High-strength Beta Titanium Alloy Research Articles

Effect of Molybdenum Content on Microstructure and Mechanical Properties of Ti-Mo-Fe Alloys by Powder Metallurgy

Titanium has many limitations in coverage and frequency of application due to its expensive alloying elements and complex manufacturing process. The biocompatible Ti-Mo-Fe ternary beta titanium alloys were designed by replacing high-cost beta-stabilizer elements (V, Nb, Zr, etc.) with low-cost Mo and Fe elements. In addition, it was attempted to obtain a low-cost, high-strength beta-titanium alloy with 800 MPa or more by applying the powder metallurgy process technology to the Ti-Mo-Fe alloy system. The added Mo element has the effect of reducing the elastic modulus of the titanium alloy without reducing its strength. In this study, Ti-Mo-Fe alloys designed with different Mo contents were fabricated using a powder metallurgy process and analyzed in connection with microstructural properties, phase changes, and mechanical properties. As Mo contents are increased, the α-lath thickness of Widmanstätten decreases and the size of prior β grain decreases. It was confirmed that the hardness and tensile strength were excellent and were compared with the ingot material of the same alloy system.

Thermomechanical processing of a high strength metastable beta titanium alloy powder, consolidated using the low-cost FAST-forge process

The high strength titanium alloy Ti-5553 has been fully consolidated and thermomechanically processed from powder using the FAST-forge process, in only three steps, both at the small and the pilot scale. Titanium alloy components are conventionally produced using a time-consuming process, which involves carbo-chlorination extraction of TiO2, triple vacuum arc re-melting, and multiple thermomechanical and heat treatment steps, before machining. The proposed FAST-forge processing route for titanium alloy components cuts out or significantly reduces these stages, and uses field-assisted sintering technology (FAST) to consolidate powder. This paper assesses the effectiveness of the process for a conventionally used high-strength beta titanium alloy, Ti-5553. Ti-5553 has been fully consolidated by the FAST process at two different dwell temperatures, 850 and 1000°C, and for a 30min dwell time. Small-scale upset forging of cylinders machined from each FAST condition has been performed at forging temperatures 785, 810 and 835°C, and strain rates 0.01, 0.1, 1 and 5s−1, in order to examine the flow stress behaviour. The flow behaviour of both FAST-produced Ti-5553 specimens was found to be very similar to conventionally produced Ti-5553, and the forged microstructures were also comparable. Large-scale forging has been performed on three double truncated cones machined from a FAST specimen produced at a temperature of 1000°C with a 30min dwell time. The cones were forged at 785, 810 and 835°C, and at a strain rate of approximately 3s−1. Each forged specimen was heat treated for 4h at a proprietary temperature between 750 and 850°C, and aged for 8h at a proprietary temperature below 650°C in an inert atmosphere, before air cooling to room temperature. The forged microstructures and heat treated microstructures were found to be comparable to that of conventionally produced Ti-5553. Microhardness measurements of the heat treated specimens, with averages of between 410 and 417Hv, were higher than that of conventionally produced Ti-5553, and were very consistent despite variations in strain and forging temperature. Successful scale-up of the process for a metastable beta titanium alloy indicates its potential to be utilised at an industrial scale.

On the effect of incremental forming on alpha phase precipitation and mechanical behavior of beta-Ti-10V-2Fe-3Al

A combination of good ductility and fatigue resistance makes β-titanium alloys interesting for many current and potential future applications. The mechanical behavior is primarily determined by microstructural parameters like (beta phase) grain size, morphology and volume fraction of primary / secondary α-phase precipitates, and this allows changing and optimizing their mechanical properties across a wide range. In this study, we investigate the possibility to modify the microstructure of the high-strength beta titanium alloy Ti-10V-2Fe-3Al, with a special focus on shape and volume fraction of primary α-phase. In addition to the conventional strategy for precipitation of primary α, a special thermo-mechanical processing is performed; this processing route combines the conventional heat treatment with incremental forming during the primary α-phase annealing. After incremental forming, considerable variations in terms of microstructure and mechanical properties can be obtained for different thermo-mechanical processing routes. The microstructures of the deformed samples are characterized by globular as well as lamellar (bimodal) α precipitates, whereas conventional annealing only results in the formation of lamellar precipitates. Because of the smaller size, and the lower amount, of α-phase after incremental forming, tensile strength is not as high as after the conventional strategy. However, high amounts of grain boundary α and lamellar αp-phase in the undeformed samples lead to a significantly lower ductility in comparison to the matrix with bimodal structures obtained by thermo-mechanical processing. These results illustrate the potential of incremental forming during the annealing to modify the microstructure of the beta titanium Ti-10V-2Fe-3Al in a wide range of volume fractions and morphologies of the primary α phase, which in turn leads to considerably changes, and improved, mechanical properties.

