–24Ti–18Si–6Ta–8Cr–4Al alloy.     

Variant selection during α precipitation in Ti–6Al–4V under the influence of local stress – A simulation study

Variant selection of α (hexagonal close-packed, hcp) phase during its precipitation from β (body-centered cubic, bcc) matrix plays a key role in determining the microstructural state and mechanical properties of α/β titanium alloys. In this work, we develop a three-dimensional quantitative phase field model to predict variant selection and microstructural evolution during β → α transformation in Ti–6Al–4V (wt.%) under the influence of both external and internal stresses. The model links its inputs directly to thermodynamic and mobility databases, and incorporates the crystallography of bcc to hcp transformation, elastic anisotropy and defects within semi-coherent α/β interfaces in its total free energy formulation. It is found that, for a given undercooling, the development of a transformation texture (also called microtexture) of the α phase due to variant selection during precipitation is determined by the interplay between externally applied stress or strain and internal stress generated by the precipitation reaction itself. For example, the growth of pre-existing α precipitates is accompanied by selective nucleation and growth of secondary α plates of certain variants that may not be the ones preferred by the initially applied stress. Possible measures to reduce transformation texture are discussed.     

Grain refinement in γ-TiAl-based alloys by solid state phase transformations

The colony size of a fully lamellar Ti–46Al–2Cr–2Mo–0.25Si–0.3B ingot was refined from 120 to 30–65 μm by well defined heat-treatments which exploit the suppression of the α → α + γ transformation or involve cyclic annealing around the α-transus temperature. In addition, an extremely fine lamellar spacing in the range of 30–40 nm was obtained. For coarse-grained fully lamellar Ti–46Al–9Nb a massive phase transformation was used for microstructural refinement. The thermal stability of the massively transformed material was tested by annealing treatments and characterized by hardness measurements and the variation of the c/a-ratio of the tetragonal γ-TiAl cell as obtained from X-ray diffraction. After annealing at 1200 °C α₂-Ti₃Al lamellae appear within the former massively transformed γ-TiAl grains parallel to all four (111)_γ-planes causing an increase in hardness.     

The kinetics of Nb(C,N) precipitation during the isothermal austenite to ferrite transformation in a low-carbon Nb-microalloyed steel

The kinetics of Nb(C,N) precipitation occurring during the isothermal ferritic (α) transformation were quantitatively measured, along with the transformation kinetics at intercritical temperatures ranging from 710 to 790 °C in a Nb-microalloyed steel by means of electrical resistivity and dilatometry. The precipitation occurred most rapidly at 750 °C, which corresponds to a bay temperature on the start curve of the ferritic transformation of the transformation–time–temperature diagram. While interphase precipitation was observed at and above the bay temperature, precipitation in the α matrix was predominantly below the bay temperature. However, precipitation in the untransformed austenite (γ) matrix during the ferritic transformation was also observed, regardless of the intercritical temperatures. It is suggested that the precipitation occurring in the untransformed γ matrix during the ferritic transformation was accelerated owing to carbon enrichment from the α matrix to the γ matrix during the ferritic transformation. The average size of Nb(C,N) particles observed in the α matrix was slightly larger than that of the γ matrix at a given intercritical temperature. This result is proposed to arise primarily from the rapid diffusion of solute Nb atoms in the body-centered cubic α matrix.     

Shape memory properties of Ti–Nb–Mo biomedical alloys

Mo is added to Ti–Nb alloys in order to enhance their superelasticity. The shape memory properties of Ti–(12–28)Nb–(0–4)Mo alloys are investigated in this paper. The Ti–27Nb, Ti–24Nb–1Mo, Ti–21Nb–2Mo and Ti–18Nb–3Mo alloys exhibit the most stable superelasticity with a narrow stress hysteresis among Ti–Nb–Mo alloys with Mo contents of 0, 1, 2 and 3 at.%, respectively. The ternary alloys reveal better superelasticity due to a higher critical stress for slip deformation and a larger transformation strain. A Ti–15Nb–4Mo alloy heat-treated at 973 K undergoes (2 1 1)〈1 1 1〉-type twinning during tensile testing. Twinning is suppressed in the alloy heat-treated at 923 K due to the precipitation of the α phase, allowing the alloy to deform via a martensitic transformation process. The Ti–15Nb–4Mo alloy exhibits stable superelasticity with a critical stress for slip deformation of 582 MPa and a total recovery strain of 3.5%.     

Investigation of early stage deformation mechanisms in a metastable β titanium alloy showing combined twinning-induced plasticity and transformation-induced plasticity effects

As expected from the alloy design procedure, combined twinning-induced plasticity and transformation-induced plasticity effects are activated in a metastable β Ti–12 wt.% Mo alloy. In situ synchrotron X-ray diffraction, electron backscatter diffraction and transmission electron microscopy observations were carried out to investigate the deformation mechanisms and microstructure evolution sequence. In the early deformation stage, primary strain/stress-induced phase transformations (β → ω and β → α″) and primary mechanical twinning ({3 3 2}〈1 1 3〉 and {1 1 2}〈1 1 1〉) are activated simultaneously. Secondary martensitic phase transformation and secondary mechanical twinning are then triggered in the twinned β zones. The {3 3 2}〈1 1 3〉 twinning and the subsequent secondary mechanisms dominate the early-stage deformation process. The evolution of the deformation microstructure results in a high strain-hardening rate (～2 GPa), bringing about high tensile strength (～1 GPa) and large uniform elongation (>0.38).     

Texture inheritance and variant selection through an hcp–bcc–hcp phase transformation

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 a common zirconium alloy. In contrast to recently reported results by other authors, there is clear evidence of a strong variant selection occurring during the α → β transformation and a weaker variant selection during the β → α transformation. Modeling of the texture change associated with the phase transformation has been successful in describing the variant selection occurring during the α → β transformation, and gives insights into the selection occurring during the β → α transformation. The results are relevant to processing of both zirconium and titanium.     

Flow field and its effect on microstructure in cold crucible directional solidification of Nb containing TiAl alloy

A 3D finite element model was established to investigate the flow field in TiAl melt under different process parameters that include the position of the solidification interface, the meniscus height, the heating power and the current frequency. A square cold crucible with an inner cross section of 36 mm × 36 mm was employed to directionally solidify Ti–46Al–6Nb–B (at.%) ingots. The flow pattern in the melt is highly parameter-related. Generally, there are two flow swirls with opposite directions in the melt. The flow near the solidification interface can be minimized with an optimal meniscus height. The flow velocity is decreased with a decrease of heating power and an increase of current frequency. A weaker lateral flow near the solidification interface is beneficial for the continuous growth of columnar grains. The microsegregation of a directionally-solidified ingot can be reduced by controlling the flow in the melt, which is one of the advantages of cold crucible directional solidification.     

Crystallographic variant selection in α–β brass

The transformation texture of α/β brass with a diffusional Widmanstätten α growth morphology has been investigated. Electron micrographs and electron backscattered diffraction was used to determine that the orientation relationship between the β phase and the α associated with nucleation at β grain boundaries was 44.3° 〈1 1 6〉. Crystallographic variant selection was observed across those prior β/β grain boundaries, but this has little effect on the transformation texture due to the crystal symmetry. The effect of the crystallographic variant selection on texture is further weakened by nucleation of diffusional transformed α in the grain interior.