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](alpha) // [011](gamma) and [110] (alpha),//[1(-)1(-)1] (gamma). 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 alpha-Fe and M2B phases are mainly low-angle with less than 5 degrees. The alpha-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 alpha-Fe in Fe-B-C-Cu alloys is (001)(alpha-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.