Mechanism of Beta-Grain Growth in Alpha/Beta Titanium Alloys During Continuous, Rapid Heating

摘要

Rapid Heat Treatment (RHT) of commercial titanium alloys is a promising novel technology ensuring a high level of mechanical properties as well as high cost efficiency of processing. One of the significant advantages of RHT is related to the possibility of employing heating to temperatures in the single-phase beta field. By this means, the technique can be used to obtain fully lamellar intragrain microstructures favorable for high creep resistance and fracture toughness with relatively small or moderate size beta-grains. Such microstructures give rise to good tensile ductility and fatigue properties, comparable to those of equiaxed or bimodal microstructures. Therefore, fully lamellar microstructures with a controlled beta grain size of around 100 micrometers are a feasible way to balance the tensile, fatigue, and creep properties of titanium alloys. With direct resistance heating, these microstructures and the corresponding properties can be produced throughout the part uniformly. With induction heating, gradient type microstructures can be produced in which the properties of surface and core volumes of the heat-treated part are selectively controlled depending on the intended service application. In either case, an understanding of beta grain growth behavior during continuous, rapid heating is the key point. Thus, the goal of this investigation was to establish the temperature dependence of beta-grain size and its dependence on crystallographic texture for the alpha/beta alloy Ti-6A1-4V. The principal variables to be investigated include processing history, heating rate, and method of rapid heating (direct resistance or induction) in order to provide an explanation of beta-grain growth mechanism during continuous rapid heating. As a final output of this effort, practical recommendations for choosing RHT parameters to ensure optimized balance of mechanical properties will be developed.