Microstructure of Cr_100−xAl_x alloys (10 at% ⪅ x ⪅ 33 at%) studied by means of transmission electron microscopy and diffraction. II. Discovery of a new phase

This paper reports on a new phase, denoted X, discovered in Cr_100–xAl_x alloys in and around the composition range 20 at% ⪅ x ⪅ 25 at%. The electron diffraction patterns of this phase appear to be commensurate with the reciprocal lattice of the disordered b.c.c. α‐phase, except in rapidly cooled samples. Dark‐field images show that the reflections originate from very small domains, 1 to 3 nm in size. High‐resolution images reveal rather ill‐defined fringes corresponding to two X‐phase orientation variants. The X‐phase is accompanied by the β‐phase in a large part of the composition range considered. The identification of the X‐phase either as a rhombohedral superstructure or a concentration modulation of the α‐phase or as a periodic anti‐phase boundary structure of the β‐phase is discussed. The influence of size‐, disorder‐, and twinning effects on the diffraction patterns and high‐resolution images is consdered. Finally, a tentative correction to the currently accepted phase diagram is presented.