Two-phase Titanium Alloy Research Articles

Unified modeling of work hardening and flow softening in two-phase titanium alloys considering microstructure evolution in thermomechanical processes

One of the most critical aspects in understanding the deformation behavior of two-phase titanium alloys subjected to thermomechanical processes (TMPs) lies in being able to describe the flow stress accurately. To this end, in this study, initially, hot tension tests were conducted on a two-phase Ti-6Al-2Zr-1Mo-1V alloy. It was observed that the flow stress at a given temperature and strain rate exhibits work hardening, followed by flow softening. Flow softening occurs at the peak strain, where the maximum stress is observed; peak strain decreases with increasing temperature and decreasing strain rate. Variations in peak strain are more obvious at low temperatures and high strain rates. Later, the microstructure of the alloy was analyzed and the results show that work hardening and flow softening are caused by dynamic recrystallization (DRX). The DRX volume fraction was found to exhibit an increasing trend and discontinuous dynamic recrystallization (DDRX) was observed at increasing temperature and decreasing strain rate. With respect to microstructure evolution, a unified model consisting of a thermally activated stress component and an athermal stress component was developed. In the athermal stress term, dislocation density and the Hall-Petch effect were used to describe the work-hardening and flow-softening behavior. In the case of the dislocation term, the DRX effects were modeled considering the critical strain for DRX initiation and the DRX rate, which are both temperature-and strain rate-dependent. In the Hall-Petch effect term, the dependence of the Hall-Petch coefficient on the processing conditions was considered and the loss of Hall-Petch strengthening with deformation was modeled. Using the proposed model, the work-hardening and flow-softening behavior and microstructure evolution in Ti-6Al-2Zr-1Mo-1V alloys subjected to TMP were predicted. A good agreement could be observed between the experimental and predicted results. This study provides a solution for modeling work-hardening and flow-softening behavior and helps us understand the deformation behavior of two-phase titanium alloys subjected to TMP.

Effect of Microstructural Features of Titanium Alloys on Fatigue Properties and Mechanism of Crack Nucleation in Region of Very High Cycle Fatigue

This work considers the problem of subsurface fatigue crack nucleation and decrease in fatigue properties in two-phase titanium alloys at very high cycle fatigue (VHCF). Analysis of microstructure of alloy VT3-1 obtained by two different technological processes, punching and extrusion, is performed. It is shown that main features of the microstructure of alloy VT3-1 are inhomogeneity of size and shape of the structural elements of the α phase. Agglomerations of globular structural elements of the α phase as well as “macrozones” of acicular plates are found for the forged alloy. As for the extruded VT3-1 it is shown that the inhomogeneity of the microstructure is connected with the regions where acicular plates of the α phase are formed within the primary β grains. In this case, their direction does not coincide with the direction of extrusion. Nonmetallic inclusions are found neither in the forged nor in the extruded titanium alloy VT3-1.

Wear resistance of two-phase titanium alloy after deformation-diffusion treatment

Abstract The influence of deformation-diffusion treatment including a combination of the previous cold surface plastic deformation (ball burnishing) and subsequent thermochemical treatment (nitriding), on the tribological behavior of the Ti–6Al–5Mo–5V–1.5Cu–1Fe two-phase titanium alloy in a pair with Cu–10Al–4Ni–4Fe bronze alloy was studied. According to the results of the tribological test, SEM and EDX analyses of the wear surfaces, wear mechanisms (or processes) those of tribo-pairs in the conditions of boundary lubrication were established. It is shown that deformation-diffusion treatment of titanium alloy provided higher tribotechnical parameters (wear resistance, friction coefficient, temperature near the friction zone) of investigated tribo-pairs compared to traditional treatments (ball burnishing or nitriding).