Effect of hot rolling and heat treatment on microstructure and tensile properties of high strength beta titanium alloy sheets

Ti–3.5Al–5Mo–6V–3Cr–2Sn–0.5Fe is a high-strength β titanium alloy that shows excellent tensile properties. This paper presents the effect of hot rolling and heat treatment on the microstructure and tensile properties of this alloy. Microstructure observation shows that the α+β rolled alloy leads to smaller β grain size than the β rolled alloy in all heat treatment conditions. Solution treatment of the alloy in the α+β field can also lead to smaller β grain than solution in the β field. Tensile results indicated that the strength of the alloy was improved greatly by aging heat treatment, which was attributed to the secondary α phase. The 790°C rolled alloy can obtain an ultimate strength of 1744MPa by solution at 800°C plus aging at 440°C. The strength decreases and ductility increases with the increase of aging temperature. Moreover, the ductility of the alloy is significantly determined by the rolling and solution procedure. Rolling and solution treated at the α+β field results in better ductility than rolling and solution at the β field. Therefore, to obtain the high ductility, the α+β rolling and α+β solution should be chosen as long as possible before aging heat treatment.

High strength beta titanium alloys: New design approach

A novel approach for development of high strength and ductile beta titanium alloys was proposed and successfully applied. The microstructure of the designed alloys is fully composed of a bcc β-Ti phase exhibiting dendritic morphology. The new Ti68.8Nb13.6Cr5.1Co6Al6.5 (at%) alloy (BETAtough alloy) exhibits a maximum tensile strength of 1290±50MPa along with 21±3% of fracture strain. The specific energy absorption value upon mechanical deformation of the BETAtough alloy exceeds that of Ti-based metallic glass composites and commercial high strength Ti-based alloys. The deformation behavior of the new alloys was correlated with their microstructure by means of in-situ studies of the microstructure evolution upon tensile loading in a scanning electron microscope.

Characterization of fracture and deformation mechanism in a high strength beta titanium alloy Ti-10-2-3 using EBSD technique

In the present study, fracture toughness tested specimens in longitudinal (LT) and transverse loading (TL) directions of beta titanium alloy have been investigated using Electron Back Scattered Diffraction (EBSD) technique. The orientation images captured at three different locations i.e. machine notch, fatigue pre-crack and final fracture have been compared. The LT sample exhibits with more facets than TL sample. The faceted aspect of the crack is generally associated with quasi-cleavage mechanism. The EBSD analysis clearly points out that in the LT specimen, the hexagonal orientation has moved towards near basal during the test. This may be the reason for the observance of facets in the LT specimen. The cracking in TL specimen might have been taken over by the bcc phase as it is oriented with harder planes of cubic {001} planes. Further, the Schmid factor has also been computed based on the defined loading conditions. Significant variation has been observed in the slip plane orientations and distributions at these three locations. The Schmid analysis has highlighted the significant contribution of different slip systems towards deformation and cracking in LT and TL specimens.

Effect of Variable Stress Intensity Factor on Hydrogen Environment Assisted Cracking

Fitness-for-service evaluations of engineered components that are subject to environment assisted cracking (EAC) often require analyses of potentially large crack extensions through regions of variable stress intensity. However, there are few EAC data and models that directly address the effects of variable stress intensity factor on EAC crack growth. The model developed here is used to evaluate stress corrosion cracking (SCC) data that were obtained on a high-strength beta-titanium alloy under conditions of variable crack mouth opening displacement (CMOD) rate. SCC of this Ti alloy in ambient temperature, near-neutral NaCl aqueous solution is thought to be due to hydrogen environment assisted cracking (HEAC). As the model equations developed here do not admit to a closed form solution for crack velocity as a function of applied stress intensity factor, K, a semiquantitative graphical solution is used to rationalize the crack growth data. The analyses support a previous suggestion that the observed crack growth rate behavior can be attributed to the effect of crack tip strain rate on rates of mechanical disruption and repair of an otherwise protective crack tip oxide film. Model elements introduced here to HEAC modeling include (1) an expression relating corrosion-active surface area to crack tip strain rate and repassivation rate, (2) an expression relating the critical grain boundary hydrogen to the applied stress intensity factor, and (3) an expression relating CTSR to both applied and crack advance strain rate components. Intergranular crack advance is modeled assuming diffusive segregation of corrosion-generated hydrogen to grain boundary trap sites causing embrittlement of the fracture process zone (FPZ). The model equations developed here provide a quantitative basis for understanding the physical significance of K-variation effects and, with additional development, will provide an engineering tool for analysis of crack growth in a variable K field.

