Globularization Mechanisms Research Articles

On the microstructure evolution during isothermal low cycle fatigue of β-annealed Ti-6242S titanium alloy: Internal damage mechanism, substructure development and early globularization

The present work deals with the room and high temperature microstructure evolutions of a near alpha titanium alloy (Ti-6242S) during isothermal low cycle fatigue. The initial fully lamellar microstructure has been purposefully annealed to form α/β interface phase layers. The shearing and displacement of the β layers is identified as the main governing internal damage mechanism. This is phenomenal owning to the very low imposed strain; and is justified considering: (i) the large colony size and thin thickness of the β layers within the initial lamellar microstructure, (ii) the presence of the interface phase, and (iii) the intensified substructure development within the α and β phases. The shearing and displacement of the β layers lead to the segmentation and this is followed by initial stage of dynamic globularization even at room temperature condition. The globularization kinetic is intensified at higher temperature cyclic loading. In this regard, the sub-boundary induced boundary splitting/grooving is characterized as an involved mechanism. Interestingly, the early dynamic globularization of the α lamellae is also traced through the high temperature fatigued microstructures. The β phase penetration along the α phase sub-boundary is realized as the main globularization mechanisms. The interface phase layer stimulates the possibility of globularization via providing the high-energy non-coherent interfaces in structure. In addition, the deformation twinning within the interface phase, so called as “twinning induced micro-plasticity effect”, appears to have a significant effect on the fatigue damage behavior via compensation of incompatibility between α and β phases and deformation twin formation.

Dynamic Globularization Behavior of O-phase Lamellae in Ti-22Al-25Nb (at%) Alloy during Deformation at Elevated Temperatures

Abstract To clarify the globularization mechanism and model of the O-phase lamellae of Ti2AlNb-based alloys during deformation at elevated temperatures, the hot deformation behavior and microstructure evolution of Ti-22Al-25Nb (at%) alloy during hot compression deformation in (α2+O+B2) region were investigated. The results show that the flow softening is relevant to the dynamic globularization of O-phase lamellae and the flow instability including 45° shear deformation, 45° shear cracking and micro-cracks. The deformation constitutive model based on the exponential law was established and the deformation activation energy was calculated to be 831 kJ·mol−1. The globularization of O-phase lamellae is hard to occur at low temperatures or at high strain rates due to slow diffusion rate or insufficient diffusion time. The main globularization mechanism is the kink and shear of the O-phase lamellae, which is confirmed to be a dynamic recrystallization behavior. The dynamic globularization kinetics was also studied, which is sensitive to deformation conditions including deformation temperatures and strain rates and follows an Avrami type sigmoid equation.

Static globularization mechanism of Ti-17 alloy during heat treatment

The mechanisms of static globularization of Ti-17 alloy during heat treatment are investigated at 820 °C and 840 °C. Static globularization fraction increases with the increasing time and temperature during heat treatment. The quantitative analysis indicates that the process of static globularization can be divided into the two distinct stages. The first is a fast globularization stage and occurs at the beginning of heat treatment, and the second is a slow globularization stage and includes microstructure change during prolonged heat treatment. The first stage consist of the globularization of alpha lamellae via boundary splitting, whereas the second stage is controlled by microstructure coarsening including termination migration and Ostwald ripening. Boundary splitting is a process in which the distortion energy is rapidly released, and this process is finished via shearing surfaces caused by shear deformation and substructure generated by recovery. The duration is about 0.5–1 h. Termination migration and Ostwald ripening are the diffusion process of elements, and they are influenced by temperature and geometrical shapes of microstructure. The driving power comes from the energy gradient among the different positions in the same alpha particle or the different alpha particles. Termination migration leads to the separation and coarsening of the lamellar alpha and Ostwald ripening causes the decreasing of amount of particles and microstructure coarsening. Boundary splitting, termination migration and Ostwald ripening jointly promote static globularization process of Ti-17 alloy.

Obtaining bimodal microstructure in laser melting deposited Ti–5Al–5Mo–5V–1Cr–1Fe near β titanium alloy

Bimodal microstructures, consist of equiaxed primary α (αp) and lamellar secondary α (αs), usually lead to good comprehensive properties for wrought titanium alloys. However, for laser melting deposited titanium alloys, only lamellar microstructures are usually obtained, which result in relative low ductility. In this paper, to improve the ductility of laser melting deposited Ti–5Al–5Mo–5V–1Cr–1Fe titanium alloy, we try to introduce equiaxed α and obtain bimodal microstructures by heat treatments. Firstly, two kinds of heat treatment are applied to obtain equiaxed α i.e., subtransus anneal treatment and subtransus multi-anneal treatment. The subtransus anneal treatment is found to be able to promote α phase globularization, and the underlying mechanism is proposed by diffusion theory. However, it only leads to the elongated α phase with aspect ratio about 3.5. Then, inspired by the globularization mechanism, a novel subtransus multi-anneal treatment is designed, which can lead to near equiaxed α with the aspect ratio about 1.7. Afterwards, the subtransus multi-anneal and aging treatment are applied to obtain bimodal microstructure with near equiaxed αp and lamellar αs, which increases the elongation of the alloy to 11.5%, compared to 6.7% for the stress-relief anneal treated samples.