Influence of alpha/beta processing on fracture toughness for a two-phase titanium alloy

Effect of alpha/beta processing on fracture toughness of Ti-17 alloy is investigated. For this purpose, Ti-17 alloy is deformed to 0, 30% and 50% height reductions at 820 °C, subsequently heat treated for 820 °C/4 h/WQ (water quenching) + 630 °C/8 h/AC (air cooling). The interrelationship of fracture toughness and microstructure is evaluated, and the fracture mechanism is analyzed. The results show that beta particles are stretched along the metal flow direction as alpha/beta deformation, and the aspect ratios for microstructures with 0, 30% and 50% deformations are 1, 2 and 4, respectively. Alpha phase presents an almost perfect basket-weave structure for undeformed material, whereas the globularized structure can reach 25% and 60% for material of 30% and 50% deformation. Fracture toughness exhibits a decreasing trend as the increasing deformation degree, which is opposite with change law of the globularization fraction. The lamellar alpha can increase the tortuosity of the crack propagation path. For undeformed material, the fracture surface is characterized by large amplitude of ravines with steep ups and downs, and a long crack propagation path is created. In this case, the highest fracture toughness is obtained. As alpha/beta processing, the fracture surface is smoother, the steep ups and downs disappear, and a shorter crack path is created. This results in decreased fracture toughness of material. The tortuosity ratios L(ε)/L0(ε) are about 1.45, 1.23 and 1.15 for 0, 30% and 50% deformations, respectively. In addition, the prediction model of fracture toughness considering tensile properties is established based on the Griffith-Orowan-Irwin relation, and it can provide a relatively reliable prediction for fracture toughness of Ti-17 alloy during alpha/beta processing.

Achieving fine-grained equiaxed alpha via thermo-mechanical loading under off-equilibrium state in two-phase Ti-alloys

An off-equilibrium hot deformation scheme was proposed to achieve fine-grained equiaxed alpha (αs) for two-phase titanium alloys. Controlled cooling and deformation was carried out at the soaking temperatures of 950–990 ℃,cooling rates of 50–140 ℃/min and strain rates of 0.01–0.1 s−1 in TA15 alloy. The results showed that fully fine-grained equiaxed αs (∼500 nm) was obtained after heating to near β-transus temperature, fast cooling (i.e., >140 ℃/min) and subsequent hot deformation at strain rates above 0.1 s-1. The transformation of nucleation mode from complete wetting to incomplete wetting promoted by hot deformation mainly resulted in the formation of fine equiaxed αs at the β/β grain boundaries. For intragranular αs phase, selective heterogeneous nucleation at dislocations with reduced nucleation barrier and weak variant selection mainly led to the formation of equiaxed α precipitates. A comparison among different off-equilibrium cases revealed that flow stress was decreased greatly by increasing the soaking temperature due to more αp → β transformation. This would provide a novel path to achieve grain refinement of equiaxed structure with decreased forming load and low cost.

Influence of Dynamic Nonequilibrium Processes on the Mechanical Properties and Corrosion Resistance of Titanium Alloy

We compare the mechanical properties and corrosion resistance of specimens made of sheet two-phase VT22 high-strength titanium alloy in the intact state, after static tension, and after realization of dynamic nonequilibrium processes with subsequent static tension. It is shown that one-time dynamic nonequilibrium processes realized under impact vibration loading significantly increase the initial plastic deformation in this alloy and almost do not affect its corrosion resistance.

Quantitative research on microscopic deformation behavior of Ti-6Al-4V two-phase titanium alloy based on finite element method

The microscopic deformation of Ti-6Al-4V titanium alloy shows great inhomogeneity due to its duplex-microstructure that consists of two phases. In order to study the deformation behaviors of the constituent phases, the 2D FE model based on the realistic microstructure is established by MSC.Marc nonlinear FE software, and the tensile simulation is carried out. The simulated global stress-strain response is confirmed by the tensile testing result. Then the strain and stress distribution in the constituent phases and their evolution with the increase of the global strain are analyzed. The results show that the strain and stress partitioning between the two phases are considerable, most of the strain is concentrated in soft primary α phase, while hard transformed β matrix undertakes most of the stress. Under the global strain of 0.05, the deformation bands in the direction of 45° to the stretch direction and the local stress in primary α phase near to the interface between the two phases are observed, and they become more significant when the global strain increases to 0.1. The strain and stress concentration factors of the two phases are obviously different at different macroscopic deformation stages, but they almost tend to be stable finally.

