Microstructure Evolution Of Ti Research Articles

Enhanced kinetics of microstructural evolution in Ti–6Al–4V through electropulsing treatment

The post-processing of Ti–6Al–4V, which has traditionally been conducted using furnace heat treatment (FHT), is crucial to attaining the optimum microstructure and mechanical performance. This study investigated the microstructural kinetics induced by the electropulsing treatment (EPT) of an extruded Ti–6Al–4V alloy from 50 to 400 s. The results were compared with the conventional microstructural evolution caused by FHT at the same temperature and duration. The remarkable energy efficiency enabled a rapid heating of EPT specimens. EPT enhances the kinetics of several aspects of microstructural evolution, such as phase transformation, static recrystallization, dislocation mobility, and the resultant suppression of grain growth. These phenomena arose from the promoted atomic diffusion due to the combination of the thermal and athermal effects of the electropulses. Grain refinement, amplified solid-solution hardening, and increased phase interfaces improved the microhardness of the EPT specimens. These results demonstrate the potential of EPT to facilitate favorable microstructural modifications in Ti–6Al–4V.

Hot deformation behaviors and dynamic recrystallization mechanism of Ti-35421 alloy in β single field

Hot deformation behaviors and microstructure evolution of Ti−3Al−5Mo−4Cr−2Zr−1Fe (Ti-35421) alloy in the β single field are investigated by isothermal compression tests on a Gleeble−3500 simulator at temperatures of 820−900 °C and strain rates of 0.001−1 s−1. The research results show that discontinuous yield phenomenon and rheological softening are affected by the strain rates and deformation temperatures. The critical conditions for dynamic recrystallization and kinetic model of Ti-35421 alloy are determined, and the Arrhenius constitutive model is constructed. The rheological behaviors of Ti-35421 alloys above β phase transformation temperature are predicted by the constitutive model accurately. The EBSD analysis proves that the deformation softening is controlled by dynamic recovery and dynamic recrystallization. In addition, continuous dynamic recrystallization is determined during hot deformation, and the calculation model for recrystallization grain sizes is established. Good linear dependency between the experimental and simulated values of recrystallized grain sizes indicates that the present model can be used for the prediction of recrystallized grain size with high accuracy.

Incipient dynamic recrystallization and adiabatic shear bands in Ti–7Al studied via in situ X-ray diffraction

The interconnected phenomena of dynamic recrystallization (DRX) and adiabatic shear bands (ASBs) are critical features of dynamic deformation and fracture in metals. Despite their importance, observations of these phenomena have largely been restricted to post-mortem analyses. Here we report on the first in situ X-ray diffraction (XRD) observations of DRX. By combining high strain rate loading via Kolsky bar with high speed X-ray diffraction, time resolved observations of microstructural evolution in Ti–7Al are made. XRD measurements are coupled with explicit finite element simulations to assess the stress, strain, and temperature dependence of the microstructural evolution. These observations reveal a continuous evolution of the microstructure with increasing plastic strain up to a maximum stress. At the maximum stress, the structure ceases evolving and the subsequent softening suggests the onset of shear banding. As a whole, these unique experiments definitively reveal the onset of DRX to be driven by accumulation of plastic strain and not the temperature rise associated with plastic deformation. The observed sequence of events further suggests DRX results in microstructural softening and precedes ASB formation.

Microstructure evolution and sintering kinetics of Ti(C,N)-based cermet

In this paper, the grain growth behavior of Ti(C,N)-based cermet, including the microstructure evolution and sintering kinetics in the sintering process at different temperatures for different holding time, was studied experimentally and theoretically. A comprehensive grain growth model for Ti(C,N)-based cermet was established based on the experimental data. Furthermore, different influencing factors of microstructure evolution during sintering for Ti(C,N)-based cermet were investigated systematically by a scanning electron microscope. The results showed that the sintering temperature and holding time have a significant effect on the microstructure evolution of Ti(C,N)-based cermet. The grain growth rate was the fastest at the temperature range of 1300°C to 1400°C. The grain growth rate was reduced slightly at higher temperatures. A comprehensive sintering kinetics model of Ti(C,N)-based cermet during the sintering process was obtained based on the experimental results via nonlinear regression method.

