SUPPLEMENTARY X-RAY STUDIES OF THE Ni-Sn-Bi SYSTEM

Phase equilibria were studied in the system Ni-Sn-Bi. Special attention has been paid to the identification of the reacently found ternary phase. For this purpose samples were synthesized using intimately mixed powders. After annealing and quenching, all alloys were analyzed by scanning electron microscope and by X-ray diffraction. The results give evidences about the existence of a ternary compound with approximate formula Ni6Sn2Bi to Ni7Sn2Bi. Overlapping of some neighbouring diffraction peaks of this phase with NiBi and Ni3Sn_LT is the reason for the difficulties related to the X-ray diffraction identification of the ternary phase.

Although the knowledge of a phase diagram is of fundamental importance for developing new materials, systematic explorations of the Ni-Sn-Bi alloys were not undertaken until now.To revamp this void, phase equilibria studies of this ternary system were performed recently by Vassilev et al. [9] and a formerly unknown ternary compound with approximate formula Ni 7 Sn 2 Bi was observed.In this study we report auxiliary X-ray studies of the nickelrich region of the ternary equilibrium diagram.

Experimental
Supplementary samples were synthesized and studied, in order to inquire the phase equilibria related with the formerly unknown Ni-Bi-Sn compound previously reported [9].For this purpose, five samples with nominal composition 68% Ni, 20% Sn and 12% Bi were annealed in evacuated and sealed quartz tube at 873 K, for two weeks, and quenched in cold water.Powders (325 mesh) of pure metals (Ni (4N), Sn (5N), Bi (4N)) (production of Goodfellow) were used.It happened that during the annealing all pellets (5 mm of diameter and 3 to 4 mm thick) stuck together, thus they were treated as one sample referred further as S1.In order to avoid some side reactions, previously observed in sealed silica tubes, a specimen with the same nominal composition (S2) was prepared and put in an alumina crucible placed into iron container.This container was heated at 873 K in a furnace for two weeks, under flowing argon atmosphere and quenched in cold water.The specimen S2, already annealed at 873 K for two weeks, was annealed for 3 months yet to verify the phase equilibria.
In addition, mechanical alloying method was applied to synthesize the ternary phase.For that purpose, a ribbon (1.5 mm thick) of Ni, Sn and Bi powders (325 mesh) was produced using manual rolling-mill.The composition of the mixture was 68% Ni, 19% Sn and 13% Bi.This way of preparation should provide more homogeneous samples.This specimen, denoted as S3, was annealed at 903 K for 3 months in evacuated and sealed quartz tubes.

Results and discussion
The ternary compound Ni 6 Sn 2 Bi was (as previously reported [9]) observed by electron probe microanalyser (EPMA) in the supplementary synthesized specimens: S1 (annealed at 873 K), S2 (annealed at 873 in iron container) and S3 (annealed at 903 K).In these three samples the following phase equilibria were found: S1 -Ni 3 Sn+NiBi+Ni 7 Sn 2 Bi (Fig. 1  (curve 3).The diffractogram was checked up for the presence of other phases (e.g. the most probable was (Ni)).Systems of diffraction peaks belonging to NiBi and Ni 3 Sn_LT and another system of 15 peaks that probably are associated with the formerly unknown ternary phase Ni 7 Sn 2 Bi were found (Table 1).
The diffractograms are noteworthy with overlapping of some neighboring diffraction peaks.Four peaks are single (these at 2θ = 27.2,29.6, 30.8, 32.0, 43.5 deg), while nine represent multiple diffraction, i.e. double and triple peaks.The overlapping of the peaks is probably due to similar crystal structure parameters of the present phases.2-4).

Fig. 1 .
Fig. 1.Micrograph of specimen S1 in back-scattered electrons.The dark phase is of Ni 3 Sn; small light phase represents NiBi and the light gray phase -Ni 7 Sn 2 Bi