Structure Distribution Controlling & Improvement Of Strength And Ductility In The Eutectoid System Alloy

High strength with loss of ductility has been demonstrated a limitation of engineering utility in bulk nanostructured materials. Y.M.Wang et al reported nano-Cu with bimodal structure obtained by cryo-rolling at liquid nitrogen, which possesses excellent tensile ductility without obvious loss of strength. It suggests micrometer-sized grains with some fractions embedded in nanostructured matrix, namely bimodal structure was possible to be an effective route to eliminate the loss of ductility. But there is no process which can fabricate multi-phase nano-alloys by quantificationally controlling the fraction anddistribution of micro-meter sized grains. Present research succeeds in achieving controllable bimodal structure according to processing routes of composition design + eutectoid-phase-transformation treatment + severe plastic deformation + annealing. Results showed that tensile ductility was greatly improved without much loss of strength. Meanwhile, effect of distribution of micro-meter sized grains on mechanical properties was first discussed. Also analysis about different formation mechanisms of bimodal structure in these two eutectoid systems was included.Experiment results suggested that micrometer-sized grains originated from the refinement during high pressure torsion and then coarsening happened after annealing, while decomposition of eutectoid martensite resulted in formation of nanostructured matrix; compared with its nanostructured counterpart, specimen with this bimodal structure performed a little loss of strength from 997MPa to 730MPa, but much higher in tensile ductility from 0 to 4%; additionally, strength and tensile ductility were improved further to 1300MPa and 3% because of dispersion of micrometer-sized grains resulted from eutectoid transformation treatment. Bimodal structure is also observed in Zn-41wt.%Al with 40% hypo-eutectoid phase in volume fraction after solid solution treatment and 4 passes ECAP. Mechanical test results showed that the tensile uniform ductility increases from 7% to 26% with loss of the strength in this alloy.