Modeling the anisotropy of hot plastic deformation of two-phase titanium alloys with a colony microstructure

Two-phase titanium alloys deform heterogeneously in primary hot working due to the strong anisotropy of plastic deformation of colony structure associated with the transformation crystal structure, morphology and orientation relationship between the constituent phases. To understand the heterogeneous deformation in primary hot working, a homogenized crystal plasticity constitutive model is developed for a single colony which relates anisotropic deformation behavior to microstructural features. Efforts are made to model the morphological effects which cause the abnormally low measured critical resolved shear stress (CRSS) of two basal <a> slip systems and the anisotropic Hall-Petch strengthening associated with the Burgers orientation relationship. The model is able to capture the deformation characteristics and texture evolution in compression of colony structure. It is found that the morphological effects cause the formation of transverse texture and continuous flow softening in hot compression.

Acceleration of globularization during interrupted compression of a two-phase titanium alloy

To characterize the plastic deformation behavior and globularization in multistage primary hot working, isothermally interrupted compression was carried out for a TA15 titanium alloy with initial colony structure at temperature of 900 °C and strain rate of 0.1 s−1. Through-process microstructural developments were examined by Electron Backscattered Diffraction (EBSD) and Scanning Electron Microscope (SEM). The globularization efficiency was analyzed via the evolution of intra-α boundaries, α/β interfaces and α grain size. The analyses show that the globularization efficiency is greatly enhanced by short time holding after a true strain of 0.73. The acceleration in globularization is associated with the formation of transverse intra-α boundaries across the α lamellae, the loss of α/β interfacial coherency, the change in deformation mode and the improvement of deformation homogeneity. Thus, the globularization efficiency increases with interrupted strain, holding time and loading pass. The results can be used to optimize the primary hot working of titanium alloys.

Deformation banding in β working of two-phase TA15 titanium alloy

Abstract To study deformation banding in β working of TA15 titanium alloy, hot simulation compression experiments were carried out on a Gleeble 3500 thermal simulator, and the microstructure was investigated by optical microscopy (OM) and electron back-scattered diffraction (EBSD). It is found that in β working of TA15 titanium alloy, deformation banding is still an important grain refinement mechanism up to temperature as high as 0.7Tm (Tm is the melting temperature). Boundaries of deformation bands (DBBs) may be sharp or diffusive. Sharp DBBs retard discontinuous dynamic recrystallization (DDRX) by prohibiting nucleation, while the diffusive ones are sources of continuous dynamic recrystallization (CDRX). Deformation banding is more significant at high strain rate and large initial grain size. The average width of grain subdivisions is sensitive to strain rate but less affected by temperature and initial grain size. Multi-directional forging which produces crossing DDBs is potential to refine microstructure of small-size forgings.

Coupled effects of deformation and cooling on the evolution of primary and secondary alpha of two-phase Ti-alloys

The concurrent hot deformation and temperature drop is an important phenomenon in hot forging of two-phase titanium alloys. Understanding the microstructure development in this process is critical to control microstructure and tailor the mechanical properties. For initial equixed structure, primary and secondary alpha evolution, and deformation behavior are revealed by designing concurrent hot compression and controlled cooling experiments. The results show that the growth of primary alpha phase is retarded by deformation at low cooling rate. The morphology of primary alpha cannot be changed by deformation. However, concurrent hot deformation and slow cooling can promote the precipitation of secondary alpha phase. The equixed secondary alpha can be obtained at low strain rates, which can be ascribed to the change in the mechanism of β→α＋β phase transformation by EBSD orientation analysis, the strain weakened anisotropic growth and globularization of alpha laths. Furthermore, without considering the change of alpha phase fraction, the relative difference between calculated and experimental flow stress even can reach 53.4%, which confirms that phase fraction has a significant influence on rheology. Moreover, the strength of transformed beta matrix is improved greatly when the precipitation of secondary alpha is considered. Finally, it can be found that flow stress increases with strain in approximately sigmoidal way. This is due to that temperature drop and increasing phase fraction lead to the increase of flow stress, whereas the rotation and globularization of secondary alpha laths can cause flow softening. The flow stress increases obviously with cooing rate, which can be attributed to fine alpha laths and significant Hall–Petch strengthening effect.

