Abstract
A metallurgical methodology has been set up to design alloys with improved mechanical properties such as strength-ductility synergy, via improved strain hardening. To this end, a multi-concepts approach including High-Entropy-Alloys (HEA), grain refinement, chemical contrast and using the transformation induced plasticity (TRIP) deformation mechanism has been implemented. The use of a conventional thermomechanical treatment comprising cold rolling followed by annealing at 650°C, produced two high entropy alloys with submicron alpha (hcp) and beta (bcc) phases. For Ti35-5-5 alloys, stress-induced martensitic transformation, and its subsequent reverse transformation during annealing at 650°C, led to a well-recrystallized state. A microstructure consisting of alpha and beta equiaxed grains was obtained, whose size increased within the submicron domain from about 300 nm to 600 nm with the holding time between 15 to 300 min. Contrariwise, for Ti35-6.5-6.5, which does not deform by the TRIP effect, the same thermomechanical treatment does not produce the recrystallization. Rather precipitation of platelets in the recovered beta matrix occurred. As for the mechanical properties, the yield strengths of the alloys with dual-phase microstructure lie between 950 and 1150 MPa for Ti35-5-5 and between 850 and 950 for Ti35-6.5-6.5 counterpart, for annealing times ranging from 15 min (higher yield strength) to 300 min (lower yield strength). This corresponds to a very large increase in the yield strength compared to that of the as-cast alloys, displaying values of about 400 MPa and 725 MPa for Ti35-5-5 and Ti35-6.5-6.5, respectively. Reasonable ductility was obtained for the alloys with optimized microstructures, that both display a tensile ductility of about 12% after annealing for 300 min at 650°C
Highlights
It is clear that the strategy for the choice of a material results in compromise between optimal physical performances for the foreseen application and the capacities of shaping and mechanical strength required
The same behavior was reported by Fellah et al (2012) in the case of UFG Co processed via Hot Isostatic Pressing (HIP) via twinning-induced plasticity (TWIP)-like effect
The influence of thermo-mechanical treatments was studied for two high-entropy alloys (HEAs)
Summary
It is clear that the strategy for the choice of a material results in compromise between optimal physical performances for the foreseen application and the capacities of shaping and mechanical strength required. In the case of Cu, discussed in their work, the bimodal microstructures were obtained through severe plastic deformation (SPD) followed by heat treatment Interesting, this two-step strategy is not flexible because it does not allow a truly in-demand bimodal microstructure with a balanced volume fraction of each phase. Improved strain hardening rate due to delocalized plasticity from the coarse-grained core to the fine-grained shell and backstress contribution has been identified as the main mechanism of plasticity occurring in such heterogeneous materials This is in line with a report by Lu (2014). Combination of the TRIP mechanism with another classical metallurgy approach, the precipitation hardening, was done by Lilensten et al (2019) and Danard et al (2019) These studies emphasize that several metallurgical design strategies can be used together to provide the best of each strategy’s properties. The developed methodology is expected to open on many other perspectives in terms of innovative microstructures and new physical and mechanical properties
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