Hot Workability of a Spark Plasma Sintered Intermetallic Iron Aluminide Alloy Above and Below the Order-disorder Transition Temperature

Abstract

The Fe-25Al-1.5Ta alloy has proved to be qualified for structural applications at and above 600°C due to a superior creep resistance to its binary counterpart. The creep resistance of the Fe-25Al-1.5Ta alloy at 650°C surpasses that of the P92 martensitic-ferritic steel, which is one of the most developed creep resistant alloys for steam turbine applications. From the viewpoint of cost-effectiveness in real-scale forgings, it is important to safely deform the material at as low as possible temperatures. Based on Thermo-Calc computations, the Fe-25Al-1.5Ta alloy shows a B2-to-A2 order-disorder transition at around 860ºC. This paper investigates the hot compression behavior and microstructural evolution of a Fe-25Al-1.5Ta alloy deformed in the disordered A2 (900-1100ºC) and ordered B2 (800-850ºC) regimes. Effects of ordering on plastic deformation, energy dissipation efficiency and instability parameters are identified using the concept of processing maps, and the underlying deformation mechanisms are characterized using scanning electron microscopy and electron back-scattered diffraction. The samples deformed in the A2 disorder region showed no flow instability for the deformation conditions tested, while the specimens deformed in the ordered B2 region revealed a region of flow instability located at 900ºC/1s-1. The observed flow instability region manifests itself in a longitudinal surface crack formed in the samples deformed at 900ºC/1s-1. The change of activation energy of hot deformation and efficiency of energy dissipation are discussed based on the ordering effect and movement of super-dislocations in the B2 regime. The current study identifies processing parameters to safely deform the Fe-25Al-1.5Ta alloy at lower temperatures of 800ºC and at strain rates below 1s-1.