Martensitic transformation and ordering in heat-resistant Ni-Co-Cr-Al β/γ eutectics

The method of thin-foil transmission electron microscopy was used to examine phase transformations in the beta-phase, representing the main heat-resistant component, of beta/gamma eutectics of the Ni-Co-Cr-Al system (mass %: 0-15 Co, 14-20 Cr. 8.5-10.5 Al). The martensitic transformation beta(B2)-->L1(0) took place in all the alloys, except those with 20 mass % Cr, when they were quenched in water from 1200degreesC. It was found for the first time that high-rank superstructures A(2)B (Ni2Al) and A(5)B(3) (Ni5Al3) could be formed in a complex-alloyed P solid solution. Phase transformations L1(0)-->A(5)B(3), L1(0)-->14M, L1(0)-->B2-->A(2)B, and L1(0)-->B2-->A(5)B(3) were observed in the beta-component of the eutectics when isothermal annealing temperature was increased from 250 to 600degreesC. At a temperature of 400degreesC and higher those transformations were accompanied by the decomposition of the 0 solid solution and the appearance of dispersed alpha-Cr particles. Structural features of the aforementioned phase transformations in heat-resistant quaternary eutectics were considered.