Effect of Precipitation Hardening on the Superconducting Properties of Aluminum

The effect of heat treatment on the superconducting properties of a precipitation hardening Al-Mg-Si alloy has been studied in some detail by ballistic induction techniques. The alloy (Alcoa 6063) contains approximately 1.1% ${\mathrm{Mg}}_2$Si which precipitates from a supersaturated solid solution to an extent controlled by the heat treatment. The precipitation process results in marked changes in the mechanical properties of the alloy, and equally pronounced changes are found to occur in the superconducting behavior. For heat treatments which produce mechanical properties similar to those of pure Al, the superconducting properties of the alloy are found to be substantially the same as pure Al. Heat treatment to the condition of maximum hardness depresses the critical field of the alloy about 8 gauss below that of pure Al, the effect being closely similar to that expected for the application of hydrostatic pressure. In conditions of intermediate hardness pronounced hysteretic effects are observed. It is proposed that the observed changes arise from coherency stress around the Guinier-Preston zones which describe the aggregation of Mg and Si atoms in the early stages of precipitation.