Ti 10V 2Fe 3Al Alloy Research Articles

The response of the high strength Ti–10V–2Fe–3Al beta titanium alloy to laser assisted cutting

Abstract Metal cutting is a process that uses tools to create new surfaces by imparting intense shear stresses and high strain rates on the work material. Consequently, the mechanical properties of the work material directly influence its machinability, and high strength materials such as titanium are notoriously difficult to cut. Laser assisted machining (LAM) is a promising solution to reduce the cutting pressures when machining difficult-to-cut materials. The method involves using a laser beam to locally heat and reduce the flow stress of the material ahead of an advancing cutting tool, making the metal shearing process easier. To date there is limited, if any, published literature on using the technology to improve the machinability of metastable β-titanium alloys. It remains unclear whether these materials will respond to laser assisted machining since many are specifically designed to exhibit high temperature strength. This paper compares the conventional and laser assisted machining method for the high strength Ti–10V–2Fe–3Al β-titanium alloy over a wide range of cutting parameters. The effect of the laser beam on the cutting force, cutting temperature and chip formation is discussed. The effectiveness of the LAM process in reducing the cutting pressure of Ti–10V–2Fe–3Al alloy is also compared against other alloys including commercial-purity titanium and Ti–6Al–4V.

Surface characteristics of anodic oxide films fabricated in acid and neutral electrolytes on Ti–10V–2Fe–3Al alloy

Anodic oxide films were fabricated on Ti–10V–2Fe–3Al alloy in acid (H2SO4/H3PO4) and neutral environmental friendly (C4H4O6Na2) electrolytes. The morphology, roughness, crystalline structure of the anodic oxide film were characterized by using scanning electron microscopy, atomic force microscopy, Raman spectroscopy and electrochemical impedance spectroscopy (EIS). The results showed that the oxide film fabricated in H2SO4/H3PO4 electrolyte had a porous structure and the thickness of the film was 3.5 µm. The oxide film fabricated in C4H4O6Na2 electrolyte presented a nonporous structure that sustained the evident microstructure of the substrate, and the thickness of the film was 6.0 µm. The surface average roughness values of the two types of films were 245 nm and 166 nm, respectively. The phase of the anodic oxide films consisted mainly of anatase and rutile. EIS results showed that the film fabricated in C4H4O6Na2 electrolyte had higher impedance of the outer layer, while the film fabricated in H2SO4/H3PO4 electrolyte had higher impedance of the inner layer. Moreover, we attempt to explain the differences in the anodizing kinetics, structure and electrochemical impedance of anodic oxide films by the different films growth processes in the two types of electrolytes. Copyright © 2012 John Wiley & Sons, Ltd.

Influence of incremental rate of anodising current on roughness and electrochemical corrosion of oxide film on titanium alloy Ti–10V–2Fe–3Al

In this study, the influence of incremental rate of anodising current VIR on the crystal structure, surface roughness and corrosion resistance of anodic oxide films on Ti–10V–2Fe–3Al was studied using Raman spectroscopy, atomic force microscopy (AFM), electrochemical impedance spectroscopy (EIS) and potentiodynamic polarisation. The results showed that the surface roughness, crystal structure and electrochemical corrosion behaviour of anodic oxide films were remarkably affected by different VIR with regard to the change in the rate of anodising voltage. In addition, results of EIS and potentiodynamic polarisation revealed that the film that formed at VIR of 0··50 A min−1 had the highest corrosion resistance. Quantitative AFM characterisation showed good correlation between surface roughness and EIS results. Furthermore, results of Raman spectroscopy showed that the relative intensity of anatase and rutile had a tendency to increase with a decrease in VIR before reaching 0··50 A min−1.