Učinek mikrolegiranja itrija in cirkonija na strukturo in lastnosti na spoje zavrov dvofazne zlitine titana

Učinek mikrolegiranja itrija in cirkonija na strukturo in lastnosti na spoje zavrov dvofazne zlitine titana

Quantification of strain localisation in a bimodal two-phase titanium alloy

High Resolution Digital Image Correlation in combination with orientation imaging microscopy has been applied to compare quantitatively strain localisation in Ti-6Al-4V alloy with a bimodal microstructure subjected to two different ageing treatments, i.e. above and below the α2 solvus temperature. Interestingly, the most pronounced strain heterogeneity was observed in the secondary-α regions for the sample heat-treated to promote α2 formation. The high local strain was associated with intense slip bands within single long secondary-α laths with low levels of neighbouring strain, which is likely to have a significant impact on the low-cycle fatigue performance.

Phase fraction evolution in hot working of a two-phase titanium alloy: experiment and modeling

In the present work, the coupled effects of initial structure and processing parameters on microstructure of a two-phase titanium alloy were investigated to predict the microstructural evolution in multiple hot working. It is found that microstructure with different constituent phases can be obtained by regulating the initial structure and hot working conditions. The variation of deformation degree and cooling rate can change the morphology of the constituent phases, but do not alter the phase fraction. The phase transformation during heating and holding determines the phase fraction for a certain initial structure. β–α–β transformation occurs during heating and holding. β to α transformation leads to a significant increase in content and size of lamellar α. The α to β transformation occurs simultaneously in equiaxed α and lamellar α. The thickness of lamellar α increases with temperature, which is caused by the vanishing of fine α lamellae due to phase transformation and coarsening by termination migration. By assuming a quasi-equilibrium phase transformation in heating and holding, a modeling approach is proposed for predicting microstructural evolution. The three stages of phase transformation are modeled separately and combined to predict the variation of phase fraction with temperature. Model predictions agree well with the experimental results.

Evolution of Defect Structure of Two-Phase Titanium Alloy Under Active Load

Evolution of Defect Structure of Two-Phase Titanium Alloy Under Active Load

Influence of Heating Temperature and Cooling Conditions on Structure and Properties of Two-Phase Titanium Alloy

The effects of heating temperature in the range 790-990 ° C and subsequent cooling in the water, in air, with the furnace on the formation of the structure, phase composition of the titanium alloy VST2 were studied. There was the metastable β-solid solution and α - phase during the quenching heating temperature to 865 °C. The phase composition in the temperature range of 865-915 °C was α + β + α''. There was only α'' - martensite in the structure at the temperatures above 940 °C. Reducing the rate of cooling (air, furnace) helped the retention of two-phase α + β - state after all the heating temperatures. The temperature - rate parameters of heat treatment which can obtain good strength and ductility properties are suggested.

Effect of electrolyte depletion on the characteristics of the anodic plasma electrolytic nitriding of a VT22 titanium alloy

Changes in the saturation capacity of an aqueous electrolyte containing ammonium chloride and ammonia during the nitriding of a VT22 two-phase titanium alloy are studied. Changes in the electrolyte composition are associated with the evaporation of water, ammonia, and ammonium chloride decomposition products and the emission of ions from the boiling solution into the envelope under the action of an electric field. It is found that the thickness of the layer exhibiting high hardness (up to 460 HV), which reaches 20 μm in the case of the treatment in a fresh electrolyte, decreases by 40% in a solution used for 5 h. It is shown that the electrolyte lifetime can be increased due to the vapors and gases from the envelope returning to the electrolyte. The use of an electrolytic cell with the possibility of the vapors and gases leaving the electrolyte during the treatment returning back to it makes it possible to maintain the saturation capacity of the electrolyte within 5 h of operation and ensuring the formation of approximately the same layer as the one formed in a fresh electrolyte. It is found that the electric conductivity of the solution and the gas–vapor envelope increases with electrolyte depletion due to the predominant loss of the dielectric components.