Hot deformation behavior and globularization mechanism of Ti-6Al-4V-0.1B alloy with lamellar microstructure

Hot deformation behavior and globularization mechanism of Ti-6Al-4V-0.1B alloy with lamellar microstructure were quantitatively studied through isothermal compression tests with the temperature range of 850–950 °C and strain rate range of 0.01–1.00 s−1. The results show that the peak flow stress and steady stress are sensitive to the strain rate and temperature. The value of deformation activation energy is 890.49 kJ·mol−1 in (α + β) region. Dynamic recrystallization is the major deformation mechanism. Flow softening is dominated by dynamic recrystallization at 850–950 °C. TiB particles promote the recrystallization of α laths. Globularization processes consist of four steps: formation of subgrain after dynamic recovery in α plates; subgrain boundary migration caused by α/β interfacial instability; interfacial migration promoting β phase wedge into α phase; disintegrating of α laths by diffusion processes; and grain-boundary sliding. Globularization mechanisms during hot deformation processes of the Ti-6Al-4V-0.1B alloy with lamellar structure are continuous dynamic recrystallization.

Evolution of globular microstructures during processing of aluminium slurries

Slurry processing experiments were performed with AlSi7Mg0.6 to identify the globularization mechanisms. The melt sample water quenched slightly above the liquidus point is predominantly dendritic while that cooled into the semi-solid temperature range internally via stirring the melt with a rotating cylindrical block of the alloy itself becomes fully globular. The globules are much smaller when internal cooling and stirring are employed longer to achieve higher solid fractions before casting. Coarse dendrite fragments of various sizes are revealed, in the case of stirring after an initial fraction of solid is first formed without the benefit of additional internal cooling.

An experimental study of deformation mechanism and microstructure evolution during hot deformation of Ti–6Al–2Zr–1Mo–1V alloy

Isothermal tensile tests have been performed to study the deformation mechanisms and microstructure evolution of Ti–6Al–2Zr–1Mo–1V titanium alloy in the temperature range 750–850°C and strain rate range 0.001–0.1s−1. The deformation activations have been calculated based on kinetics rate equation to investigate the hot deformation mechanism. Microstructures of deformed samples have been analyzed by electron backscatter diffraction (EBSD) to evaluate the influences of hot deformation parameters on the microstructure evolution and recrystallization mechanism. The results indicate that deformation mechanisms vary with deformation conditions: at medium (800°C) and high (850°C) temperature, the deformation is mainly controlled by the mechanisms of dislocation creep and self-diffusion, respectively. The microstructure globularization mechanisms also depend on deformation temperature: in the temperature range from 750 to 800°C, the high angle grain boundaries are mainly formed via dislocation accumulation or subgrain boundaries sliding and subgrains rotation; while at high temperature of 850°C, recrystallization is the dominant mechanism. Especially, the evolution of the recrystallization mechanism with the deformation temperature is first observed and investigated in TA15 titanium alloy.

Mechanism and kinetics of static globularization in TA15 titanium alloy with transformed structure

The static globularization behavior and mechanism of transformed structure during heat-treatment of hot worked TA15 alloy were investigated. It is found that boundary splitting and microstructure coarsening are two competing mechanisms for static globularization. Boundary splitting is significant in the initial stage of annealing while coarsening occurs throughout the annealing process. Static globularization kinetics increases with annealing temperature and prestrain, but is independent on strain rate. The rate of static globularization kinetics decreases with annealing time. The asymptotic equation can be used to model the static globularization kinetics.

Optimization of hot process parameters of Ti–6.7Al–2Sn–2.2Zr–2.1Mo–1W–0.2Si alloy with lamellar starting microstructure based on the processing map

Isothermal compression of BT25 titanium alloy with lamellar starting microstructure was carried out in the deformation temperatures ranging from 940 to 1000 °C, strain rate between 0.01 and 10 s−1, and the height reduction of 60%. The hot deformation behavior of BT25 alloy was characterized based on the analysis of the stress–strain behavior, kinetics and processing map for obtaining optimum processing windows and achieving desired microstructure during hot working. The apparent activation energy of deformation was calculated to be 545.4 kJ/mol and constitutive equation that describes the flow stress as a function of the strain rate and deformation temperature was proposed for high temperature deformation of the alloy. The processing map was constructed to evaluate the efficiency of the forging process in the temperatures and strain rates investigated and to recognize the instability regimes. The processing map exhibits two domains with the peak efficiencies of 36–40% occur at the strain rate of 0.01 s−1, the temperatures of 960 and 1000 °C, respectively. The domain (at 960 °C/0.01 s−1) is the one in which the mechanism of globularization mainly operates and the power is dissipated by the globularization of α lamella. The other domain (at 1000 °C/0.01 s−1) is the one in which the mechanisms of globularization and α + β → β phase transformation work simultaneously. The both domains are all the optimum processing regions. Plasticity instability was expected in the regime of strain rate higher than 1.0 s−1 and entire temperature range.

