Synthesis and characterization of mechanically alloyed Ni-Sn powders

Ni1-xSnx alloys were synthesized by mechanical alloying (MA) from pure nickel and tin powders. The reaction products were characterized by X-ray diffraction, differential scanning calorimetry, and scanning and transmission electron microscopy. In the composition range studied (0.2 < x < 0.4), a single-phase amorphous alloy was obtained for Ni75Sn25. At all other compositions, the powders were mixtures of amorphous and microcrystalline phases. Amorphous Ni75Sn25 alloy, heated at 10 K min-1, crystallizes polymorphously into the intermetallic Ni3Sn at 608 K. The crystallization enthalpy and the apparent activation energy of crystallization are 3.4 and 213 kJ mol-1 respectively. A free-energy diagram was calculated as a function of composition for T = 240 K (the ball milling temperature) using a thermodynamic model and our data. Comparison of this diagram with the products from the MA tests indicates that the formation of amorphous Ni1-xSnx. competes with the formation of crystalline Ni3Sn2 and the terminal nickel-rich solid solution. The amorphous Ni75Sn25 powder had a diamond pyramid hardness of 730 kg mm-2. This powder was hot-pressed at 523 K and 0.86 GPa into cylinders 4.8 mm in diameter and 3 mm in length. The amorphous compacts had a density 98% of that of crystalline Ni3Sn and a diamond pyramid microhardness of 750 kg mm-2. Corrosion-resistance measurements showed no weight losses after exposure to HNO3 or H2SO4 for 6 days. © 1988.