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Body-centered cubic (BCC) lightweight refractory high-entropy alloys (LWRHEAs) with Al contents have attracted much attention due to their low density and excellent mechanical properties. However, these typical lightweight alloys often suffer from poor room temperature plasticity. In this study, we prepared TiZrNbTa0.1Alx LWRHEAs by using a high-vacuum arc-melting technique and investigated the influence of Al content on the phase structures and mechanical properties. It was found that the TiZrNbTa0.1Al1 alloy showed a BCC solid solution matrix with some micrometer-sized Al3Zr5 precipitates and exhibited a density of 6.110 ± 0.003 g/cm3. The TiZrNbTa0.1Al1 alloy had a low mixed enthalpy of −20.831 kJ/mol, a compressive yield strength of 1037 ± 178 MPa, and a fracture plasticity of ∼6%. As a result of reducing the Al content, the TiZrNbTa0.1Al0.2 alloy showed a simple BCC phase structure without any precipitates and maintained a low density of 6.743 ± 0.008 g/cm3. The TiZrNbTa0.1Al0.2 alloy had a relatively high mixed enthalpy of −4.5577 kJ/mol, a high yield strength of 1022 ± 51 MPa, and a plasticity of >70%. The TEM analysis results demonstrated that the excellent mechanical properties of this LWRHEA were mainly attributed to the reducing Al content, which could elevate the mixed enthalpy of the alloy to eliminate the brittle Al3Zr5 phase and induce the formation of dense network dislocations at the grain boundaries. [ABSTRACT FROM AUTHOR]