Hot deformation characteristics and microstructure evolution of Ti–5Al–3Mo–1.5V alloy

A detailed investigation was conducted to study the concurrent effect of temperature and strain rate on the microstructure evolution in Ti–5Al–3Mo–1.5V dual-phase Titanium alloy. By applying varying strain rates from $$10^{-3} $$ to 10 $$\hbox {s}^{-1}$$ between 1098 and 1298 K at an interval of 50 K, isothermal compression characteristics and microstructural changes were recorded. The sizes of globules, concentrated predominantly at the lamellar kinks, were found to be inversely proportional to the strain rate. Further, a dynamic material model was employed to assess and plot the processing map displaying the safe hot working regime. The apparent hot-working activation energy in the $$\alpha +\beta $$ and $$\beta $$ phase field was 636 kJ/mol and 379 kJ/mol, respectively. A higher activation energy than the self-diffusion threshold of the $$\alpha +\beta $$ and $$\beta $$ field was attributed to lamellae breakup and dynamic recrystallization in the respective phase fields. The microstructure analysis and identified softening mechanisms further helped in concluding the safe hot working regime to be 1248 K and 10 $$^{-3}$$ $$\hbox {s}^{-1}$$ .

Microstructural evolution in Ti–10Al–1Zr–1Mo–1Nb alloy under thermomechanical treatment in β-region

The influence of temperature and strain rate on microstructural evolution in near α-type Ti–10Al–1Zr–1Mo–1Nb alloy was studied under uniaxial compression in the β single-phase-region. Hot compression tests were conducted for up to 75% reduction at temperatures between 1273 K and 1333 K and strain rates between 0.01 s−1 and 5 s−1. The steady-state of the flow behavior revealed that dynamic recovery in the β single-phase was dominant in the microstructural evolution process, and few dynamically recrystallized grains were detected, except in the vicinity of the pre-existing β grain boundaries. Two types of configurations such as lamellar αL and acicular αAC microstructures were developed, which were consistent with the compressive deformation mechanism in the β-region. The αL microstructure consisted of α platelets that were developed via growth of a specific variant, whereas the αAC microstructure consisted of individual α platelets with a different variant in the prior-β matrix. The stress exponent and activation energy were evaluated to determine the deformation mechanism. The heterogeneous nucleation of individual α platelets in the cellular structure (sub-grains) without a specific orientation in the β matrix resulted in the αAC microstructure. In contrast, the substructure as uniformly distributed dislocations in prior-β grains under the viscous motion of dislocations promoted a single variant selection to form αL platelets in the transformation. When the viscous drag dislocation motion in the β-region is dominant, a large crystal rotation is detected in the acicular (side plate) αSP platelets in the vicinity of the prior-β grain boundary.

Influence of Controlled Cooling Rates During Thermal Processing of Ti 6% Al 4% V Alloys Using In-Situ Scanning Electron Microscopy

ABSTRACTWe describe experimental approaches to real time examination of the microstructural evolution of Ti 6%Al 4%V upon cooling from above the beta transus (~995 °C) while imaging in the scanning electron microscope. Ti 6%Al 4%V is a two phase, α+β titanium alloy with high strength and corrosion resistance. The β →α transformation on cooling can give rise to different microstructures and properties through various thermal treatments. Fully lamellar microstructures, bi-modal microstructures, and equiaxed microstructures can each be obtained by accessing different cooling rates upon the final treatment above the beta temperature, each resulting in uniquely enhanced material properties.Utilizing the capabilities of a heating/ tensile stage developed by Kammrath & Weiss Inc., are able to apply real-time imaging techniques in the scanning electron microscope to monitor the development of the microstructure. Annealing temperatures up to 1100 °C are attainable, with cooling rates ranging from 0.1 ° C per second to 3.3 °C per second. This has allowed us to directly observe the formation of lamellae at different annealing temperature/ cooling rate combinations to determine the lamellar microstructure width, separation, and colony size.

