Effects of interstitial impurities on phase equilibria

Examples of the effects of impurities on phase equilibria in alloys of Th, Zr, V, Nb have been considered and discussed by Carlson and Smith (1987). Phase stabilization and also phase de-stabilization (carbon effect on Zr4Sn) processes have been described. 

For the niobium-copper system different phase diagrams of the simple eutectic type (with the eutectic point very close to Cu) have been proposed, either with an S-shaped near horizontal liquidus line or with a monotectic equilibrium. It was stated that the presence of about 0.3 at.% O can induce the monotectic reaction to occur, whereas if a lesser amount of oxygen is present no immiscibility gap is observed in the liquid. 

A similar behaviour has been suggested for the vanadium-copper system. Examples of shifts of solid-solution boundaries due to small amounts of interstitial impurities have also been discussed with special reference to the Y-Th and Th-Zr alloy systems. 

The purities in mass%, declared by worldwide commercial suppliers, range from 99.9% for alkaline-earth metals and rare earths metals, to 99.999% for Al, Cr, Mn, Fe. .., to 99.9999 for Au, Ag, Pt. In certain cases, these values can be misleading since they refer to the metal contents, disregarding the non-metallic impurities contents (interstitial H, C, N, O, etc.). Moreover, the concentration is expressed in mass% and, for low atomic mass impurities, such as H or O, this can result in a much lower at.% purity value. The elements for special use, such as for spectrog-raphy, may require very high purities, which can be attained only by specialized laboratories. 

A number of intermetallic phases may be prepared directly starting from the pure components suitably mixed together. The synthesis reaction may then be performed 

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