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This study focuses on improving the mechanical properties of Co-Cr-W alloys by applying Harmonic Structure Design – bimodal grain size distribution with an interconnected framework of ultra-fine-grained (UFG) regions, called the “shell region”, surrounding isolated coarse-grained (CG) regions. Harmonic structure Co-Cr specimens were successfully fabricated by Powder Metallurgy (PM) that consisted of controlled mechanical milling and spark plasma sintering. The sintered compacts revealed an outstanding combination of strength and total elongation. Moreover, the sintering dwell time significantly improved densification and led to large total elongation. PM improved the mechanical properties of Co-Cr-W alloys and offered an attractive approach to fabricate harmonic structures for commercial applications.

Co–Cr–Mo alloy (CCM) is widely used as structural material for biomedical applications, such as coronary stent, hip and knee replacements and dental implants, due to their excellent biocompatibility, wear resistance, corrosion and mechanical properties. In the present work, ‘harmonic structure’ design is proposed for CCM with improved mechanical properties. The harmonic structure consists of coarse-grained (CG) regions called ‘core’ surrounded by a continuously connected 3D network of ultra-fine grain (UFG) regions called ‘shell’. Harmonic structure CCM compacts were fabricated by a combination of severe plastic deformation and powder metallurgy process. Mechanical milling was used to produce UFG region on the surface of powder by controlling the milling conditions, leading to bimodal structure in the milled powder. Subsequently, the mechanically milled bimodal powder was sintered by Spark Plasma Sintering. Sintered compacts of milled powder exhibited ‘harmonic structure’ and they demonstrated out...

Pure copper (Cu) having bimodal ‘harmonic structure’ (HS) was fabricated by a technique based on severe plastic deformation of powders, which involved tailored mechanical milling and spark plasma sintering. The harmonic-structured Cu demonstrates a unique combination of high strength and large elongation superior to its homogeneous as well as bimodal heterogeneous counterparts. Specific features of harmonic structure, i.e. continuous network of ultra-fine grained (UFG) regions encompassing coarse-grained areas, lead to the extension of uniform elongation. The optimum combination of properties in pure Cu is found to be in the harmonic-structured material having 40 % UFG fraction.

Co-Cr-Mo alloy powders were subjected to controlled mechanical milling at room temperature under Ar atmosphere to fabricate bimodal microstructure in the MM powders, having nanosized grains in the surface region and micron-sized coarse grains in the center of the milled powders. Subsequently, the MM powder was compacted by spark-plasma sintering (SPS) process. The sintered compacts indicated two structure areas: (i) ultra-fine grained (UFG) regions, called shell, and (ii) the coarse grained regions called core. The shell and the core correspond to the surface and center of the MM powders, respectively. The shell regions established a continuous three dimensional network of high strength ultra-fine grained regions, which surrounded the discrete coarse grained ductile regions. Such a microstructure is referred as Harmonic Structure. The sintered Co-Cr-Mo alloy compacts exhibited outstanding mechanical properties. The yield strength increased from 605 to 635 MPa, and ultimate tensile strength increased from 1201 to 1283 MPa. Moreover, the elongation was maintained more or less same as that of coarse grained compacts. Therefore, the harmonic structure design leads to the new generation microstructure of Co-Cr-Mo alloy, which demonstrates outstanding mechanical properties, i.e. superior strength and excellent ductility as compared to conventional materials. Keywords: mechanical milling, Co-Cr-Mo alloys, mechanical properties, harmonic structure.