Constitutive behavior and microstructural evolution in Ti–Al–Si ternary alloys processed by mechanical milling and spark plasma sintering

Severe plastic deformation is closely related to the microstructural behaviors of intermetallics or grain refinement by mechanical milling and subsequent rapid sintering process, i.e. high energy ball milling and spark plasma sintering. In this study, the Ti–Al–Si ternary powders were synthesized to investigate their phase transformation by plastic deformation during the mechanical milling in each process time and composition. The sintered-bodies Ti–Al–Si ternary alloys were fabricated by spark plasma sintering among the rapidly consolidated powders, except for the distribution of single phases in accordance with the Al-melting. The microstructures of intermetallics in Ti–Al–Si ternary alloys were composed of titanium aluminides and titanium silicides, and even ternary compounds as a τ1-phase. In particular, the twining or stacking faults induced by severe plastic deformation were revealed through crystallographic patterns and microstructural evidence. These defects were caused by the slip system for TiAl3 (fcc) or Ti5Si3 (hcp) phases, depending on the grain refinement and generation of ambient intermetallics. To estimate the possibility of these defects, various approaches were taken to obtain experimental measurement of the twin probability and stacking fault energy derived from TEM and XRD analyses.

Phase transformation and evolution of dislocation structure in the β phase of Ti-17 alloy during hot deformation

The hot deformation behavior and microstructural evolution of Ti–5Al–2Sn–2Zr–4Mo–4Cr (Ti-17) alloy in the β-phase field were examined. Hot compression tests were conducted at temperatures ranging from 700 and 1100 °C and strain rates ranging from 10-3 to 1 s-1. The obtained true stress–true strain curves exhibited steady-state flow behavior at temperatures above the β-transus temperature (Tβ ~ 890 °C). The microstructural observations suggested that the dynamic recovery process represents a dominant mechanism for the β-phase of the alloy. In contrast, the flow softening was identified at 700 °C at a strain rate of 10–3 s–1. The formation of fine acicular α microstructures was noticed in such a deformation condition, indicating that dynamic phase transformation can occur in the metastable β-phase during hot deformation. Such a dynamic phase transformation also occurred preferentially at the initial β-grain boundaries as well as sub-boundaries that created within the βmatrix via dynamic recovery. The β-processing conditions were also examined by constructing processing maps based on the dynamic materials model (DMM).

Effect of alloying additions on work hardening, dynamic recrystallization, and mechanical properties of Ti–44Al–5Nb–1Mo alloys during direct hot-pack rolling

There is significant gap in the understanding of the effect of alloying elements, particularly V and B, on the dynamic recrystallization (DRX) behavior, and the corresponding microstructure evolution in Ti–44Al–5Nb–1Mo-(V, B) alloys during uniaxial hot compression. In this regard, we have studied here the relationship of work hardening rate (θ) and critical strain of dynamic recrystallization (εc) on strain, deformation parameters. The θ values showed typical dynamic recrystallization characteristics, and were sensitive to temperature and strain rate. Deformation mechanism involving twinning-dislocation interactions and phase transformation contributed to characteristics of θ. DRX occurred in Ti–44Al–5Nb–1Mo–2V-0.2B alloys even at higher rolling speed and at ambient temperatures over a wide range, which implied a wider processing window during the direct hot-pack rolling process. Crack-free sheets were directly obtained from the as-HIPed alloys at 1250 oC with rolling speeds of 0.3 m/s, and 10%–20% reduction per pass. Their microstructure, high-temperature tensile properties and fracture mechanisms are discussed. As rolling progressed, Ti–44Al–5Nb–1Mo alloy was characterized by residual lamellae with locally dissolved α2 laths, broadening γ laths, and fine sub-grains. Dislocation-induced DRX and phase transformation of α2/γ lamellae were the primary deformation modes. In contrast, on the addition of V and B, deformation twinning was activated. Multi-phase microstructure was characterized by equiaxed grains of ~ 5–20 μm. The as-rolled sheets of both the alloys exhibited superior performance at 800 oC. Ti–44Al–5Nb–1Mo alloy showed a higher ultimate tensile stress of ~689 MPa, while superior ductility of greater than 75% was obtained for Ti–44Al–5Nb–1Mo–2V-0.2B alloy. The microstructure with different degree of dynamic recrystallization contributed to various aspects of high-temperature tensile properties.

