H. Kwon

Microstructural characteristics and thermal stability of ultrafine grained 6061 Al alloy fabricated by accumulative roll bonding process

Abstract An accumulative roll bonding process was employed to introduce a ultrafine grained structure into a commercial 6061 Al alloy. In performing the accumulative roll bonding process, the alloy was rolled with a 50% reduction ratio. Then, the rolled sheet was cut, stacked to be the initial thickness and the stacked piece was rolled again with the same reduction ratio. This procedure was repeated five times so that an effective strain of 4 was accumulated into the alloy. By 5-passes rolling, the grain size of ∼0.4 μm was obtained when the grain size was measured on the rolling plane and a remarkable enhancement in hardness was achieved, over twice than that before rolling. The microstructural examination revealed that, at relatively low strain, fine dislocation cells were formed by an operation of multi-directional slip. With increasing strain, dislocation cells were developed into ultrafine subgrains. At large strains, the microstructural change was dominated by a conversion of low-angled subboundaries to high-angled boundaries, rather than grain refinement. The 1 h static annealing treatment was carried out at temperatures of 373–773 K in order to examine the thermal stability of ultrafine grained 6061 Al alloy. The present ultrafine grained 6061 Al was found to be thermally stable up to 473 K. The microstructural change of 6061 Al alloy during accumulative roll bonding was compared with that observed in ultrafine grained Al alloys fabricated by the equal channel angular pressing technique which is another representative technique for fabricating ultrafine grained bulk materials. In addition, thermal stability of ultrafine grained 6061 Al alloy was discussed in terms of the grain growth kinetics.

Effects of austenitizing treatments and inclusions on secondary hardening and fracture behavior for high Co–Ni steels containing W

Abstract When the inclusions were modified by additions of La and Mn in the WCrCoNi steel, La and Mn sulfides were formed, respectively. When no attempt to modify inclusions was performed, Cr sulfides were formed. While the secondary hardening peak was investigated at 510°C in the 1200 and 1000°C austenitizing conditions, it was shifted down to 475°C in the 899–816°C double-austenitizing condition. The aging acceleration in the latter condition of low-temperature austenitizing correlated with the presence of undissolved carbides containing great amounts of W, increasing the Cr/W ratio in the matrix. In the as-quenched condition, the fracture surfaces of impact specimens exhibited the transgranular dimple mode. While the combined action of impurities and coarse cementite at the grain boundaries leading to the intergranular fracture decreased the impact toughness in the underaging condition, however, the dissolution of cementite eliminated the intergranular fracture and recovered the impact toughness in the peak-aging condition in spite of an increase in hardness. In the 1000°C austenitizing and peak-aging condition, the impact toughness to hardness ratio was relatively high in the presence of large, spheroidal La sulfides with the large interparticle spacing. Its ratio was lowered in the presence of small, spheroidal Cr sulfides and large Mn sulfides of rod shape, due to the small interparticle spacing and the easy stress concentrations at the interfaces, respectively.

A simple route for the synthesis of copper nanowires

We have synthesized Cu nanowires via a simple process using both thermal evaporation and CuCl powders. In this method, complicated processes and special instruments are not required to synthesize Cu nanostructures. In addition, we have used commercial CuCl powders as a precursor. There have been no reports on the preparation of Cu nanostructures using simple instruments together with a commercial source material as in this study. Cu nanostructures of various morphologies were formed at relatively low temperature. The microstructure of the Cu nanostructures was investigated using HRTEM images and SAD pattern analysis.

Interpretation of the strengthening of steel with lower bainite and martensite mixed microstructure

A mixed microstructure consisting of lower bainite and martensite was introduced in an alloyed medium carbon steel (SCM 435) through an appropriate isothermal treatment and the variation in microhardness was measured as a function of lower bainite fraction. It was shown that a maximum hardness peak appeared when the steel contained about 20% lower bainite. A double etching technique using picric acid to reveal the prior austenite grain and Le Pera etchant to reveal the lower bainite phase showed that the microstructure became manifest through the partition of the prior austenite grain by the lower bainite plates. Based on such microstructural characteristics, a model interpreting the strengthening of the steel through the presence of lower bainite was constructed and shown to be well-fitted with the experimental results. The model took into consideration of three factors: the refinement of the martensitic substructure through the presence of lower bainite, the carbon shifting from bainitic ferrite into untransformed austenite during lower bainite transformation and the mixture rule.

Fabrication of AA6061/Al2O3p composites from elemental and alloy powders

The tensile properties and microstructures of AA6061/Al2O3p composites fabricated by the pressureless infiltration method under a nitrogen atmosphere were examined. Since the spontaneous infiltration of molten metal into elemental powders bed as well as alloy powders bed occurred at 700°C for 1 hour under a nitrogen atmosphere, it was possible to fabricate 6061 Al matrix composite reinforced with Al2O3p irrespective of the type of metal powders. Both MgAl2O4 and MgO were formed at interfaces between Al2O3 and the matrix. In addition, MgAl2O4 was formed at within the matrix by in situ reaction during composite fabrication. Fine AlN was formed by in situ reaction in both composites. A significant strengthening in the composites occurred due to the formation ofin situ AlN particle and addition of Al2O3 particles, as compared to the commercial alloy, while tensile properties in the both elemental and alloy powders composites showed similar trend.

Microstructural effects on discrepancy between toughness under sharp cracks and blunt notches in Ti-added 8Ni-13Co secondary hardening steel

In this study, the toughness under sharp cracks and blunt notches is investigated in terms of the prior austenite grain size (PAGS) and inclusion particles as a function of the austenitizing treatment in a Ti-added 13Co-8Ni secondary hardening steel. For the quantitative analyses of the inclusion and precipitate particles, small-angle neutron scattering analyses are conducted under austenitizing conditions of 1050 °C, 1200/1050 °C, and 1200/1200 °C; the impact toughness values are 32, 30, and 24 J for each austenitizing condition, respectively. In contrast, the fracture toughness values under the same conditions are 66, 78, and 103 MPa·m1/2. Thus, the fracture toughness significantly improves under 1200/1200 °C austenitizing conditions with coarse PAGS; however, the impact toughness deteriorates. The adverse effect of the grain size on the toughness under sharp cracks and blunt notches is elucidated in terms of the effective microstructural factors that control the fracture process inside the plastic zone, the size of which varies with the notch sharpness. In particular, through considering the density of the slip bands as a function of the grain size in the small confined plastic zone before the sharp crack, the complicated problem regarding an increase in the fracture toughness with an increasing grain size is described from micromechanical and microstructural perspectives.