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New non-equiatomic Ti_(25+x)-Zr₂₅-Nb₂₅-Ta_(25-x) (x = 0, 5, 10, 15, 20, in at%) medium entropy alloys (MEAs) have been designed using the atomic mismatch approach and fabricated through a conventional arc-melting process. These novel MEAs were derived from a recently developed equiatomic Ti-Zr-Nb-Ta MEA by gradually replacing its Ta content with Ti. Each non-equiatomic MEA solidified as a single solid-solution phase, which was characterised in detail and compared with Pandat™ simulation and empirical rules. Systematic tensile mechanical property data revealed the existence of a brittle-to-ductile transition for Ti-Zr-Nb-Ta MEAs, i.e., when 15 at% of Ta in the equiatomic Ti₂₅-Zr₂₅-Nb₂₅-Ta₂₅ MEA was replaced by Ti to become a Ti₄₀-Zr₂₅-Nb₂₅-Ta₁₀ MEA. The transition occurs corresponding to a small reduction in atomic mismatch from 4.72% to 4.65% but a signficant drop in nanoindentation …