Multi-Length Scale Characterization of Microstructure Evolution and Its Consequence on Mechanical Properties in Dissimilar Friction Stir Welding of Titanium to Aluminum

In dissimilar friction stir welding (FSW), the weld nugget is composed of two elements, which are mechanically mixed. This microstructure helps enhance the mechanical properties when they are homogeneously mixed, and the particles are sub-micron in size. Therefore, it is important to understand the mechanism of the particle formation and their distribution for engineering the mechanical properties of the weld. In the present investigation, dissimilar FSW between commercial purity Al and Ti has been carried out. The weld nugget consisted of distributed Ti particles in an Al matrix. The distribution of the Ti particles in the weld nugget was characterized using X-ray micro-computed tomography (XCT). Microstructural evolution in Ti and Al was examined using a scanning electron microscope (SEM) equipped with an energy dispersive spectrometer (EDS), X-ray diffraction and electron back-scattered diffraction (EBSD). Hardness and tensile tests were carried out to determine the integrity of the welds. The XCT result shows that the weld nugget contains Ti particles of variable size. The finer particles of Ti are distributed homogeneously in the weld nugget, unlike large particles. The deformation mechanisms and microstructural evolution of the Ti interface and Al matrix are investigated using EBSD. It is observed that the microstructure of both Ti and Al is substantially refined. However, for a given grain in Al, the boundary is of mixed character, namely low- and high-angle boundaries. Hardness data of the weld indicate large variation within the nugget region. The tensile test sample revealed that the failure of the sample occurs on the Al side of the weld. The fractograph indicates ductile and brittle modes of fracture with a bimodal distribution of dimples at the surface. Lack of twining and fine grains (40 to 5 µm) at the Ti interface indicates high-temperature deformation. Deformation of Ti at low temperature and high strain rate is caused by adiabatic shear banding (ASB). In these shear bands, a high level of grain refinement is observed and is a path for easy crack propagation. It is proposed that the ASB-controlled deformation of Ti leads to a recrystallized microstructure at the interface and fragmentation of Ti. These Ti particles undergo further fragmentation to form smaller particles. On the other hand, microstructural evolution in Al is gradual because of the high stacking fault energy, which leads to continuous dynamic recrystallization (CDRX) through the dynamic recovery (DRV) mechanism. The mechanical properties of the weld depend on the characteristics of particles in the Al matrix. Proper control of the fragmentation and distribution of Ti particles and interface property can lead to superior mechanical properties of the weld.

Microstructure evolution and fracture behavior of innovative Ti-(SiCf/Al3Ti) laminated composites

The microstructure evolution and phase formation in novel Ti-(SiCf/Al3Ti) laminated composite were emphatically investigated via exploring the microstructures concerning different processing stages. The mechanical properties and fracture behavior of the composite were studied using compression, three-point bend (TPB) and single-edge notched beam (SENB) methods. The results indicated that the microstructure evolution of Ti-(SiCf/Al3Ti) laminated composite involved the solid-state and solid-liquid reaction of Ti and Al during hot pressing. In addition, Ti-(SiCf/Al3Ti) laminated composite generally possessed superior fracture toughness, compressive and bending strength than those of Ti-Al3Ti due to the addition of SiC fibers. The fiber breakage and interface debonding behavior was observed, confirming the expected toughening mechanisms associated with arrested and deflected behavior of the crack. Furthermore, when subjected to external bending and compressive stresses, the laminated composite displayed similar failure characteristic, containing the fragmenting of Al3Ti and cracking or buckling of Ti. Moreover, the main cracks oriented at 45° to layers in the bending and compressive specimens under perpendicular load. Nevertheless, the main crack carved all layers, propagating with a tortuous path in the edge-notched specimen along the loading direction. Additionally, the specimens were catastrophically fractured under compressive load and non-catastrophically fractured under bending load.

