On the Possibility of Detecting Shock-Induced Second-Order Phase Transitions in Solids. The Equation of State of Invar

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Abstract
First‐order phase changes in shock‐loaded solids have been previously detected from the formation of multiple‐wave structures in the solids. It is shown theoretically that a multiple‐shock structure under shock loading also may be produced by a second‐order phase transition. Accordingly, the possibility of detecting shock‐induced demagnetization of ferromagnetic metals and alloys is discussed and an experimental attempt to detect such a demagnetization in shock‐loaded Invar (64 Fe, 36 Ni) is reported. Extrapolation of data by Patrick suggests a Curie point transition in Invar in the neighborhood of 50 kbar. The Hugoniot equation of state of Invar has been obtained over a pressure range of 35 to 160 kbar. No double‐shock structure was observed, but a decrease in compressibility observed in the neighborhood of 60 kbar is thought to be evidence of demagnetization.