Static Globularization of TC11 Alloy during Hot Working Process

Abstract The microstructure evolution of TC11 alloy with lamellar structures deformed at temperatures of 980–850°C and strain rates of 0.001, 0.01 and 0.1 s−1, and the static structure evolution and mechanism of the alloy annealed at different temperatures were investigated. The results show that dynamic recrystallization process at higher reduction, higher temperature and lower strain rate is almost complete, and the globularized fraction of the annealed microstructure mainly depends on the dynamic recrystallization during deformation. But the dynamic recrystallization process at lower reduction, lower temperature and higher strain rate is restrained, and the globularized fraction of the annealed structure depends on both of the deformation state and the static recrystallization process during annealing. α-grain size mainly depends on annealing temperature and holding time mainly. EBSD tests of dynamic and static globularized microstructures show that the mechanism of static globularization during annealing involves the static recrystallization of β-phase and the static recovery of α-phase, and subsequent globularization and coarsening.

Heat-treated microstructure and mechanical properties of laser solid forming Ti-6Al-4V alloy

The effects of heat treatment on the microstructure and mechanical properties of laser solid forming (LSF) Ti-6Al-4V alloy were investigated. The influences of the temperature and time of solution treatment and aging treatment were analyzed. The results show that the microstructure of LSFed samples consists of Widmanstatten α laths and a little acicular in columnar prior β grains with an average grain width of 300 µm, which grow epitaxially from the substrate along the deposition direction (Z). Solution treatment had an important effect on the width, aspect ratio, and volume fraction of primary and secondary α laths, and aging treatment mainly affects the aspect ratio and volume fraction of primary α laths and the width and volume fraction of secondary α laths. Globular α phase was first observed in LSFed samples when the samples were heat treated with solution treatment (950°C, 8 h/air cooling (AC)) or with solution treatment (950°C, 1 h/AC) and aging treatment (550°C, above 8 h/AC), respectively. The coarsening and globularization mechanisms of α phase in LSFed Ti-6Al-4V alloy during heat treatment were presented. To obtain good integrated mechanical properties for LSFed Ti-6Al-4V alloys, an optimized heat treatment regimen was suggested.

Effect of Preheating Temperature of Multiplex-Type Slurry Cup on the Morphology of Primary Solid Phase of Semi-Solid Slurry

Semi-solid slurry for a rheocasting process requires a fine and globular structure of primary solid phase. In the present study, in order to find an optimum preheating temperature of a multiplex-type slurry cup in the slurry maker that we recently developed for the rheocasting process, the semi-solid slurry was held for a various time of up to 60 min. in a multiplex-type slurry cup preheated at a different temperature. A size and morphology of primary solid phase of the semisolid slurry was measured and analyzed with the difference of the recorded cooling curve during the solidification with respect to a preheating temperature of the slurry cup. Also, a microstructural evolution and globularization mechanism occurred during the isothermal holding were discussed. Finally, as considering the effect of a different casting condition on the morphology of primary solid phase the optimum manufacturing condition of the semi-solid slurry for the rheocasting process was proposed.

Reheating process of cast and wrought aluminum alloys for thixoforging and their globularization mechanism

A thixoforging process has numerous advantages compared to die casting, squeeze casting, and conventional forging. For the thixoforging process, beside a co-existing solidus–liquidus interval, the reheating conditions to obtain a fine globular microstructure are very important. During reheating, the eutectic must be remelted completely in order to obtain good mechanical properties. The solid fraction distribution over temperature, the phase distribution and the morphology of structure are important for the reheatability of SSM. Moreover, the reheating of the billet in the semi-solid state as quickly and homogeneously as possible is one of the most decisive aspect. To obtain the globular microstructure in the cross-section of the billet, the optimal design of the induction coil for variations of alloys and specimen sizes is necessary. Thus in this study, the optimal coil design of A356 (ALTHIX) and Al2024 with d× l=60 mm×90 mm to reduce the temperature gradient of the billet and to obtain a globular microstructure was theoretically proposed, and then the coil was manufactured. The suitability of the coil design will be demonstrated by reheating experiments. The microstructure characteristics were investigated by changing the heating time, the heating temperature, the holding time, and the heating capacity.