Cast CrCoNiAlx (x=0-1.2) medium-entropy alloys (MEAs) were produced by arc melting and flip cast to investigate the alloying effect of Al addition on the microstructure,phase constituent and mechanical properties.The crystal structure changes from an initial face-centered cubic (FCC) to duplex FCC and body-centered cubic (BCC) and finally a single BCC with increasing Al content.In the duplex region,FCC and BCC phases form under a eutectic reaction in the interdendrite region.In the single BCC region,the dendrites transform to ordered B2 and disordered A2 BCC phases resulting from spinodal decomposition.Corresponding to their phase constituents,yield strength increases accompanied with an elongation reduction with increasing Al addition.A very interesting phenomenon of very weak ordered FCC (001) spots appearing in Al-0.4 alloy was observed,indicating a local ordering of FCC phase.The changes of fracture surfaces after the tensile deformation are also corresponding to the variations in mechanical properties.