Enhancement of athermal α″ martensitic transformation in Ti–10V–2Fe–3Al alloy due to high-speed hot deformation

This work examines the origin of the athermal α″ martensitic transformation in Ti–10V–2Fe–3Al alloys arising from high-speed hot deformation on the basis of chemical thermodynamics and the change in stored energy introduced by deformation. The value of the martensitic transformation starting temperature (Ms) is strongly affected by various microstructural factors. This work reveals that a high accumulation of dislocation density, greater than 1015 m−2, arising from high-speed deformation, resulted in an enhancement of athermal α″ martensitic transformations.

Tuning the stress induced martensitic formation in titanium alloys by alloy design

Two novel titanium alloys, Ti–10V–2Cr–3Al and Ti–10V–1Fe–3Al (wt%), have been designed, fabricated, and tested for their intended stress-induced martensitic (SIM) transformation behavior. The results show that for Ti–10V–1Fe–3Al the triggering stress for SIM transformation is independently affected by the β domain size and β phase stability, when the value of the molybdenum equivalent is higher than ~9. The triggering stress was well predicted using the equations derived separately for the commercial Ti–10V–2Fe–3Al alloy. For samples containing β with a lower molybdenum equivalence value, pre-existing thermal martensite is also present and this was found to have an obstructive effect on SIM transformation. In Ti–10V–2Cr–3Al, the low diffusion speed of Cr caused local gradients in the Cr level for many heat treatments leading even to martensite free zones near former β regions.

Thermal stability of bulk nanocrystalline Ti–10V–2Fe–3Al alloy

Abstract Thermal stability of the nanocrystalline Ti–10V–2Fe–3Al alloy produced by cold-forging and corresponding hardness variations were investigated. When the annealing temperature was lower than 400 °C, the primary features were the decline of lattice defect density and the increase of lattice ordering level in the vicinity of grain boundaries, named as grain boundary relaxation. The nanocrystalline grains grew slowly. In comparison, when the annealing temperature was higher than 400 °C, the decomposition of the deformation-induced α″ martensites and the subsequent rapid grain growth were observed. Vickers hardness measurement shows an initial hardening followed by a softening with the increase of annealing temperature. The hardening peak appeared at 400 °C. The critical grain growth temperature, i.e., Tcgg, in the nanocrytalline Ti–10V–2Fe–3Al alloy, was slightly lower than those in other nanocrystalline alloys. That could be attributed to the martensitic decomposition-induced grain growth in this alloy.

Microstructure characterisation of biphasic titanium alloy Ti–10V–2Fe–3Al and effects induced by heterogeneities on X-ray diffraction peak’s broadening

The present study deals with a microstructural characterisation of a biphasic titanium alloy Ti–10V–2Fe–3Al. The near β titanium alloy is primarily used for high strength applications, such as components for aeronautical industry. The forging process and the heat treatments of Ti–10V–2Fe–3Al induce a complex microstructure, which consists of an equiaxial primary phase αp (hcp) and a secondary phase αsec (hcp) precipitating in the cubic phase β matrix. The forged material exhibits some structures hundreds of micrometres in size. Microstructure evolutions of forged and milled samples are analysed using a scanning electron microscope. Superficial treatments on forged and aged components induce a microstructure flow on the surface. Crystallites’ sizes and strain between phases are heterogeneous, and thermomechanical treatments may induce preferred crystallographic orientations. These phenomena broaden peaks strongly and generate difficulties in analysing residual stresses by X-ray diffraction. The present paper presents the microstructure’s evolution, linked to diffraction peaks broadening and crystallographic textures analyses.

Correlation of Dynamic Response with Adiabatic Shearing Behavior in Ti–10V–2Fe–3Al Alloy

A Split Hopkinson Pressure Bar system was employed to investigate the compressive dynamic mechanical behaviors of Ti-10V-2Fe-3Al (Ti-1023) alloy with lamellar microstructure, over a broad strain rates ranging from 1500/s to 5100/s. The results reveal that the strain rate has a significant effect on the flow stress of Ti-1023 alloy, and there exists serious thermal softening as the strain rate exceeds 3200/s. The critical strain rate of fracture for this alloy is 2300/s. The microstructure examination indicated that adiabatic shear bands (ASBs) bifurcate more intensely with the increasing of strain rate. Micro-voids nucleate either in the ASB or interface between shear band and matrix bulk. Finally, fracture of this alloy proceeds through the nucleation, growth and coalescence of these voids and cracks along the ASBs.

