Atypical phase transformation behavior of Fe-33%Ni alloys induced by shot peening

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The phase transformation behaviors of two Fe-33 mass%Ni alloy specimens subjected to shot peening (SP) were investigated. The first sample consisted solely of an austenite (γ) phase while the second also contained a high proportion of a martensite (α′) phase. When the specimen containing only the γ phase was subjected to SP, a stress-induced martensitic transformation occurred. In contrast, the second sample underwent the reverse transformation from the α′ to γ phase. Moreover, the volume fraction of the γ phase in the vicinity of the peened surface of each specimen was found to plateau as the SP treatment was prolonged. The final volume fraction of the γ phase was the same in both specimens regardless of their initial microstructure prior to the SP process. In addition, the volume fraction of the γ phase was found to increase with increasing peening pressure, meaning that the reverse transformation was promoted by SP at higher pressures. Calculations of the deformation heat generated during SP indicated that this heat was not primarily responsible for the reverse transformation induced by SP. Estimation of the change in the phase transformation temperature resulting from the residual compressive stress imparted by SP showed that this treatment changed the value of T0 (the arithmetic mean of the martensitic transformation start temperature and the reverse transformation start temperature) for the Fe-33 mass%Ni alloy from 328.5 K to 287.8 K. Because T0 after SP was lower than the temperature to which the specimens were raised during SP, this process could have caused stress-induced reverse transformation. Therefore, the decrease in the phase transformation temperature stemming from residual compressive stress is believed to explain the reverse phase transformation observed during SP of this alloy.