Microstructure, mechanical properties and strengthening mechanisms of 5Cr5MoV modified by aluminum

• The polygonal ferrite is obtained in 5Cr5MoV with Al concentration of 1.6 wt.%.
• The twin martensite presents in 5Cr5MoV with Al concentration between 1.6 and 2.2 wt.%.
• The tensile strength increases as Al concentration varies between 0.01 and 1.6 wt.%.
• The secondary hardening peak of 5Cr5MoV–1.6Al exhibits hardly any decrease.

The influence of aluminum on the microstructure and tensile properties of 5Cr5MoV was systematically investigated for improving the mechanical properties at room temperature. Microstructure was characterized by optical microscope (OM), scanning electron microscopy (SEM), field-emission transmission electron microscopy (FE-TEM), and additionally the uniaxial tensile tests were performed on a MTS-810 testing system. Tensile strength as a function of aluminum concentration exhibits an obvious two-stage characteristic. In stage I, 5Cr5MoV is continuously strengthened with the increase of aluminum, mainly owing to the refinement of lath martensite, the precipitation of small carbides and the presence of fine twins in martensite. The maximum tensile strength is 1303 MPa at the aluminum concentration of 1.63 wt.%. In stage II, the tensile strength rapidly drops down with the excessive addition of aluminum due to both the presence of large carbides at the boundaries between ferrite and martensite and the excessive ductile phase of irregular ferrites. As a result, the appropriate addition of aluminum can improve the mechanical properties of 5Cr5MoV by introducing the cryptocrystalline martensite as well as the dispersive polygonal ferrite in the microstructure.

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