Harmonic alloys constitute a new class of heterogeneous materials with a grain size gradient. Processed by powder metallurgy, they demonstrate enhanced mechanical properties compared to their conventional counterparts. Using mechanical testing and transmission electron microscopy observations, especially in-situ, we show, in the biomedical Ti-Nb-Zr alloy, that the elementary deformation mechanisms are different in conventional and harmonic alloys. These findings inform both on the hardening and the ductility of this alloy.
The elementary deformation mechanisms have been studied both after mechanical tests and in-situ in a transmission electron microscope (TEM), in a conventional and harmonic microstructure. We show that the deformation mechanisms are quite different: the harmonic one deforms by dislocations, glinding from the core and pilling up against the shell, before being transmitted. In the conventional alloy, besides dislocation glide, the deformation is accommodated by bands composed of several unusual twins for bcc alloys. By combining automated orientation mapping in TEM and straining experiments, we were able to show that twinning occurs after the formation of a martensite phase. The sequence of twinning and phase transformation leads to bands with the same morphology as the ones observed post-mortem. This indicates that twinning is probably a consequence of martensite relaxation. In both case, we show that the initial hardening is controlled by the pinning of gliding dislocations on clusters of solute atoms. Stress measurements at dislocation scale were found comparable to the ones performed during macroscopic mechanical tests. The difference in mechanical behavior between the two microstructures can be attributed to both the existence of chemical heterogeneities arising from the material processing and the existence of strain redistribution mechanisms in the shell of the harmonic structure.
Conventional vs Harmonic-structured β-Ti-25Nb-25Zr alloys: a comparative study of deformation mechanisms,
F. Mompiou, D. Tingaud, Y. Chang, B. Gault, G. Dirras,
Acta Materialia, Volume 161, December 2018, Pages 420-430,
Dr. Frédéric MOMPIOU, CEMES (CNRS)