Microstructure and Mechanical Strength of Surface Ods Treated Zircaloy-4 Sheet Using Laser Beam Scanning

Zirconium alloys have been used as a fuel cladding material for a long time because they have good corrosion resistance and irradiation stability in a reactor environment. However, it was recognized that the severe oxidation and creep ballooning behaviors of zirconium alloy cladding tubes at an accident condition of a nuclear reactor have to be considered to improve nuclear plant safety after the Fukushima accident event [1, 2]. The cladding, having an accident tolerant performance, will need to have sufficient oxidation resistance at elevated temperatures to decrease the hydrogen generation needed to prevent hydrogen-related explosions [1], as well as having sufficient strength at elevated temperatures to preserve the fuel rod geometry in order to maintain the core coolability [2]. The coating concept on zirconium alloy cladding is considered to improve the high temperature oxidation resistance [1, 3]. Although the oxidation resistance of zirconium alloy can be improved by the application of coating technology [1, 3], the low strength of zirconium alloy cladding at high temperatures remained as a weak point. This is because the severe deformation of the cladding at increased temperatures is an inherent physical property of zirconium alloy. Thus, the mechanical strength of zirconium-based alloy at high temperature will be increased so as to increase the reactor core coolability. The oxide dispersion strengthened (ODS) concept is considered to increase the strength of the zirconiumbased alloy up to high temperatures. At present, ODS alloys can be applied as structural materials for components in nuclear power plants, since such alloys have a high mechanical strength at high temperatures of up to 700oC [4-6]. This type of alloy is generally manufactured through mechanical alloying from its source metal and Y2O3 powders. The mechanical alloyed powders are subjected to the hot isotatic pressing (HIP) or hot extrusion;. The product is then heat treated at the target temperature and time. Thus, the Y2O3 particles are dispersed in a metal matrix. The manufacturing processes of ODS alloy are very complex and expensive. In addition, it is necessary that special techniques be used to obtain a uniform dispersion and volume control of Y2O3 particles. Another problem is the final product formation such as a tube or sheet since the intermediated-product has a high mechanical strength owing to the dispersion of Y2O3 particles. From this, we considered an LBS technique as an alternative method for making the ODS of zirconium-based alloys. http://dx.doi.org/10.5516/NET.07.2014.027