There is a widespread interest in understanding the properties of
Al-base alloys so that progress can be made toward extending their present
applications in the aircraft industry. Al-Zn-Mg is precipitation hardened to
gain its high strength; however, during aging the formation of heterogeneous
precipitates on the grain boundaries creates a precipitate-free zone in the
adjacent region. Since high angle grain boundaries are not easily
characterized, it is difficult to establish a relationship between the
precipitate and the boundary structure. Therefore, this study involves
precipitation on low angle grain boundaries where the boundary and the
precipitate can be fully analyzed.
The plastic deformation mechanism of Ti-55531 alloy with bimodal microstructure was investigated by compression testing at room temperature. The bimodal microstructure was composed of equiaxed primary α phase (αp) and transformed β (βtrans) that consisted of acicular secondary α phase (αs) and residual β phase (βr). In the initial stage of deformation, the αp grains first underwent plastic deformation, the dislocations germinated and increased, forming the dislocation loop with the dislocation free zone in αp at the true stain of 0.083. With the true strain subsequently increasing to 0.105, the dislocation tangle and dislocation pile-up occurred in αp, and a lot of dislocations were also activated in most of αs. Moreover, the dislocation density was increasing gradually in βr with the adding of strain. Finally, the dislocation pile-up and dislocation tangle appeared in αs and βr at the true strain of 0.163. The whole deformation process was coordinated by αp, αs and βr. They accommodated mutually and completed deformation together.
Large size (>10000 μm2) precipitate-free zones in the absence of microsegregation were observed in the near-β Ti-55531 titanium alloy after furnace cooling from high temperature and longtime annealing in the single-β phase field. To reveal the formation mechanism of the large size precipitate-free zone, continuous cooling and isothermal heat treatment were carried out to investigate the β-α phase transformation process. It was found that the large size precipitate free zone is attributed to the heterogeneous nucleation of α phase. The nucleation site evolves in three different modes: I-random nucleation inside the β grain, II-network nucleation inside the β grain and, III-heterogeneous nucleation on the precipitated α phase. Modes I and II lead to homogeneous transformed structure while Mode III results in the large size precipitate-free zone. Both modes II and III are promoted at high annealing temperature, rapid cooling above 600 °C or slow cooling below 600 °C. Mode II is common as it can minimize the strain energy in phase transformation. As a result, the formation of the large size precipitate-free zone is not deterministic.
In this paper, comparison studies of the hydrogen effect on the structural and phase state, deformation behavior, and mechanical properties of the fine- (average grain size 4 µm) and ultrafine-grained (average element size 0.3 and 0.4 µm) Zr–1wt.%Nb (hereinafter Zr–1Nb) alloy under tension at temperatures in the range of 293–873 K were conducted. The formation of an ultrafine-grained structure is established to increase the strength characteristics of the Zr–1Nb alloy by a factor of 1.5–2 with a simultaneous reduction of its resistance to the localization of plastic deformation at the macro level and the value of deformation to failure. The presence of hydrogen in the Zr–1Nb alloy in the form of a solid solution and hydride precipitates increases its resistance to the localization of plastic deformation at the macro level if the alloy has an ultrafine-grained structure and decreases if the structure of the alloy is fine-grained. In the studied temperature range, the Zr–1Nb alloy in the ultrafine-grained state has a higher resistance to hydrogen embrittlement than the alloy in the fine-grained state.
Experimental and numerical investigation of key microstructural features influencing the localization of plastic deformation in Fe-TiB2 metal matrix composite