Reactive Synthesis of Polycrystalline Ti 3 AlC 2 and Its Sintering Behavior

A method for fabrication of ternary compound Ti 3 AlC 2 powder by pressureless calcining (PC) technique was developed. Powders of Ti, Al and TiC were mixed as staring materials. The effects of calcining temperature and aluminum content on the purity of Ti 3 AlC 2 were discussed. The reaction mechanism and the microstructure evolution of Ti 3 AlC 2 prepared at different temperatures were investigated by X-ray diffraction, field emission scanning electron microscopy and high resolution transmission electron microscopy. Results indicate that the single-phase polycrystalline Ti 3 AlC 2 powders are obtained at 1300 °C, which possesses a typical laminated structure, and the best molar ratio of raw Ti/Al/TiC materials is 1:1.2:2. The bulk samples sintered by hot pressing were from the Ti 3 AlC 2 powders. After hot pressing of different temperatures, the fully dense bulk Ti 3 AlC 2 shows the best properties when prepared at 1300 °C. The density of bulk Ti 3 AlC 2 reaches to 99.9%. The average Vickers hardness is around 5.7 GPa, and the maximum flexural strength is over 630 MPa. Finally the strengthening mechanism of the bulk Ti 3 AlC 2 was also proposed.

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.

Microstructure evolution and interfacial reaction of TiAl–Si alloy solidified in alumina crucible

The microstructure evolution of Ti–43Al–3Si (at-%) alloy solidified in alumina crucible was investigated by directional solidification technology. After directional solidification, the microstructure of the alloy is consisted of γ-TiAl, α2-Ti3Al, ξ-Ti5Si3 phases and Al2O3 particles. There are three morphologies of ξ phases formed in the alloy, namely, long rod-like, cluster-like with eutectic morphology, and needle-like shape. The volume fraction of ξ phases decreases with increasing growth rates. Al2O3 particles broke from the crucible and enter into the melt by the thermal physical erosion. Al2O3 particles enrich in the liquid phase with the moving of solid-liquid interface, and are captured or entrapped by dendrites during solidification. The Al2O3 particles mainly distributed in the interdendritic region, and some particles exist in dendrites.

Microstructural evolution of Ti–47Al–2Cr–2Nb–0.8B alloy prepared by semi-solid process

The refinement of large boride ribbons in the as-cast TiAl alloy is the essential issue for aerospace industry application, which is difficult to avoid by classical casting techniques. The present paper seeks to explore the possibility of the semi-solid process in manufacturing Ti–47Al–2Cr–2Nb–0.8B (at%) alloy. An important result is that, except forming a nondendritic globular structure, the semi-solid process also plays a crucial role in refining large borides for the TiAl alloys with boron. In the current alloy, the long ribbon borides can be successfully converted to fine, equiaxed particles with an average size of 2–5 μm. Furthermore, subsequent proper heat treatment is necessary to control and achieve a fine, nearly fully lamellar microstructure. The key microstructure-refinement mechanism is due to the fine borides obtained during semi-solid process, which act as the effective nucleation centers for the formation of interdendritic fine γ grains, providing the pinning effect on prior α-grain boundaries. These results prove that semi-solid process can be an ideal candidate for the microstructure refinement in TiAl alloys.

