Seshacharyulu Tamirisakandala

Processing, microstructure, and properties of β titanium alloys modified with boron

The development of next-generation βTi alloys is expected to involve very attractive combinations of strength-toughness-fatigue resistance at large cross sections, improved and affordable thermomechanical processing, and enhanced elevated temperature capability. This article describes the development of βTi alloys that are modified with small boron (B) additions to achieve these goals. Two important aerospace alloys, Ti-15Mo-2.6Nb-3Al-0.2Si and Ti-5Al-5V-5Mo-3Cr microalloyed (0.1%) with B were considered. Ingots that were 70 mm in diameter and 500 mm in length were cast using induction skull melting. A detailed microstructural characterization and tensile property evaluation were conducted. Microalloying with B refines the cast grain size to about 50 µm, which enhances strength and ductility. The effect of B additions on the microstructural stability and properties in the as-cast condition was established. The implications of B additions on the microstructural evolution and affordability of subsequent processing is also discussed.

Thermomechanical response of a powder metallurgy Ti-6Al-4V alloy modified with 2.9 pct boron

The thermomechanical response of Ti-6Al-4V modified with 2.9 pct B produced by a blended powder metallurgy route was studied with isothermal constant strain-rate hot compression tests in the temperature range 850 °C to 1200 °C and strain rate range 10−3 to 10 s−1. Detailed analyses of the flow stress data were conducted to identify various microstructural deformation and damage mechanisms during hot working by applying available materials modeling techniques. In the α + β phase field, cavitation at the matrix/TiB interfaces and TiB particle fracture occurs at low strain rates (<10−1 s−1), while adiabatic shear banding also occurs at high strain rates. At low strain rates, the β phase deforms superplastically due to the stabilization of a fine grain size by the TiB particles. Grain boundary and matrix/TiB interface sliding with simultaneous diffusional accommodation are observed to contribute to the β superplasticity. The deformation behavior at high strain rates in the β-phase field is similar to that of the α + β phase field, with microstructural manifestations of extensive cavitation at the matrix/TiB interfaces and TiB particle fracture.

Beta phase superplasticity in titanium alloys by boron modification

Addition of boron to titanium alloys produces fine TiB whiskers in situ with excellent thermal stability and good chemical compatibility with the matrix. These whiskers stabilize a fine-grain microstructure by restricting grain growth at high temperatures in the β phase field. The hot deformation behavior in the β phase field (temperature range 1050–1200 °C) of Ti-6Al-4V alloys modified with two different levels of B additions (1.6 and 2.9 wt.%) produced by powder metallurgy was investigated using hot compression tests in the strain rate range of 10−3 to 10−1 s−1 and hot tensile tests at a nominal strain rate of 6×10−4 s−1. The β phase exhibits superplasticity, which occurs due to stabilization of a fine-grain microstructure by the TiB. Matrix grain boundary sliding and β/TiB interface sliding appear to contribute to the β superplasticity. The ability to achieve superplasticity at higher temperatures enable lower flow stresses, improved chemical homogeneity, and high strain rate capability due to enhanced accommodation processes.

Solidification Microstructure and Texture in Grain-Refined Titanium Alloys

In the present study, solidification microstructure and texture evolution in grain-refined Ti-6Al-4V and γ-TiAl alloys via trace boron addition are compared with their baseline counterparts. Boron addition resulted in dramatic grain refinement by almost an order of magnitude. The texture developed in these alloys is also markedly different from the baseline alloys.