High-Temperature (1023 K to 1273 K [750 °C to 1000 °C]) Plastic Deformation Behavior of B-Modified Ti-6Al-4V Alloys: Temperature and Strain Rate Effects

A minor addition of B to the Ti-6Al-4V alloy, by ~0.1 wt pct, reduces its as-cast prior β grain size by an order of magnitude, whereas higher B content leads to the presence of in situ formed TiB needles in significant amounts. An experimental investigation into the role played by these microstructural modifications on the high-temperature deformation behavior of Ti-6Al-4V-xB alloys, with x varying between 0 wt pct and 0.55 wt pct, was conducted. Uniaxial compression tests were performed in the temperature range of 1023 K to 1273 K (750 °C to 1000 °C) and in the strain rate range of 10^–3 to 10⁺¹ s^–1. True stress–true strain responses of all alloys exhibit flow softening at lower strain rates and oscillations at higher strain rates. The flow softening is aided by the occurrence of dynamic recrystallization through lath globularization in high temperature (1173 K to 1273 K [900 °C to 1000 °C]) and a lower strain rate (10^–2 to 10^–3 s^–1) regime. The grain size refinement with the B addition to Ti64, despite being marked, had no significant effect on this. Oscillations in the flow curve at a higher strain rate (10⁰ to 10⁺¹ s^–1), however, are associated with microstructural instabilities such as bending of laths, breaking of lath boundaries, generation of cavities, and breakage of TiB needles. The presence of TiB needles affected the instability regime. Microstructural evidence suggests that the matrix cavitation is aided by the easy fracture of TiB needles.     

Heat-Treatment Effects on the Microstructure and Tensile Properties of Powder Metallurgy Ti-6Al-4V Alloys Modified with Boron

The Ti-6Al-4V (Ti-64) alloys modified with two levels of boron (1B and 1.7B (wt pct)) representing hypoeutectic and hypereutectic compositions, produced via a prealloyed powder metallurgy approach, were subjected to various standard heat treatments of Ti-64 to study the microstructural evolution and its influence on tensile properties. Boron-modified Ti-64 (Ti-64B) alloys exhibited differences in microstructural response to heat treatment compared to that of Ti-64 due to variations in constituent phase fractions and the influence of TiB on the beta-to-alpha phase transformation kinetics. The tensile elastic modulus of Ti-64B alloys increased nearly linearly with the boron content (or TiB volume fraction) and the increase could be satisfactorily predicted with an isostrain rule of mixtures (ROMs) and the Halpin–Tsai model. The Ti-64-1B possessed a good combination of tensile strength (1200 to1370 MPa) and ductility (10 to 13 pct), while Ti-64-1.7B exhibited high strength (1300 to 1695 MPa) and modest ductility (2 to 3.5 pct). Coarse primary TiB particles present in Ti-64-1.7B were found to initiate premature failure. Strength modeling revealed that load sharing by the micron-sized TiB whiskers provides the major contribution for the increase in yield strength.     

Structure and properties of a β solution treated,quenched, and aged si-bearing near-α titanium alloy

The microstructure, tensile properties, and fractographic features of a near-α titanium alloy, IMI 829(Ti-6.1 wt pct Al-3.2 wt pct Zr-3.3 wt pct Sn-1 wt pct Nb-05 wt pct Mo-0.32 wt pct Si) have been studied after aging over a temperature range of 550°C to 950°C for 24 hours following solution treatment in the β phase field at 1050°C and water quenching. Transmission electron microscopy studies revealed that aging at 625°C and above produced discrete silicides at α′ interplatelet boundaries. However, aging at 900°C and above has also resulted in the precipitation of β phase along the lath boundaries of martensite. The silicides have been found to have a hexagonal structure withc=0.36 nm anda=0.70 nm (designated as S₂ by earlier workers). There is a significant improvement in yield and ultimate tensile strength after aging at 625°C, but there is less improvement at higher aging temperatures. The tensile ductility is found to be drastically reduced. While the fracture surface of the unaged specimen shows elongated dimples, the aged samples show a mixed mode of fracture, consisting of facets, featureless parallel bands, and extremely fine dimples.     

Effect of Combined Addition of Cu and Aluminum Oxide Nanoparticles on Mechanical Properties and Microstructure of Al-7Si-0.3Mg Alloy

In this study, an ultrasonic cavitation based dispersion technique was used to fabricate Al-7Si-0.3Mg alloyed with Cu and reinforced with 1 wt pct Al₂O₃ nanoparticles, in order to investigate their influence on the mechanical properties and microstructures of Al-7Si-0.3Mg alloy. The combined addition of 0.5 pct Cu with 1 pct Al₂O₃ nanoparticles increased the yield strength, tensile strength, and ductility of the as-cast Al-7Si-0.3Mg alloy, mostly due to grain refinement and modification of the eutectic Si and θ-CuAl₂ phases. Moreover, Al-7Si-0.3Mg-0.5Cu-1 pct Al₂O₃ nanocomposites after T6 heat treatment showed a significant enhancement of ductility (increased by 512 pct) and tensile strength (by 22 pct). The significant enhancement of properties is attributed to the suppression of pore formation and modification of eutectic Si phases due to the addition of Al₂O₃ nanoparticles. However, the yield strength of the T6 heat-treated nanocomposites was limited in enhancement due to a reaction between Mg and Al₂O₃ nanoparticles.     