Microstructural evolution of cast TiAlNb intermetallics for third generation gas turbine components

The microstructural evolution of Ti–45·9Al–8Nb and Ti–45·9Al–8Nb–0·2C (at-%) alloys fabricated by centrifugal investment casting is studied. It is found that both Ti–45·9Al–8Nb and Ti–45·9Al–8Nb–0·2C are mainly composed of γ phase and a small amount of α2 phase, and the microstructure is mainly fully lamellar in C containing alloy but again the C free alloy is mainly nearly fully lamellar. Nb distributes homogeneously in the γ and α2 phases but is depleted in the interdendritic regions. The addition of C reduces the lamellar spacing of Ti–45·9Al–8Nb alloy. The mean interlamellar spacings between α2 and α2 of Ti–45·9Al–8Nb and Ti–45·9Al–8Nb–0·2C alloys are about 950 and 700 nm, and those between γ and α2 lamellae are about 320 and 220 nm respectively. Oil cooling from the α phase field after 1h treatment at 1360°C leads to massive transformation to γm for both Ti–45·9Al–8Nb and Ti–45·9Al–8Nb–0·2C alloys. Subsequent aging in the α+γ phase field results in α phase precipitates within the γm phase, which leads to convoluted microstructures. However, the C addition retards the precipitation of α2 phase.

Effect of 0.5wt.% hydrogen addition on microstructural evolution of Ti–6Al–4V alloy in the friction stir welding and post-weld dehydrogenation process

Abstract The α + β titanium alloy, Ti–6Al–4V, was thermohydrogen processed with 0.5 wt.% hydrogen and friction stir welded using a W–Re pin tool. Defect-free joints were obtained with proper parameters. Hydrogen was removed from the joint through a post-weld dehydrogenation process. The microstructures of the as-welded and dehydrogenated joints were examined. The effect of 0.5 wt.% hydrogen addition on microstructural evolution of Ti–6Al–4V alloy in the friction stir welding and post-weld dehydrogenation process was revealed.

Plastic flow and microstructural evolution in Ti–6Al–2Zr–1Mo–1V alloys during hot deformation

The hot deformation behaviour and microstructural evolution in Ti–6Al–2Zr–1Mo–1V alloys have been studied using isothermal hot compression tests. The processing map was developed at a true strain of 0·7 in the temperature range 750–950°C and strain rate range 0·001–10 s−1. The corresponding microstructures were characterised by means of a metallurgical microscope. Globularisation of lamellae occurring to a greater extent in the range 780–880°C and 0·001–0·01 s−1 had a peak power dissipation efficiency of 58% at about 850°C and 0·001 s−1. The specimens deformed in 750–880°C and 0·01–10 s−1 showed an instability region of processing map, whereas the specimens deformed in 880–950°C and 1–10 s−1 indicated three kinds of flow instabilities, i.e. macro shear cracks, prior beta boundary cracks and flow localisation bands.

Study of the dynamic recrystallization of Ti–6.5Al–3.5Mo–1.5Zr–0.3Si alloy in β-forging process via Finite Element Method modeling and microstructure characterization

By integrating the thermomechanically coupled simulation with the mathematically modeling of microstructure evolution using Finite Element Method (FEM), the study of the dynamic recrystallization (DRX) of Ti–6.5Al–3.5Mo–1.5Zr–0.3Si alloy in β-forging process is conducted. Through physical experiment, microstructure characterization and FEM-based microstructure modeling, the DRX behavior of the Ti-alloy in β-forging process is extensively explored. The effects of plastic deformation strain, strain rate and deformation temperature on the DRX of the Ti-alloy in terms of DRX volume fraction, DRX grain size and the average grain size are systematically investigated. The simulation results show that the increase of plastic deformation strain, deformation temperature, and strain rate contributes to the DRX of the alloy. The simulation and experimental results further reveal that the FEM-based microstructure evolution modeling is able to predict the DRX behavior and the microstructure evolution of Ti–6.5Al–3.5Mo–1.5Zr–0.3Si alloy in β-forging process.