On ripple-load, stress-corrosion, and sustained-load cracking behavior in a high strength beta titanium alloy

Both overaged and peakaged TIMETAL 21S beta titanium alloys exhibit significant ripple-load cracking susceptibility in salt water and in ambient air environments. At R = 0.90, the ripple-load cracking thresholds of the overaged alloy are 67% and 72% lower than the stress-corrosion cracking and sustained-load cracking thresholds. For the peakaged alloy, the reductions are 55% and 61%. The stress-corrosion cracking threshold in salt water and the sustained-load cracking threshold in air of peakaged TIMETAL 21S are significantly lower while the ripple-load cracking threshold is slightly lower than those of the overaged alloy. The stress-corrosion cracking, sustained-load cracking, and ripple-load cracking resistance of peakaged TIMETAL 21S are significantly inferior to those of both beta-annealed Ti-6Al-4V and Ti-15V-3Cr-3Al-3Sn. The ripple-load cracking resistance of overaged TIMETAL 21S, though better than Ti-15V-3Cr-3Al-3Sn, is still inferior than that of beta-annealed Ti-6Al-4V.

Aqueous environmental crack propagation in high-strength beta titanium alloys

The aqueous environment-assisted cracking (EAC) behavior of two peak-aged beta-titanium alloys was characterized with a fracture mechanics method. Beta-21S is susceptible to EAC under rising load in neutral 3.5 pct NaCl at 25 °C and −600 mVSCE, as indicated by a reduced threshold for subcritical crack growth (K TH ), an average crack growth rate of up to 10 μms, and intergranular fracture compared to microvoid rupture in air. In contrast, the initiation fracture toughness (K ICi ) of Ti-15-3 in moist air is lower than that of Beta-21S at similar high σYS (1300 MPa) but is not degraded by chloride, and cracking is by transgranular microvoid formation. The intergranular EAC susceptibility of Beta-21S correlates with both α-colonies precipitated at β grain boundaries and intense slip localization; however, the causal factor is not defined. Data suggest that both features, and EAC, are promoted by prolonged solution treatment at high temperature. In a hydrogen environment embrittlement (HEE) scenario, crack-tip H could be transported by planar slip bands to strongly binding trap sites and stress/strain concentrations at α colony or β grain boundaries. The EAC in Beta-21S is eliminated by cathodic polarization (to −1000 mVSCE), as well as by static loading for times that otherwise produce rising-load EAC. These beneficial effects could relate to reduced H production at the occluded crack tip during cathodic polarization and to increased crack-tip passive film stability or reduced dislocation transport during deformation at slow crack-tip strain rates. High-strength β-titanium alloys are resistant, but not intrinsically immune to chloride EAC, with processing condition possibly governing fracture.

Selective surface ageing in the high-strength beta titanium alloy beta-C: Gregory, J.K., Wagner, L. and Muller, C. Proc. Conf. Surface Engineering, Bremen, Germany, 1993, pp. 435–440

Selective surface ageing in the high-strength beta titanium alloy beta-C: Gregory, J.K., Wagner, L. and Muller, C. Proc. Conf. Surface Engineering, Bremen, Germany, 1993, pp. 435–440

Strengthening of Ti-15V-3Cr-3Sn-3Al by Thermo-mechanical Treatments.

Mechanical properties of beta titanium alloys strongly depend on the precipitation behavior of alpha, which is controlled by thermo-mechanical treatments. To obtain high strength beta titanium alloys, uniform distribution of fine alpha precipitates is required, and various thermo-mechanical treatments have been developed for this purpose.This paper discusses strengthening by fine alpha precipitation using a thermo-mechanical process which combinates cold rolling and un-recrystallized heat treatments.High tensile strength of over 1 700 Mpa can be attained by the thermo-mechanical process combining oveer-aging, cold rolling and aging for 86.4 ks at 723 K. Higher strength can be attained after aging by alpha-beta solution treatment than by beta solution treatment because fine alpha particles precipitate in the aging after the former treatment. It seems that un-recrystallized beta matrix supplies the effective nucleation sites of alpha particles.Ultra high tensile strength of 1 940 Mpa can be attained through the aging after repeated applications of the cold rolling and recovering process. The microstructure after 723 K aging displayed uniform distribution with very fine alpha precipitates 2-5 nm in thickness.