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The article brings out the correlation between the microstructure and mechanical properties of Ti5.4Al3Mo1V (VT14 alloy) high-strength alpha beta titanium alloy. The specimens were solution treated above the beta transus temperature of the alloy (980°C and 1000°C) for 45 min and cooled at different rates (water quenched, air cooled, and furnace cooled) to ambient temperature. The specimens were polished, and the microstructural observations under optical microscopy resulted in a discontinuous alpha phase and a continuous network of alpha for 980°C and 1000°C beta-annealed conditions, respectively. Mechanical properties like tensile and impact were evaluated for a 980°C beta-annealed condition. The fracture morphology features of failed impact specimens were compared with a mill annealed product. From the tensile strength data, it was observed that the strength achieved is maximum in the water-quenched condition (1291-1308 MPa) and lowest in the furnacecooled condition (895-905 MPa), whereas the impact strength is highest in the furnace-cooled condition (8.03-8.7 gm/cm 2 ), which is comparable with the aircooled condition (7.3-8.5 kg/cm 2 ), and lowest for the water-quenched condition (2.5-3.1 kg/cm 2 ). The current study is an attempt to correlate the mechanical properties to the heat treatment and thereby to the microstructure.

The beta titanium alloys are highly cold workable in annealed condition, due to presence of single phase bcc structure (beta) at ambient temperature. The Ti15V3Al3Sn3Cr alloy is a metastable beta alloy retains single beta phase at ambient temperature by beta annealing. The beta alloys are most hardenable among titanium alloys, due to the formation of hard alpha (hcp) precipitates in beta (bcc) grains in solution treated and aged (STA) conditions. The present paper brings out the hot forging and rolling studies carried above beta transus temperature and correlating microstructure with mechanical properties in heat treated conditions (a. 800°C for 30 minutes and b. 800°C for 45 minutes, subsequent water quenched from single phase beta region plus aged at 482°C/538°C). The results conclude that solution treatment carried for 45 minutes and aged at 482°C/538°C achieved high tensile strength with improvement in ductility. This is due to less nucleation sites of alpha precipitates along the grain boundaries for the 45 minutes solution treated specimens. The Young’s modulus evaluated for solution treated (78GPa), aged at 482°C (105GPa) and 538°C (103GPa), the increase in aged conditions is due to the formation of alpha precipitates throughout the matrix and makes the alloy two phase alpha-beta system.Keywords: Metastable beta, alpha precipitates, solution treatment, tensile strength, Young’s modulus.

Titanium alloys are used for high-pressure gas bottles / propellant tanks and structural applications owing to their high specific strength, good fabricability / weldability and compatibility with various working fluids. For these applications at ambient temperature, the workhorse Ti6Al4V alloy is extensively used. For the applications at low temperatures, two ELI grades of titanium alloys namely Ti6Al4V and Ti5Al2.5Sn are used as these retain toughness down to 77K and 4K respectively. Due to this inherent advantage, Ti5Al2.5Sn-ELI alloy has been selected as high pressure helium gas bottle submerged in liquid hydrogen (20K temperature). The gas bottle is spherical in shape and is made by electron beam welding of two machined hemispherical shells of 500 mm nominal diameter. The hemispherical shells for the difficult-to-forge Ti5Al2.5Sn-ELI alloy are developed through controlled closed-die forging operations. Shells are subsequently characterized for microstructures and mechanical properties at ambient temperature. Substantial increase in tensile strength with reasonably good ductility with respect to ambient temperature is achieved at 20K temperature. Multi point necking is observed at 20K. The present paper briefly outlines the process control devised for development of these domes and discusses the various characterization results obtained on forged hemispherical shells.