Effect of strengthening mechanisms on cold workability and instantaneous strain hardening behavior during grain refinement of AA 6061-10 wt.% TiO2 composite prepared by mechanical alloying

The mechanical alloying (MA) of AA 6061 alloy reinforced with 10 wt.% fine anatase-titania composites powder milled with different timings (1, 5, 10, 20, 30, and 40 h) was cold consolidated and sintered. The main purpose of this study is to investigate the effect of microstructure and the various strengthening mechanisms such as solid solution, grain size, precipitate, dislocation and dispersion strengthening during grain refinement of AA 6061-10 wt.% TiO2 composite via MA on cold working and strain hardening behavior. The sintered composite preforms were characterized by X-ray diffraction, scanning electron microscope, and transmission electron microscope. The strengthening mechanisms were estimated by using simplified models available in the literatures. The evaluation of cold deformation behavior under triaxial stress condition through room temperature cold-upsetting tests (incremental loads) was studied by correlating the strengthening mechanisms. Among the developed strengthening mechanisms the grain size and dislocation strengthening mechanisms diminished the deformation capacity of the composites. The strain hardening behavior was also examined by proposing instantaneous strain hardening index (ni). The value of maximum instantaneous strain hardening index for ultrafine-grained composite was to be about twice that of coarse-grained composite at lower true axial strain value. Also, the coarse grained 5 h composite was observed to be the best one as it exhibited a better strain hardening or deformation behavior.

Research highlights▶ Various strengthening mechanisms such as solid solution, grain size, precipitate, dislocation and dispersion strengthening promoted yield strength of the composites ▶ The 5 h sintered composite yielded a large plastic strain (23%) at ambient temperature. ▶ The domination of interparticle friction effects, grain size and dislocation strengthening diminished the deformation capacity of the composites greater than 5 h of milling. ▶ Ultra-fine grained composite (40 h) yielded a high strength (>1000 MPa). ▶ The proposed instantaneous new Poisson's ratio and the instantaneous strain hardening index used to study the extent of plastic zone and strain levels of the composite.