Effect of High-pressure Quenching on Pure-iron Martensite Transformation and Its Strengthening Mechanism

Abstract

Industrial pure iron samples were austenitized and quenched (6°C/s cooling to room temperature) under hydrostatic pressure of 3-5 GPa. The morphology, phase transformation and strengthening mechanism of high-pressure quenched martensite are analyzed by the method of SEM, XRD, EBSD, and TEM. Lath martensite with hierarchical packet-block-lath structure is induced in industrial pure iron by high pressure, which keeps the Kurdjumov-Sachs (K-S) orientation relationship with the face-center-cubic (FCC) phase. Pressure refines the size of prior austenite by depressing the mobility of grain boundaries, leading to the decrease of the type of martensite variants. with the increment of pressure, the dislocation density increases gradually (2.04 × 10 13 to 3.14 × 10 14 m − 2 ) and the martensite blocks are refined from 3.3 to 0.9 μm. In addition, an enormous number of twin boundaries and high-density dislocations are observed in 5 GPa-samples, which is fairly rare in lath martensite of low carbon steels. Superior tensile performances are obtained in industrial pure iron, especially in 5 GPa-sample with ultra-high yield strength of 700 MPa and excellent ductility of 27%. The strengthening mechanism is quantitatively analyzed by Olson's strengthening model, and the results show that both of dislocation strengthening and Hall-Petch strengthening enhances with the increase of pressure. Based on the above findings, martensite transformation can be effectively controlled by hydrostatic pressure, which extends the knowledge into martensitic transformation mechanism and offers a new avenue for developing high performance metal materials.