The deformation, damage and rupture mechanisms at cryogenic temperature of an extra low interstitial Ti-6Al-4V alloy obtained by powder metallurgy are investigated through tensile tests, scanning electron microscopy observations, electron backscattered diffraction (EBSD) mapping and finite elements analysis. Crystallographic orientations, measured through EBSD, allow three different twinning modes to be identified. Among them, one has never been indexed in titanium alloys. At 20 K, half of the deformation is carried by twinning. Damage is controlled by the cavities nucleation step, which is also strongly related to twinning. These cavities appear at the very end of the deformation process and their growth and coalescence are rapid. Correlations between these results and finite element calculations are used to build damage as well as rupture maps that help to interpret the failure of the alloy.The study has presented experimental and numerical data related to the deformation and damage processes of a PM Ti-6Al-4V strained at 20 K. The associated mechanisms were analysed. The main results are summarized as follows: 1.) Three types of twins were identified in Ti-6Al-4V ELI PM strained at 20 K and account for practically half of the plastic deformation. 2.) The {'5-61-3'} twin, experimentally observed for the first time, leads to the nucleation of cavities by a twin plane debonding process. This twinning mode is the major damage precursor at a microscopic scale. {10-11} twins, identified inside the {'5-61-3'} from 10% strain, lead to internal stresses which promote the {'5-61-3'} twin plane debonding. 3.) Void nucleation is the critical step of ductile damage: the growth and the coalescence steps are very short. 4.) A rupture criterion based on two parameters, the plastic strain and the maximum principal stress, is proposed for Ti-6Al-4V ELI PM loaded at 20 K and interpreted with the help of microstructural observations. Only one mechanism leads to the rupture: the {'5-61-3'} twins debonding. Consequently, rupture is promoted for a critical stress of 3 GPa. Strong internal stresses coming from secondary twins may be added to macroscopic stress to reach locally the critical stress. 5.) A damage map was build and a physical parameter called critical damage volume (CDV) is proposed to explain several features identified in this map.
Plastic deformation, damage and rupture of PM Ti-6Al-4V at 20 K under monotonic loading
Plastische Verformung, Schädigung und Bruch von PM Ti-6Al-4V bei 20 K bei gleichmäßiger Belastung
Acta Materialia ; 55 , 1 ; 105-118
2007
14 Seiten, 17 Bilder, 6 Tabellen, 34 Quellen
Article (Journal)
English
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