Site Occupancy of the B2 Phase in Ti-25Al-zMo Alloys

1. Introduction

The intermetallic compounds with B2 (ordered bcc, CsCl type) structure have been observed in several binary, ternary and multi-component alloys (Banerjee, 1993; Banerjee, 1997; Kim, 1989; Bradley & Taylor, 1937). The stoichiometric composition of this phase in binary alloy is AB while in ternary alloys is A₂BC or A₄B₃C. However, these alloys often exist over a wide range of compositions on either side of the stoichiometric composition. The deviations from the stoichiometric composition are accommodated by constitutional defects namely vacancies on the different sublattice sites and/or antisites (Bradley & Taylor, 1937). The properties of the B2 phase change drastically with deviation from stoichiometric composition (Westbrook, 1967). This behavior is quite different from that of solid solution for which properties normally vary smoothly with compositions (Westbrook, 1967).

The existence of B2 phase has been identified in several Ti₃Al, Ti₂AlNb and TiAl based alloys as well as in Nb and Ta based alloys (Naka et al., 1992; Naka et al., 1993; Singh et al., 2007; Bendersky et al., 1990; Bohm & Lohberg, 1958; Hamajima et al., 1972; Azad et al., 2006; Das & Das, 2003; Chaumat et al., 1999; Banerjee et al., 1987; Nandy et al., 1988; Singh et al., 2008; Haddad et al., 1994). Naka et al. (1993) have proposed a classification of alloying elements of titanium based B2 phase into three groups which are X (Ti, Zr, Hf), Al and M (V, Ta, Nb, Cr, Mo, W etc.). Both the X and M may correspond either to an individual element or to a combination of elements of each group. The elements in group X and M exhibit close packed hexagonal (cph) and body centred cubic (bcc) structures at room temperature, respectively.

Site occupancy of the B2 phase can be experimentally determined by several competing or supplementary techniques and also can be predicted theoretically. This has been summarized by Jones in his review article on the determination of locations of chemical species in ordered compounds using atom location by channeling enhanced microanalysis (ALCHEMI) (Jones, 2003). Rietveld refinement of X-ray and neutron diffraction data is one of the techniques which has been used to determine site occupancy of the B2 phase for stoichiometric and non-stoichiometric alloys (Chaumat et al., 1999; Singh et al., 2007; Singh et al., 2008).

The B2 phase has been observed in Ti-Al-Mo system in a series of alloys which possess both the stoichiometric as well non-stoichiometric compositions either having single B2 phase or mixture of two or three phases (α₂ + B2, α₂ + γ + B2) (Bohm & Lohberg, 1958; Naka et al., 1993; Das & Das, 2003; Azad et al., 2006; Singh et al. 2007, 2008). Singh et al. (2007, 2008) have determined the site occupancy of the B2 phase in Ti-Al-Mo alloys using X-ray and neutron diffraction data. These results are classified into two groups: (1) the alloys containing Ti<50 atom % and (2) the alloys containing Ti>50 atom %. In former case, A sites are occupied by Ti atoms while the B sites are occupied by Al and Mo atoms. The small amount of Mo atoms can also occupy A sites either due to excess of B sites or anti site effect in non-stoichiometric alloys. In later case, the A sites are occupied by Ti atoms while the B sites are populated by Al and Mo atoms. The excess Ti atoms are occupied B sites.