Amorphous Zr-Based Foams with Aligned, Elongated Pores
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INTRODUCTION
BULK metallic glasses (BMGs) have very high specific strength but lack room-temperature plasticity and fail catastrophically from highly localized shear bands.[1,2] Improvements in fracture stress and ductility can be achieved by arresting shear bands through addition of a ductile metallic second phase.[3,4] Shear band arrest can also be achieved by pores, leading to improved compressive ductility in porous BMGs.[5] At high porosity, BMG foams exhibit an interconnected network of thin amorphous metal struts deforming in bending where shear-band stabilization becomes active: as strut diameter decreases, shear bands become shallower, thus relaxing a smaller volume and enabling higher band density while creating smaller shear offsets, which reduce the probability of crack nucleation. This results in very high compressive ductility[6] with noncatastrophic damage accumulation.[7] As reviewed recently,[8,9] BMG foams have been created by the liquid route for alloys based on Pd,[10,11] Zr,[6,12,13] Fe,[14] Ti,[15] La,[16] and Mg.[17] The high contamination tendency and limited casting diameters of BMG alloys are major limitations when using liquid state processing to create BMG foams. These issues have been addressed by various authors by using amorphous metallic powders to create porous metallic glasses. Partial sintering of amorphous powders MARIE E. COX, Graduate Student, and DAVID C. DUNAND, Professor, are with the Department of Materials Science and Engineering, Northwestern University, Evanston, IL, 60208. Contact e-mail: [email protected] SUVEEN N. MATHAUDHU, Researcher, is with the United States Army Research Laboratory, Weapons and Materials Research Directorate, Aberdeen Proving Ground, MD 21005. K. TED HARTWIG, Professor, is with the Department of Mechanical Engineering, Texas A&M University, College Station, TX 77843. This article is based on a presentation given in the symposium ‘‘Bulk Metallic Glasses VI,’’ which occurred during the TMS Annual Meeting, February 15–19, 2009, in San Francisco, CA, under the auspices of TMS, the TMS Structural Materials Division, TMS/ASM: Mechanical Behavior of Materials Committee. Article published online November 24, 2009 1706—VOLUME 41A, JULY 2010
by spark plasma sintering resulted in amorphous Zr-based foams with 14 to 33 pct porosity.[18] Consolidation by warm extrusion or hot pressing of amorphous powders blended with space-holder particles, which are subsequently removed, has been used to create amorphous Ni-based foams with porosity p = 40 pct using brass as a space holder,[19] Zr-based foams with p = 70 pct using salt as a space holder,[20] and Cu-based nanoporous foams with p = 25 pct using crystalline Cu.[21] Consolidation by hot isostatic pressing of amorphous powders and a blowing agent (magnesium carbonate) followed by expansion of the consolidated precursor has produced amorphous Pd-based foams with porosities up to p = 86 pct.[22] Of these investigations, only two reported mechanical properties for the foams. First, the Ni-based, extruded fo
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