Transformation Superplasticity of Cast Titanium and Ti-6Al-4V

Samples of unalloyed titanium and Ti-6Al-4V with a cast, coarse-grain structure were subjected to simultaneous mechanical loading and thermal cycling about their transformation range to assess their capability for transformation superplasticity. Under uniaxial tensile loading, high elongations to failure (511 pct for titanium, and 265 pct for Ti-6Al-4V) and an average strain-rate sensitivity exponent of unity are observed. Samples previously deformed superplastically to a strain of 100 pct show no significant degradation in room-temperature mechanical properties as compared to the undeformed state. Biaxial dome bulging tests confirm that transformation superplasticity is activated under thermal cycling and faster than creep deformation. The cast, coarse-grained titanium and Ti-6Al-4V have similar transformation-superplasticity characteristics as wrought or powder-metallurgy materials with finer grains. This may enable superplastic forming of titanium objects directly after the casting step, thus bypassing the complicated and costly thermomechanical processing steps needed to achieve fine-grain superplasticity.

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Comparison of the Microstructure, Tensile, and Creep Behavior for Ti-22Al-26Nb (At. Pct) and Ti-22Al-26Nb-5B (At. Pct)

The effect of the addition of 5 at. pct boron on the microstructure and creep behavior of a nominally Ti-22Al-26Nb (at. pct) alloy was investigated. The boron-modified alloy contained boride needles enriched in titanium and niobium, and because to these borides, this material was considered to be a discontinuously reinforced metal matrix composite. These needle-shaped borides made up to 2 pct of the volume and were up to 158-μm long and 22-μm wide. The effect of boron on the mechanical properties was evaluated through in-situ creep testing and tensile testing at room temperature (RT) and 650 °C. Overall, the addition of 5 at. pct boron proved to be detrimental to the tensile and creep behavior. The composite exhibited a brittle failure and lower elongations-to-failure than the monolithic material. The in-situ tensile and creep experiments revealed that the deformation process initiated in the boride needles, which cracked extensively, and significantly greater primary creep strains and creep rates were exhibited by the composite.

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A Study on Alpha-Case Depth in Ti-6Al-2Sn-4Zr-2Mo

Isothermal oxidation experiments in air were performed on Ti-6Al-2Sn-4Zr-2Mo (Ti-6242) with a bimodal microstructure in the temperature range 811 K to 922 K (538 °C to 649 °C) for up to 500 hours, and α-case depths were quantified using metallography. Alpha-case depth followed a parabolic variation with time. Alpha-case depths in excess of 10 μm formed above 811 K (538 °C) and 100-hour exposures. An activation energy of 244 kJ/mol was estimated for diffusion of oxygen in the α phase of Ti-6242.     

The effect of deformation-induced transformation on the fracture toughness of commercial titanium alloys

The effect of deformation-induced transformation of metastableβ phase on the ductility and toughness of four commercial titanium alloys was investigated. Tensile tests, Charpy impact tests, and both static and dynamic fracture toughness tests were carried out at temperatures between 77 and 473 K on four titanium alloys containing metastableβ phase. Deformation-inducedα″ (orthorhombic martensite) was observed in an (α + β)-type Ti-6Al-2Sn-4Zr-6Mo alloy. The dynamic fracture toughness of this alloy increased considerably at 223 K compared to those at other temperatures. In another (α + β)-type Ti-6A1-4V alloy, the static fracture toughness at 123 K and the dynamic fracture toughness at 223 K were increased considerably by the presence of deformation-induced martensite compared to those at other temperatures. The strength increased as the temperature decreased in this alloy. An abnormal elongation of aβ-type alloy, Ti-15V-3Al-3Sn-3Cr, at 123 K was attributed to the mechanical twinning of theβ phase. However, the effect of deformation-induced transformation on the fracture toughness of Ti-3Al-8V-6Cr-4Mo-4Zr alloy was not observed. Formerly Visiting Associate Professor, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, Pittsburgh, PA. Formerly with the Department of Production Systems Engineering, Toyohashi University of Technology.     

Effect of microstructure on fracture characteristics of Ti-6Al-2Sn-2Zr-2Mo-2Cr-Si

The effects of intermetallic compounds of Ti₃Al (α ₂) and silicide separately on fracture characteristics of Ti-6Al-2Sn-4Zr-2Mo-0.1Si (Ti-62222S) alloy were investigated in this study. The alloys with only Ti₃Al and only silicide precipitated were established by aging treatments at temperatures of 913 K followed by air cooling and 1088 K followed by water quenching, respectively. X-ray diffraction analysis results showed that the volume fraction of either Ti₃Al or silicide increases with increasing aging time. Tensile properties, namely, yield stress (0.2 pct proof stress), ultimate tensile strenght, and elongation of as-received alloy are much better than those of the aged alloys. The strength of the alloy with only Ti₃Al is better than that of the alloy with only silicide, while elongation of the alloy with only silicide is better than that of the alloy with only Ti₃Al. Fracture toughness, J _IC, of the alloy with only silicide is better than that of the alloy with only Ti₃Al. The intergranular fracture appears in the alloy with only Ti₃Al. Coarsening of Widmanstätten α structure and increasing ductility of β phase during aging are considered to be effective for increasing fracture toughness.