Influence of cold creep and stress/strain redistribution on dwell sensitivity in titanium alloys

• Main Author: Sackett, Elizabeth Eileen
• Published: Swansea University 2008
• Subjects: 620.1
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.638822

A model, large grained variant of the titanium alloy Timetal 685 was utilised to investigate strain anisotropy and stress redistribution in a near alpha titanium alloy. Due to the heat treatment applied to the material, the maximum grain size was typically 15mm in diameter. Static and cyclic tests on cylindrical and flat plate specimen designs were performed. All forms of mechanical test illustrated significant scatter in performance, presumably an 'artefact of the relatively large size in comparison to the volume of critically stress volume within the laboratory specimen designs. Specimens with miniature strain gauges attached within individual grains have demonstrated the presence of "hard" and "soft" grain orientations within the bulk material response. Electronic speckle pattern interferometry (ESPI) was successful in demonstrating strain distributions and locating regions of very early strain concentration which ultimately lead to failure at the same location. However, the technique was discounted as a method of quantifying strain due to instability in the system over extended test periods. ESPI together with detailed electron backscatter diffraction (EBSD) measurements around sites of typical cold creep and dwell fatigue fractures have identified the key role of planar slip on prismatic systems. With reference to a previous model proposed by workers from Swansea University, describing the process of stress redistribution and quasi cleavage facet formation, this has led to an important conclusion: that basal planes do not necessarily have to be implicated in the fracture process as previously thought.