<p>The phase constitution, microstructure, damping capacity, and mechanical properties of as-cast Al_<i>x</i>CrFe₃Ni (<i>x</i>=0.5, 0.52, 0.54, and 0.56, respectively) medium entropy alloys were investigated. It is found that the volume fraction of BCC phase increases while that of FCC decreases with increasing the Al content. When the content of Al is 0.54, the alloy is composed of 82.1vol.% BCC matrix and 17.9vol.% FCC phase. Wherein the FCC phase is distributed on the BCC matrix, forming a structure where the hard BCC matrix is surrounded by soft FCC phase. This results in a hindering effect on the propagation process of vibration waves. The damping performance of Al_0.54CrFe₃Ni alloy, characterized by an internal friction of <i>Q</i>⁻¹ is as high as 0.059, is higher than that of most FeCr damping alloys. The volume fraction of the BCC phase and the peculiar distribution of the FCC phase are identified as the key factors affecting the damping capacity. In addition, the Al_0.54CrFe₃Ni alloy exhibits a high yield strength of 811.16 MPa.</p>