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Oh-ishi, K.,Zhang, H.W.,Ohkubo, T.,Hono, K. Elsevier Sequoia 2007 Materials science & engineering. properties, micro Vol.456 No.1
The microstructure of bulk nanocrystalline Fe-C alloy produced by mechanical milling (MM) and spark plasma sintering (SPS) has been characterized using transmission electron microscopy (TEM) and a three-dimensional atom probe (3DAP) to understand the origin of the unusually high yield strength and plastic strain observed from the sample. A bimodal grain structure consisting of fine (∼250nm) and coarse (∼900nm) grains was observed as a result of partial recrystallization. The fine grained region was found to be a duplex phase structure comprised of ferrite and cementite grains. From TEM and 3DAP analyses, the presence of fine oxide particles containing chromium was confirmed. The presence of the recrystallized coarse grains is attributed to the large plastic strain in compression.
Microstructures and mechanical properties of extruded and heat treated Mg-6Zn-1Si-0.5Mn alloys
Mendis, C.L.,Oh-ishi, K.,Ohkubo, T.,Shin, K.S.,Hono, K. Elsevier Sequoia 2012 Materials science & engineering. properties, micro Vol.553 No.-
Si-rich intermetallic particles stabilize the fine grained microstructure in extruded Mg-6Zn-1Si-0.5Mn alloy, resulting in a yield strength (YS) of ∼200MPa, ultimate tensile strength (UTS) of ∼310MPa with an elongation of ∼20%. Ca additions further refine the grain size leading to an increase in UTS by 30MPa with no decrease in YS and elongation. The extruded Mg-6Zn-1Si-0.5Mn alloy can be age-hardened with the precipitation of fine [0001]<SUB>Mg</SUB> rod-like β<SUB>1</SUB>' (Mg<SUB>4</SUB>Zn<SUB>7</SUB>) particles and YS∼290MPa and UTS∼320MPa were achieved in the peak aged condition. The mechanical properties were further enhanced by pre-aging the alloy at 70<SUP>o</SUP>C due to the refinement of the precipitate microstructure with YS∼350MPa and UTS∼360MPa.
Mendis, C.L.,Jhawar, H.P.,Sasaki, T.T.,Oh-ishi, K.,Sivaprasad, K.,Fleury, E.,Hono, K. Elsevier Sequoia 2012 Materials science & engineering. properties, micro Vol.541 No.-
A high strength bulk nano-crystalline Al-1Fe-(0-1)Zr alloys were processed by mechanical alloying and spark plasma sintering. The high yield strength of ∼850MPa was achieved in an Al-1Fe-0.5Zr alloy with 19% strain in compression. The high strength was attributed to the nanosized Al-Fe grains and the dispersion of fine Al<SUB>6</SUB>Fe particles. High temperature compressive yield strength of 455MPa and elongation of 16.7% was achieved at 250<SUP>o</SUP>C.
Sasaki, T.T.,Hono, K.,Vierke, J.,Wollgarten, M.,Banhart, J. Elsevier Sequoia 2008 Materials science & engineering. properties, micro Vol.490 No.1
Amorphous Al<SUB>85</SUB>Ni<SUB>10</SUB>La<SUB>5</SUB> powders were consolidated to cylindrical samples by spark plasma sintering (SPS), and their microstructures and mechanical properties were investigated. When the powders were consolidated below the crystallization temperature, an amorphous phase was retained in the consolidated sample. Sintering above the crystallization temperature caused full crystallization. The Vickers hardness of the amorphous-containing sample was about 350HV in the as-sintered state and increased up to 450HV by a subsequent heat treatment just below the crystallization temperature. The highest hardness was achieved in a nanocrystalline microstructure. Compression tests revealed the brittle nature of the consolidated samples although the fracture and yield strength was higher than 1GPa. The brittleness is due to the low relative density of the amorphous-containing samples and the presence of a large amount of intermetallic compounds in the fully crystallized sample.
Bimodally grained high-strength Fe fabricated by mechanical alloying and spark plasma sintering
Srinivasarao, B.,Oh-ishi, K.,Ohkubo, T.,Hono, K. Elsevier Science 2009 Acta materialia Vol.57 No.11
Nanocrystalline iron containing a certain fraction of coarse grains with nanosized oxide dispersoids has been processed by mechanically milling Fe powder and subsequent spark plasma sintering. Sintered samples exhibited a high tensile strength of 2100MPa with 5% ductility; by optimizing the sintering conditions, it was possible to tune the strength-ductility balance. The optimally sintered material showed a tensile strength of 1500MPa and 15% elongation. The microstructure consists of nanograined (<100nm) as well as coarse-grained regions (>1μm) with uniform dispersion of nanosized chromium oxide particles (∼10nm). The strength and elongation show strong dependence on the volume fraction of the coarse grains, and the high strength can be attributed to the ultrafine grain size of the nanograined regions and precipitation hardening by the oxide dispersoids. The ductility is considered to be due to the presence of coarse grains.