Effect of Cu on microstructure, crystallography and mechanical properties in Fe-B-C-Cu alloys

In order to improve the hardenability and toughness of high boron alloys, XCu (X = 0, 0.5, 1, 1.5, 2, 3, weight %) were added into high boron alloys. The Cu precipitation behavior and its effect on hardenability, microstructure, crystallography and mechanical properties of as-cast Fe-B-C-Cu alloys have been investigated systematically by means of SEM, XRD, TEM, EPMA and EBSD. The results indicate that Cu improves the hardenability of Fe-B-C-Cu alloys, while excessive Cu atoms segregating at the matrix attaching to boride promote the appearing of nano-Cu precipitation with body-centred cubic (bcc)structure, which can act as heterogeneous nuclei of the pearlite, leading to the reduction of hardenability. Therefore, the Cu concentration in Fe-B-C-Cu alloys should be controlled below 2 wt%. Observation of TEM reveals that the orientation relationship(OR) between martensite and retained austenite are [001]_α // [011]_γ and [110] _α//[1(−)1(−)1] _γ. As well, the OR between ferrite and M₃C are (100)_M// (010)_M3C and [011]_M//[001]_M3C. EBSD analysis shows that the misorientation angles of the α-Fe and M₂B phases are mainly low-angle with less than 5°. The α-Fe phase has the preferred crystal plane (111) // ND-type grain, while the M₂B phase displays a preferred growth orientation [002] on the M₂B crystal plane (001). A possible OR between borides and α-Fe in Fe-B-C-Cu alloys is (001)_α-Fe //(001)_M2B. In addition, the bulk hardness and impact toughness increase obviously with 1–2 wt% copper concentration owing to the increasing of volume fraction of martensite(V_m), which maybe a guiding copper addition in Fe-B-C-Cu alloys.