A New Face-Centered-Cubic Superlattice Structure in Rapidly Solidified Cu-4 Wt Pct Ti Alloy and Its Relevance to the Ordering Process in Ni-Mo and other 1 1/2 0 Ordering Alloys

摘要

Ordering processes in alloys showing short-range order (SRO) intensity maxima at { 1 1/2 0 } and equivalent positions in the fcc reciprocal space exhibit several intriguing characteristics and therefore have been the subject of numerous theoretical and experimental investigations. The binary Ni-Mo~[1-8] and some of its ternary derivatives ~(9,10) are classic examples of such alloys. In alloys with composition close to Ni_(80)Mo_(20), for instance, ordering initiates by the appearance of SRO intensity maxima at { 1 1/2 0}_(fcc) and equivalent positions (Figure 1(a)) in selected area diffraction patterns (SADPs). On aging, there is initially an increase in the intensity of the SRO maxima. ~[7,8] This is followed by a Continuous transformation to the equilibrium long-range ordered (LRO) Ni_4Mo (strukturbericht symbol D1_a) phase. Wing-shaped streaks of intensity extending from the SRO maxima to the superlattice positions of D1_a can be seen in Figure 1(b). Prplonged aging ~[2,3,8] leads to disappearance of both the streaks and the SRO maxima, and sharp superlattice reflections belonging to two different orientational variants of D1_a appear at l/5{420} and 2/5{420} (and also at 3/5 {420} and 4/5{420}) positions, as shown in the key in Figure 1(e). One characteristic feature of Ni_4Mo (D1_a) and other equilibrium or metastable superlattice structures ~[2,5,9,11] of the 1 1/2 0 family, such as Ni_3Mo (D0_(22)), Ni_2Mo (Pt_2Mo-type), or the hypothetical equiatomic N_2M_2 structure, ~[12,13] is that they comprise all Ni or all Mo atomic layers on {420}_(fcc) planes. As a consequence, the superlattice reflections arising from these are found to be placed periodically along <420>* directions in the reciprocal space of fcc. 1/4{420}, 2/4{420}, and 3/4{420} are the superlattice positions of D0_(22); 1/3{420} and 2/3{420} belong to Pt_2Mo, while N_2M_2 shows superlattice reflections at 1/4{420} and 3/4{420}. Some of these structures are shown in Figure 2, and their schematic diffraction patterns are shown in the keys in Figures 1(e) and (f). Note that the patterns of D0_(22) and N_2M_2 differ in only one respect--the absence of {210} reflections (equivalent to {100}) in the latter.