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Platinum Metals Rev., 1975, 19, (3), 95

Intercrystalline Rupture of Platinum Alloys

Grain Boundary Migration and Intercrystalline Slip Near Welds

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The fabrication of articles from platinum alloys generally requires the use of welding techniques but the difficulty sometimes arises that intercrystalline cracking occurs in the vicinity of the welded seam. The exact conditions under which this problem arises have yet to be fully defined. However, progress in this direction has been made by a team of workers at the Ural Polytechnic Institute named for S. M. Kirov (V. V. Stepanov, T. A. Chernyshova and V. V. Shevelev, Fiz. Metal. Metalloved., 1975, 39, (1), 183–188).

The alloys studied were 7 per cent rhodium-platinum, 10 per cent rhodium-platinum and 5 per cent rhodium–15 per cent palladium-platinum. Results were obtained for the effect of chemical composition on the tendency to crack formation and for the process of grain growth during repeated welding cycles.

A large number of welds and their surrounding zones were observed in all three alloys. These indicated that twice as much cracking occurred in the ternary alloy as in 7 per cent rhodium-platinum but that the least amount of cracking occurred in 10 per cent rhodium-platinum. Small additions of magnesium, iron, calcium, aluminium, and silicon and of their combinations were made to the alloys. It was found that more cracking resulted, particularly induced by additions of silicon, aluminium and calcium.

Fractographic analysis was undertaken to establish the mechanism of cracking before attempting to eliminate it. Two features became apparent. First, lines of intercrystalline slippage develop and, secondly, secondary phases in the form of fine dark deposits occur on the facets and boundaries of the crystals. The latter incorporate the deleterious additions.

A suggested mechanism for crack formation is that voids form between crystals because of slip. The links between crystals are thus broken and cracking is initiated. To verify this mechanism grain growth was observed closely in the weld and in the surrounding area. The thermal effects of welding induce grain growth in the alloys and this in turn may cause the intercrystalline slip which leads to voids and rupture at certain temperatures of welding, especially for impure alloys.

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