Towards a better understanding of the deformation in harmonic Ti-Nb-Zr alloys

Monday, October 29, 2018

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.

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In the context of more demanding metallic materials for structural applications, a new concept has been settled to develop original heterogeneous microstructures with grain size gradient, called “harmonic”. This approach exploits mechanical milling and sintering methods to form a multimodal microstructure composed of a matrix core of large grains embedded in an ultra-fine grained interconnected shell. These alloys show enhanced mechanical strength while keeping a good ductility, two rather antagonist properties. Within the framework of the ANR project HighS-Ti (LSPM, IJL PPRIME, CEMES), the harmonic design has been applied to Ti alloys and especially to a beta Ti-Nb-Zr alloy. These type of alloy are indeed excellent candidates for implant materials as they present a stiffness comparable to the bone, insuring a good mechanical biocompatibility.

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,







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