题目：Elevating the tensile ductility of high-strength metals and glasses

　　报告人：Evan Ma , Johns Hopkins University, USA 

　　时间： 2015年7月23日（周四）， 上午10:00

　　地点：李薰楼468会议室

　　摘要：

　　Using three examples, in this talk we discuss how to make a monolithic hard material ductile in tension at room temperature. This (uniform) elongation capability is important for the formability of a material. The first example is for a high-strength single-phase metallic material, suffering from plastic instability. We report a new route of nanostructuring: nanoscale domains (7 nm) were produced in Ni during electro-deposition, occupying only ~2.4% of the total volume. Yet the resulting Ni achieves a yield strength approaching 1.3 GPa, together with a uniform elongation as large as ~30% in uniaxial tension. TEM and MD simulations demonstrate that the spread-out nanodomains effectively block dislocations, akin to the role of precipitates for Orowan hardening in alloys. In the meantime, the abundant domain boundaries provide effective dislocation trapping for efficient dislocation multiplication and storage in the ample room left inside the grain interior. This approach uses nanodomains to achieve “self-precipitation” hardening, by creating "circularly twisted" grains with one contiguous grain boundary not in contact with others. It mimics grain size strengthening, but without using contiguous nanocrystalline grains that lead to excessive dislocation sinks and hence diminishing strain hardening.

　　The second example is for a metallic glass. In this case, without dislocations the flow is due to shear transformations facilitated by liquid-like soft spots enriched in GUMs. With highly rejuvenated “young glass” structure, the glass can exhibit extensive necking, rather than catastrophic shear banding. However, due to the lack of intrinsic strain hardening mechanism, in uniaxial tension necking onsets rather early. Strain rate hardening is also weak, as the material flows in a deeply non-Newtonian regime. In other words, the autocatalytic shear softening renders the useful tensile ductility (uniform elongation) rather limited.

　　Nevertheless, there are ways to artificially and dynamically rejuvenate the glass structure to induce uniform deformation much like in high-temperature Newtonian viscous flow. We show superplastic deformation using a normally brittle glass, amorphous silica, by subjecting it to e-beam induced radiolysis inside a TEM/SEM.

　　Taken together, these three examples demonstrate that for materials with different levels of intrinsic brittleness, new approaches/mechanisms may be invoked to make them ductile under high flow stresses in uniaxial tension at room temperature.　　 

　　报告人简历：  

　　E. Ma is a professor of Materials Science and Engineering at Johns Hopkins University. Prior to JHU, he was an assistant and associate professor at LSU. Prof. Ma has published ~300 papers (w/ ~17,000 citations and h index=69) and presented ~110 invited talks at international conferences. He is an elected Fellow of American Society for Metals (ASM), American Physical Society (APS), and Materials Research Society (MRS). His current research interests include amorphous metals (metallic glasses), chalcogenide phase-change alloys for memory applications, nanostructured metals, plasticity mechanisms, and in situ transmission electron microscopy of small-volume materials.