High strain rate deformation and failure of boron modified AS-CAST Ti-6Al-4V alloys

Abstract

Ti6Al4V alloy is the workhorse of all the Titanium alloys owing to its high specific strength, fatigue strength, and oxidation resistance at moderately high temperatures; thus making it a pre ferred material in the aerospace industry. Despite the advantages, their widespread use is limited because of the high costs involved in their wrought processing which typically involves ele vated temperature mechanical working above the β-transus to achieve a fine grain structure. Re searchers have recently observed that trace addition of Boron to Ti6Al4V substantially reduces the grain size of the alloy, thereby avoiding the need to perform high temperature mechanical working. It is observed that the quasi-static mechanical properties of the Boron modified Ti64 alloys are equivalent to the wrought Ti64 alloys. Often in applications, the components made of these alloys experience impact loading conditions. Hence, investigating the failure mecha nisms under impact loading conditions is essential in designing microstructures with improved resistance to fracture. With this aim, we have investigated the dynamic compressive strength at very high strain rates of the order of 104 s -1 and dynamic fracture toughness at a stress intensity factor rate of 106 MPa√ ms-1 on the B modified Ti64 alloys (containing 0, 0.04, 0.09, 0.3, and 0.55 wt%B addition) primarily to investigate the role of Boron on the dynamic compression and fracture behavior. A Split Hopkinson Pressure Bar (SHPB) is chosen to conduct the experiments by employing a cylin drical specimen and a 3 Point bend (3PB) geometry specimen to study the dynamic compression and fracture behavior. The stress-strain response of the material is determined from the compres sion studies. The dynamically compressed samples are further taken for detailed microstructural and fractographic studies. The crack growth dynamics (nucleation and propagation events) in the dynamic fracture tests are captured using a high-speed camera. The dynamic fracture initiation toughness (KId) is computed from the elementary fracture mechanics theory, using the input load acting on the specimen and the exact time of fracture initiation. An increase of ∼10% and ∼15% is seen in the flow stress values with addition of ∼0.1 wt % B addition at strain rates of 10000 s-1 and 20000 s-1 respectively. It has been observed that there is influence of TiB particles on the crack nucleation when present in the ASB. But, overall they have very little effect on dynamic deformation behavior of the material. It has been observed that the KId decreases with increase in Boron content. A decrease of about 20% was noted in the KId of 0.55B sample compared to the standard Ti64.