Development in Preparing Methods of the Nano-Structured ODS RAFM Steels

The Mechanical alloying method, the sol-gel-spark plasma sintering method and the vacuum casting method are researched which are used to prepare nano-structured oxide dispersion strengthened (ODS) RAFM steels. Nano-structured ODS RAFM steels can be obtained by The Mechanical alloying methods and the sol-gel-spark plasma sintering methods. Micron-structured ODS RAFM steels can be obtained by vacuum casting method. The microstructure of the material manufactured by different methods are compared. Introduction As a candidate structural material for future fusion power reactors, reduced activation ferritic/martensitic (RAFM) steels have good properties[1]. But its strength will drop at higher operating temperature. So the conventional RAFM steels limit the reactor operating temperature to around 550°C. Nano-structured oxide dispersion strengthened (ODS) RAFM steels allow increased operating temperatures (650 °C or higher) due to the high densities of the Y2O3-rich clusters/precipitates, sub-micron grain sizes and high densities of dislocations. Further, the dispersed nano-scale oxide particles may provide a large number of trap sites for transmutant helium and radiation-induced defects. The classic method of manufacturing ODS alloys is powder metallurgy(PM), Which consists of powder mixing, pressuring mould and sintering. However, this classic method has some inherent defects involved with ODS production, including: inducing anisotropy to its micro-structural and mechanical properties, introducing impurities, rendering batch production complicated and expensive[2,3]. The impact toughness and the fracture toughness of ODS alloys are also lower. So the powder metallurgy has been improved by the researcher. And the more methods have been used to manufacture the ODS RAFM alloys. In this paper, three methods of manufacturing ODS-RAFM alloy are introduced. The microstructure manufactured by different methods are compared. The Methods of Preparing ODS RAFM Steels The Mechanical Alloying Method (MA) There are three methods used in manufacturing ODS-RAFM alloys in recent years. The first one is improved classic power metallurgy method which consists of mixing powders, mechanical alloying, canning and degassing of the milled powders, compaction of the powders by hot isostatic pressing(HIP)[4]. This method can be summarized in figure 1. The improved measure compared to the classic powder metallurgy includes using Mechanical alloying instead of powder mixing, using HIP or hot extrusion instead of ordinary pressure and sintering. So this method is named Mechanical alloying method. Dissolution of Y2O3 during mechanical alloying (MA) and precipitation of Y2O3-rich nano-scale clusters/precipitates during hot isostatic pressing/hot extrusion play key roles in producing nano-structured ODS steels. So preparing ODS alloys by adding Y2O3 to RAFM to