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Seminar - Damage-Tolerance in Multi-Element Metallic Alloys by Prof. Robert Ritchie
日期:2016 年 01 月 22 日 ( 星期五)
Time:11:00 a.m. - 12:00 noon
Venue:EF305, The Hong Kong Polytechnic University
Abstract:
Structural materials invariably must possess damage tolerance with good combinations of strength and toughness. Unfortunately, these properties are generally mutually exclusive and so the development of new structural materials has traditionally involved seeking a compromise between hardness and ductility. This presentation will focus on recently developed, advanced multi-element, metallic alloys, specifically bulk-metallic glasses and high-entropy alloys, that show particularly good combinations of strength and toughness at levels comparable with the best structural materials on record. Strength levels often well above 2 GPa, fracture toughness values up to 200 MPa.m1/2, and fatigue limits up to 25% of the ultimate tensile strength make certain metallic glasses, and their composites, intriguing candidates for many structural applications. Coupled with their ease of processing, these alloys show particular promise although the reproducibility of their properties and the ability to realistically measure high fracture toughness values in locally strain-softening materials still pose problems. High-entropy
alloys represent a similar class of multi-element alloys with the distinction that these materials are fully crystalline and in principle single phase. Like metallic glasses, certain high-entropy
alloys can display remarkable damage tolerance. Specifically, we show that a nominally equiatomic, single phase medium- and high-entropy alloys can display strengths in excess of 1
GPa with fracture toughness values well above 200 MPa.m1/2. We further use high-resolution transmission electron microscopy to discern their deformation modes involving a unique synergy
of dislocation activities. Moreover, due to the onset of deformation nano-twinning, especially at cryogenic temperatures, these properties can actually improve with decrease in temperature – a
trend that is contrary to the behavior of the vast majority of materials. 

Abstract:

Structural materials invariably must possess damage tolerance with good combinations of strength and toughness. Unfortunately, these properties are generally mutually exclusive and so the development of new structural materials has traditionally involved seeking a compromise between hardness and ductility. This presentation will focus on recently developed, advanced multi-element, metallic alloys, specifically bulk-metallic glasses and high-entropy alloys, that show particularly good combinations of strength and toughness at levels comparable with the best structural materials on record. Strength levels often well above 2 GPa, fracture toughness values up to 200 MPa.m1/2, and fatigue limits up to 25% of the ultimate tensile strength make certain metallic glasses, and their composites, intriguing candidates for many structural applications. Coupled with their ease of processing, these alloys show particular promise although the reproducibility of their properties and the ability to realistically measure high fracture toughness values in locally strain-softening materials still pose problems. High-entropyalloys represent a similar class of multi-element alloys with the distinction that these materials are fully crystalline and in principle single phase. Like metallic glasses, certain high-entropyalloys can display remarkable damage tolerance. Specifically, we show that a nominally equiatomic, single phase medium- and high-entropy alloys can display strengths in excess of 1GPa with fracture toughness values well above 200 MPa.m1/2. We further use high-resolution transmission electron microscopy to discern their deformation modes involving a unique synergyof dislocation activities. Moreover, due to the onset of deformation nano-twinning, especially at cryogenic temperatures, these properties can actually improve with decrease in temperature – atrend that is contrary to the behavior of the vast majority of materials. 

Bio-sketch:

Robert O. Ritchie is the H.T. & Jessie Chua Distinguished Professor of Engineering, Professor in the Department of Materials Science & Engineering, and Professor of Mechanical Engineering at the U i it f C lif i B k University of California, Berkeley. He is also Senior Faculty Scientist in the Materials Sciences Division of the Lawrence Berkeley National Laboratory. He was Chairman of the UC Berkeley Materials Science & Engineering Department from 2005 to 2011.

Dr. Ritchie received a B.A. degree in physics and metallurgy in 1969, the M.A. and Ph.D. degrees in Materials Science in 1973, and the Doctor of Science (Sc.D.) degree in 1990, all from Cambridge University. Following periods as the Goldsmith's Research Fellow in Materials Science at Churchill College, Cambridge (1972-1974) and as a Miller Research Fellow for Basic Research in Science at the University of California in Berkeley (1974-1976), he joined the faculty in Mechanical Engineering at M.I.T. where he became the Class of 1922 Associate Professor in 1979. In 1981, he returned to Berkeley where he has been Professor of Materials Science since 1982. He was also Deputy Director of the Materials Sciences Division at the Lawrence Berkeley Laboratory from 1990 to 1994, and Director of the Center for Advanced Materials there from 1987 to 1995.

Dr. Ritchie is known for his research in the fields of materials science, fracture mechanics and particularly fatigue, having authored or co-authored over 700 papers and edited 19 books in the technical literature (he is one of ISI’s Highly Cited Authors in Materials Science with an “h-index” of 73 [91 on Google Scholar]). He is a Member of the National Academy of Engineering, the U.K. Royal Academy of Engineering (FREng), the European Academy of Sciences, the Russian Academy of Sciences (Foreign Member), and the Royal Swedish Academy of Engineering Sciences (Foreign Member). He has also been the recipient of several awards, including the Journal of Engineering Materials & Technology Best Paper Award from ASME in 1979, the Marcus A. Grossmann Award from ASM in 1980, the Most Outstanding Scientific Accomplishment Award from the U.S. Department of Energy in Metallurgy in 1982 and in Ceramics in 1989, the Champion H. Mathewson Gold Medal from TMS in 1985, the George R. Irwin Medal from ASTM in 1985, the Curtis. McGraw Research Award from ASEE in 1987, the Rosenhain Medal from the Institute of Materials (UK) in 1992, the Structural Materials Distinguished Materials Scientist/Engineer Award from TMS in 1996, the ASTM Journal of Testing and Evaluation Award for Most Outstanding Article in 1998, the Nadai Medal from ASME and the ASTM Fatigue Lectureship in 2004, the Wöhler Medal from European Structural Integrity Society in 2006, the A. A. Griffith Medal from the Institute of Materials in 2007, the Sir Alan Cottrell Gold Medal from the International Congress on Fracture and Alexander von Humboldt Senior U.S. Scientist Award in 2009, and the Institute of Metals/Robert Franklin Mehl Award from TMS, the Edward DeMille Campbell Memorial Lectureship Award from ASM, the A. Cemel Eringen Medal from the Society of Engineering Science in 2010, the David Turnbull Award from MRS in 2013, and the Acta Materialia Gold Medal and the Klaus Halbach Award for Innovative Instrumentation in 2014. He was also named as one of America's Top 100 Young Scientists by Science Digest magazine in 1984. He is an Honorary Professor of Beihang University, Xi’an Jiaotong University and the South Central University Changsha, in China, and the University of Birmingham in England. He was President (1997-2001) and is an Honorary Fellow of the International Congress on Fracture, and is also a Fellow of MRS, the Institute of Physics, ASM, ASME, American Ceramic Society, and the U.K. Institute of Materials, and a Fellow and Life Member of TMS.