Controlling crystallographic ordering in Mo–Cr–Ti–Al high entropy alloys to enhance ductility

Abstract Refractory high entropy alloys (RHEAs) from the Mo–Cr–Ti–Al system exhibit promising strength at elevated temperatures. However, at room temperature they possess a B2-ordered crystal structure and, therefore, exhibit reduced ductility. Guided by thermodynamic calculations, the Al concentration was systematically reduced in MoCrTi-xAl (x = 25, 15, 10, 5, 3 at%) in order to achieve a disordered, solid solution with A2-type crystal structure even at room temperature. The alloys were manufactured by arc melting followed by a homogenization treatment. To evaluate the chemical homogeneity, backscatter electron (BSE) imaging and energy dispersive X-ray spectroscopy (EDS) were performed. Dynamic differential scanning calorimetry (DSC) and selected area diffraction utilizing transmission electron microscopy (TEM-SAD) were performed for the: (i) detection of the characteristic heat signatures of the disorder-order transformations and (ii) determination of the ordering state at room temperature of the synthesized alloys, respectively. In case of MoCrTi–3Al, a single-phase A2 crystal structure without indications of a disorder-order transformation was obtained. Compression tests at temperatures ranging from room temperature up to 800 °C reveal a significant improvement of plastic deformability at room temperature as compared to the formerly investigated equiatomic MoCrTiAl with its B2 crystal structure.