Laser directed energy deposition of mixed Ti65 alloy and CoCrNi medium entropy alloy: process, microstructure and mechanical property

Laser directed energy deposition of near-α titanium alloys often leads to coarse columnar prior-β grains and strong crystallographic texture, which limit mechanical performance. To address this, an equiatomic CoCrNi medium-entropy alloy was added with Ti65 alloy to achieve the grain refinement and columnar-to-equiaxed transition, which led to a balanced strength-plasticity combination. According to the thermodynamic calculations, adding a CoCrNi content of 1.5 wt% effectively widens the solidification temperature range while avoiding the occurrence of brittle intermetallic phases. A homogeneous CoCrNi and Ti65 mixed powder with CoCrNi uniformly distributed in the interstices of Ti65 particles and both maintaining high sphericity, was fabricated via low energy ball milling. The influence of the CoCrNi content and laser processing parameters on the microstructure and mechanical properties of the CoCrNi-Ti65 alloy was investigated. The calculated growth restriction factor revealed that the grain-refining effectiveness of the alloying elements follows the order Co > Ni > Cr. The addition of CoCrNi enhances constitutional supercooling ahead of the solid/liquid interface, providing a strong thermodynamic driving force for equiaxed grain nucleation. Furthermore, CoCrNi randomizes grain boundary character distribution by altering variant selection in the β → α transformation. Quantitative assessment showed that grain refinement strengthening effect constituted the primary contribution the yield strength increment in the 1.5 wt%CoCrNi-Ti65 alloy. In the 2 wt%CoCrNi-Ti65 alloy, the precipitation and grain refinement strengthening effects together provided the dominant contribution.