Deformation-induced microstructure refinement in primary α phase-containing Ti–10V–2Fe–3Al alloy

Microstructural evolution in primary α phase-containing Ti–10V–2Fe–3Al alloy subjected to cold forging under different applied strains was studied. Experimental results showed that, even at a strain of 0.1, stress-induced α″ martensites were abundantly produced within the β matrix, resulting in alternative α″/β lamellae. Shear bands initiated and grew across α″/β lamellae as the strain increased to 0.35. When the strain increased to 1.2, the volume fraction of shear bands significantly increased and the grains were almost occupied by the shear bands. Interestingly, nanocrystallines were observed inside shear bands. While in the primary α phase, slip was always the predominant plastic deformation mode and dislocations were accumulated to a high density within the strain range from 0.1 to 0.35. When the strain was up to 1.2, the dislocation density was further increased and α/β interface boundary became ill-defined. However, no grain refinement was observed in the α phase. The microstructure refinement in the β matrix could be attributed to that stress-induced α″ martensitic transformation promoted the initiation, thickening and coalescence of shear bands. The plastic deformation combined with martensitic phase transformation could provide a potential effective technique to produce nanocrystalline materials.

Effect of electrolyte concentration on morphology, microstructure and electrochemical impedance of anodic oxide film on titanium alloy Ti-10V–2Fe–3Al

Anodic oxide films were fabricated on titanium alloy Ti-10V–2Fe–3Al in ammonium tartrate solutions at the concentrations: 1, 3, 5, 10 and 15 g L−1. The morphological characteristics and microstructures of the films of the alloy were studied by optical microscopy (OM) and Raman spectroscopy (Raman), respectively. The electrochemical impedances of the films in 0.5 mol L−1 H2SO4 solution were investigated by electrochemical impedance spectroscopy (EIS). It was showed that different electrolyte concentrations led to different change rates of anodizing forming voltage. The change rate significantly affected the morphology, microstructure and electrochemical impedance of anodic oxide film. When electrolyte concentration was 5 g L−1, anodic oxide film was the most uniform, exhibited by the least and smallest breakpoints on the film. In addition, the amount of crystal phase of the film was the largest at 5 g L−1, showed by the highest intensity of Raman peaks. Furthermore, the electrochemical impedance of the film of the alloy was the greatest at 5 g L−1, demonstrated by the highest values of polarization resistances and lowest values of capacitances. These phenomenon were associated with the minimum value of the change rate of anodizing forming voltage at 5 g L−1.

Effect of prestrain on microstructure and mechanical behavior of aged Ti–10V–2Fe–3Al alloy

The effect of prestrain on microstructure and mechanical behavior of aged Ti–10V–2Fe–3Al alloy was investigated. The results showed that prestrain caused the tensile strength to decrease by 5%, but the elongation to fracture significantly improved by about 200%, in comparison with the unstrained samples, using a much shorter aging time. Transmission electron microscopy investigations showed that nano-sized alpha (α) particles homogeneously precipitated in the beta (β) matrix, and continuous α films formed along grain boundaries in the unstrained and aged samples. However, in the prestrained samples, the coarse stress induced martensite laths decomposed into α- and β-phases in the form of alternately arranged plates, which suppressed formation of the continuous grain boundary α films during aging. The hardness of the prestrained samples was lower than that of the unstrained samples after the same aging treatments. The enhancement of ductility can be mainly attributed to the suppression of grain boundary α films and the reduced hardness in prestrained samples.

