Hidenori Nako

Influence of experimental parameters on the composition of precipitates in metallic alloys.

We report on the change in the shape, size and composition of spherical precipitates, found in an Fe-Cu and an Al-Ag alloy, as the base temperature, pulsing mode and parameters are adjusted. In the case of the Al-Ag alloy, the difference in the evaporation field between precipitates and matrix can be minimised at low temperature, while in the Fe-Cu alloy minimal changes are observed within the range of experimental conditions investigated. Comparison with transmission electron microscopy shows that some experimental conditions yield accurate precipitates size, while others enable reaching accurate composition measurements.

Formation of Martensite Austenite Constituent in Continuously Cooled Nb-Bearing Low Carbon Steels

Fe-0.15%C-1.5%Mn-0.2%Si (Nb-free alloy) and Fe-0.15%C-1.5%Mn-0.2%Si-0.03%Nb (Nb-added alloy) were continuously cooled to room temperature at constant cooling rates in the range from 0.1 to 20K/s. At lower cooling rates, such as 0.1K/s, the Nb addition retards the ferrite transformation, resulting in a decrease in the transformation temperature and an increase in the volume fraction of bainite. The fraction of martensite-austenite constituent (MA) increases by the Nb addition and the largest fraction of MA, about 0.5 %, is observed in the Nb-added specimen cooled at 5K/s. In the specimens cooled at 5K/s, relatively coarse bainite without cementite precipitation is formed near the austenite () grain boundary in both alloys. Most of MA is localized between such relatively coarse bainitic ferrite (BF). On the other hand, MA is hardly observed in the bainite formed with cementite precipitation in  grain. Based on microstructure observation of the continuously cooled specimens down to intermediate temperatures followed by quenching, it is concluded that small-sized untransformed  near  grain boundary partly remains as MA whereas relatively larger untransformed  in the  grain decompose into bainite with cementite precipitation.

Effect of Mg or Ag addition on the evaporation field of Al.

It is known that the distribution of the charge-states as well as the evaporation field shift to higher values as the specimen temperature is decreased at a constant rate of evaporation. This study has explored the effect of Mg or Ag addition on the evaporation field of Al in terms of the charge state distribution of the field evaporated Al ions. The fractional abundance of Al(2+) ions with respect to the total Al ions in Al-Mg alloy is lower than that in pure Al, whereas it shows higher level in the Al-Ag alloy at lower temperatures. The temperature dependence of the fractional abundance of Al(2+) ions has been also confirmed, suggesting that Al atoms in the Al-Mg alloy need lower evaporation field, while higher field is necessary to evaporate Al atoms in the Al-Ag alloy, compared with pure Al. This tendency is in agreement with that of the evaporation fields estimated theoretically by means of measurements of the work function and calculations of the binding energy of the pure Al, Al-Mg and Al-Ag alloys.

Comparison of Microstructures at As Welded Zone and Reheated Zone in 9%Ni Steel Similar Composition Weld Metal

9%Ni steel similar composition weld wires are applied for constructions of liquefied natural gas storage tanks by gas tungsten arc welding and the weld metals show high toughness and strength level. In this study, the microstructures of 9%Ni steel similar composition weld metals have been investigated. It was found that fine microstructure was formed at the reheated zone and contributed to the high toughness level. The nucleation rate of blocks and packets from prior-austenite grain boundaries was larger at the reheated zone than the as-welded zone and resulted in the fine microstructure of the reheated zone. Our results showed that the lowering of Ni concentration at and near the prior austenite grain boundaries by the reverse transformation caused the larger nucleation rate at the reheated zone.