Structure and Properties of TiAl-Based Alloys Doped with 2 at.% Mo

Abstract

Arc-melted Ti100–xMo2Alx alloys (where x is 44, 46, 48, and 50 at.% Al) that were produced from pure components were examined by X-ray diffraction and scanning electron microscopy with electron microprobe analysis (SEM/EDX). The melting points and solid-phase transformations were studied by differential thermal analysis (DTA) and the mechanical properties by fracture toughness, bending, and compression tests. The CALPHAD approach was used for thermodynamic calculation of phase equilibria in the composition region under study. The cast alloys mainly consist of a lamellar structure formed by lamellas of γ-TiAl and α2-Ti3Al phases with submicron thickness and a cubic Ti55Mo4–6Al39–40 phase of A2 type (β) or B2 type (β0). In the γ-TiAl-based alloys, molybdenum behaves as a low-melting dopant, enriching the grain periphery. The standard mechanical characteristics of the alloys were determined and their structural sensitivity was analyzed. All studied alloys demonstrate excellent high-temperature strength. The yield stress is 400–600 MPa in the temperature range 20–750°C. The strength slightly increases at 300 and 600°C under dynamic strain ageing. The strain hardening parameters were established over a wide range of test temperatures. The temperature dependence of the strain hardening coefficient and strain hardening index was analyzed for alloys in different phase and structural states. In the temperature range from 20 to 600°C, the strain hardening index and coefficient were found to vary slightly. The strain hardening index increased from n = 0.6 to n = 0.95 with aluminum content changing from 44 to 50 at.%, which indicates that the strain hardening mechanism changes with variation in the alloy phase constituents.