Effect of initial microstructure on plastic flow behaviour during isothermal forging of Ti–10V–2Fe–3Al

The plastic flow behaviour and microstructural development during isothermal forging was determined for near beta alloy Ti–10V–2Fe–3Al. Two different initial microstructures were employed: (i) a beta forged billet with a large prior beta grain size with Widmanstätten alpha platelets; and (ii) an alpha–beta forged billet with prior globular primary alpha. The beta forged condition exhibited a peak stress followed by intense flow softening, which is attributed to the break up of the Widmanstätten alpha platelets. Evidence suggests that peak hardening at low strains is due to dislocation pile-ups at alpha platelet/subgrain beta interfaces and subsequent flow softening is attributed to the transmission of beta phase through the alpha platelets. A strain rate ‘jump’ test investigation provided sufficient evidence to suggest that there is a transition from dislocation dominant deformation mechanisms at high strains rates to diffusional dominant deformation at low strain rates in Ti–10V–2Fe–3Al during forging.

Thermomechanical processing of Ti–5Al–5Mo–5V–3Cr

The constitutive behaviour and microstructural evolution of the near-β alloy Ti–5Al–5Mo–5V–3Cr in the α + β condition has been characterised during isothermal subtransus forging at a range of temperatures and strain rates. The results indicate that Ti–5Al–5Mo–5V–3Cr has a shallower approach curve, and therefore, offers a more controllable microstructure than the near-β alloy Ti–10V–2Fe–3Al. Flow softening is small in magnitude in both alloys in the α + β condition. The steady state flow stresses obey a Norton–Hoff constitutive law with an activation energy of Q = 183 kJ mol −1, which is similar to the activation energy for self-diffusion in the β phase, suggesting deformation is dominated by dynamic recovery in the β matrix. Good evidence is found for the existence of ω phase after both air cooling and water quenching from above the β transus. In addition, dissolution of the α phase is found to be slow at near-transus temperatures.

Superelasticity in Ti–10V–2Fe–3Al alloys with nitrogen addition

Metastable β titanium alloy Ti–10V–2Fe–3Al is known to exhibit a shape memory effect due to a stress-induced α″ martensitic transformation at room temperature. In the present study, nitrogen (N) was added to this alloy to strengthen the β matrix. The effect on the deformation behavior of β phase was studied. Thermally induced martensite is formed in the N-free alloy by quenching after β solutionizing but it is not observed in the alloys containing nitrogen of more than 0.1 mass%. Superelasticity due to stress-induced formation of α″ martensite was observed in the 0.1 and 0.2N alloys. N addition is moreover effective in increasing a true elastic strain up to 1% by strengthening of the β matrix.

Effect of β grain size on stress induced martensitic transformation in β solution treated Ti–10V–2Fe–3Al alloy

Abstract Grain size in β-solution treated Ti–10V–2Fe–3Al alloy has been shown to have a profound effect on the triggering stress of the stress induced martensitic (SIM) transformation. The triggering stress is shown to increase with increasing β grain size up to a grain size of 300 μm, after which it was found to become constant. Dependence of the triggering stress on β grain size has been explained in terms of the free energy change during such a transformation.

Optimisation of hot die forging processes of Ti–10V–2Fe–3Al alloy

In order to optimise the hot die processes of Ti–10V–2Fe–3Al alloy, the effects of forging processes on the microstructures and mechanical properties were studied and processing maps were used to try to aid the process design. The results show that processing maps for Ti–10V–2Fe–3Al alloy exhibit three process ranges, but only the range near β transus is suitable for hot die forging of this alloy. The alloy forged at Tβ−30°C has a higher strength and ductility while the alloy forged at Tβ+30°C offers higher fracture resistance than the former. Process C, where the alloy was forged at Tβ+30°C to form the acicular αp and then at Tβ−30°C to get globular αp, was designed and the total amount of deformation equal to 50% was chosen. Through process C the alloy has a better combination of tensile properties and fracture resistance than any other processes because a proper mixture between globular αp and acicular αp is obtained.