The influence of the indentation size in relation to the size of the microstructure of three polycrystalline materials indented with a Berkovich indenter

Three different polycrystalline materials, a fine-grained martensitic steel (CrMoV), a coarse-grained high-purity copper (C110), and a two-phase microstructure titanium alloy (Ti-6Al-4V), have been selected to investigate the heterogeneity of deformation following indentation using a depth-sensing indentation instrument fitted with a Berkovich indenter. The geometry of the pile-up profiles, measured with an atomic force microscope, were observed to be very sensitive to the indentation size with respect to the size of the microstructure and the material properties and crystallographic plane of the indented grain. In contrast, neither the recovery of the area of indentation nor the degree of piling-up were affected by the presence of indentation size effects (ISE). Furthermore, based on the results of a full-3D finite element simulation, it was concluded that the misalignment of the indenter alone does not explain the significantly asymmetric piling-up in highly anisotropic materials, e.g. C110 copper, but that this is due to the crystallographic orientation of the single grain tested. In addition, the experimental results revealed that, although a thicker mechanically hardened layer formed during polishing is more prone to recovery during unloading, leading to a smaller residual indented area, the degree of piling-up is unaffected provided that the ratio of maximum depth (hmax) to the thickness of the strain-hardened layer is above unity. Moreover, on the same premise, the surface roughness and the thickness of the strain-hardened layer can be discarded as length parameters affecting hardness measurements.

Effect of microtexture on short crack propagation in two-phase titanium alloys

Short crack propagation is difficult to predict because it is highly microstructure sensitive. In this paper, the short crack propagation behaviour in a two-phase titanium alloy Ti-6Al-4V with the presence of strong microtexture and macrozones is investigated using four-point bend fatigue test. We used four factors to characterize the resistance of grain boundaries to short crack propagation: the favourability of slip systems, the slip transmission through grain boundaries, the alignment of crack planes in neighbouring grains and the effective driving force on tilted crack plane. The influences of these factors on the short crack propagation resistance are investigated by detailed interpretation of the short crack propagation through grain boundaries between grains with similar orientations and through grain boundaries between grains with different orientations. The effect of microtexture and macrozones on short crack propagation resistance in two-phase titanium alloys is also discussed. Based on our new methodology we find that favourable slip systems in the outgoing grain, good alignment of the crack planes and high effective driving force on the outgoing crack plane are ideal for the crack transmission onto a slip plane through grain boundaries with minimal resistance. In contrast, low alignment of the favourable slip plane with the incoming crack plane increases the resistance at the grain boundary and changes the direction of the short crack propagation. In a macrozone oriented favourably for basal 〈a〉 or prismatic 〈a〉 slip systems, the short crack growth is non-deviated with minimal resistance on the grain boundaries. In this study, a further understanding on the short crack propagation behaviour and effects of microtexture and macrozones on short crack propagation in titanium alloys are provided.

Effect of the presence of macrozones on short crack propagation in forged two-phase titanium alloys

Macrozones, which are regions containing grains with similar orientations, are commonly reported to be present in two-phase titanium alloys and to be detrimental to their fatigue resistance. The effect of macrozones on fatigue crack initiation is established, but their effect on short crack propagation remains unknown. In this study, the effect of the presence of macrozones on short crack propagation is investigated under four-point bend fatigue testing on both as-forged and powder hot isostatic pressed Ti-64. Micronotches were machined into the samples as crack initiation sites and the short crack propagation behaviour was recorded by replica. It was found that the crack propagation in macrozones can be faster than in random texture because most of the crack planes in the grains of the same macrozone are aligned. Furthermore, macrozones oriented favourably for basal 〈a〉 slip systems are less resistant to short crack propagation than macrozones oriented favourably for prismatic 〈a〉 slip systems. This is because the basal planes of the grains within the same macrozone are aligned, while the prismatic planes of the grains within the same macrozone may not be aligned. This study provides an important fundamental understanding of the effect of macrozones on short crack propagation in forged two-phase titanium alloys and thus on fatigue resistance of titanium alloys and could be useful for the titanium industry in predicting the effects of the presence of macrozones in their products.