Duality of the S–N fatigue curve caused by competing failure modes in a titanium alloy and the role of Poisson defect statistics

The mechanism of the duality of the S–N fatigue data (S is stress and N is cycles to failure) or the grouping of data into two distinct S–N curves as a result of two competing failure modes, occurring during fatigue of some α + β microstructures of the metastable beta-titanium alloy Ti–10V–2Fe–3Al, is quantitatively assessed in this research. The competing failure modes result in two separate fatigue curves in this material: one for failures from surface-nucleated cracks and the other for the subsurface-nucleated. The consequence of this behavior is that one can not predict exactly how a specimen or component would fail, regardless of the number of samples tested. This work discusses in detail the microstructural conditions that trigger this phenomenon in the titanium alloy studied. Based on Poisson defect statistics and Monte-Carlo simulations, it is shown that this phenomenon can arise from a Poisson spatial pattern of low populations of microscale defects. An interesting finding is that there should be a combination of average defect density and specimen area at which this phenomenon is observable in any material having sparsely populated and Poisson-distributed defects. The complete analysis presented here also helps to explain the duality of fatigue failures caused by competing failure modes observed in steels and superalloys.

An unusual fatigue phenomenon: duality of the S– N fatigue curve in the β-titanium alloy Ti–10V–2Fe–3Al

An unusual fatigue behavior, observed in the α+ β microstructures of the β-titanium alloy, Ti–10V–2Fe–3Al, is reported. It is the duality of the S– N fatigue curve where the fatigue data of specimens failing from cracks nucleated at the surface and subsurface regions grouped in to two separate S– N curves. The microstructural factors causing this effect are analyzed.

Hot deformation mechanisms in metastable beta titanium alloy Ti–10V–2Fe–3Al

The mechanisms of hot deformation in the β titanium alloy Ti–10V–2Fe–3Al have been characterised in the temperature range 650–850°C and strain rate range 0·001–100 s-1 using constant true strain rate isothermal compression tests. The β transus for this alloy is ∼790°C, below which the alloy has a fine grained duplex +β structure. At temperatures lower than the β transus and lower strain rates, the alloy exhibits steady state flow behaviour while at higher strain rates, either continuous flow softening or oscillations are observed at lower or higher temperatures, respectively. The processing maps reveal three different domains. First, in the temperature range 650–750°C and at strain rates lower than 0·01 s-1, the material exhibits fine grained superplasticity marked by abnormal elongation, with a peak at ∼700°C. Under conditions within this domain, the stress–strain curves are of the steady state type. The apparent activation energy estimated in the domain of fine grained superplasticity is ∼225 kJ mol-1, which suggests that dynamic recovery in the β phase is the mechanism by which the stress concentration at the triple junctions is accommodated. Second, at temperatures higher than 800°C and strain rates lower than ∼0.1 s-1, the alloy exhibits large grained superplasticity, with the highest elongation occurring at 850°C and 0.001 s-1; the value of this is about one-half of that recorded at 700°C. The microstructure of the specimen deformed under conditions in this domain shows stable subgrain structures within large β grains. Third, at strain rates higher than 10 s-1 and temperatures lower than 700°C, cracking occurs in the regions of adiabatic shear bands. Also, at strain rates above 3 s-1 and temperatures above 700°C, the material exhibits flow localisation.

Application of novel technique to examine thermomechanical processing of near β alloy Ti–10V–2Fe–3Al

A novel specimen design and testing strategy has been exploited to determine the effect of thermomechanical processing on the microstructural development of the near β titanium alloy, Ti–10V–2Fe–3Al. A double truncated cone test geometry was isothermally deformed at near βtransus (∼795°C) temperatures, to obtain microstructural information for a range of strains within a single specimen. A finite element modelling package was employed to produce strain profiles, which readily correspond to the equivalent microstructural profiles of the test specimens. A parametric study of the effects of process (e.g. friction) and material (e.g. strain rate sensitivity) parameters on the strain distributions obtained during the test was investigated. Finite element modelling was conducted to interpolate the friction ring compression test. The effectiveness of this testing strategy is illustrated with a qualitative description of the microstructural evolution with strain, for various strain rates, at the sub-βtransus forging temperature of 760°C.