Influence of lanthanum on enhancement of mechanical and electrical properties of Cu-Al_2O_3 composites

Two kinds of Cu-Al_2O_3 composites(with and without La) were prepared via mechanical alloying-spark plasma sintering(MA-SPS) method. Microstructure, mechanical properties and electrical resistivity were investigated systematically using metallography, scanning electron microscopy, transmission electron microscopy, mechanical and electrical properties testing. The results indicate that an appropriate amount of La can homogenize the distribution of Al_2O_3. As such, yield strength, ultimate tensile strength and elongation of Cu-Al_2O_3-La are greatly increased. Some semi-coherent interface between Cu and Al_2O_3 is found, which means a low interface energy. The grain shape of Cu changes to irregular band with the addition of La. This change results in a density decrease of grain boundary and reduces electrical resistance. Lanthanum may exist in the form of La_2O_3.

Formation and evolution of layered structure in dissimilar welded joints between ferritic-martensitic steel and 316L stainless steel with fillers

Dissimilar high-energy beam(HEB)welding is necessary in many industrial applications.Different composition of heat-affected zone(HAZ)and weld metal(WM)lead to variation in mechanical properties within the dissimilar joint,which determines the performance of the welded structure.In the present study,appropriate filler material was used during electron beam welding(EBW)to obtain a reliable dissimilar joint between reduced-activation ferritic-martensitic(RAFM)steel and 316L austenitic stainless steel.It was observed that the layered structure occurred in the weld metal with 310S filler(310S-WM),which had the inferior resistance to thermal disturbance,leading to severe hardening of 310S-WM after one-step tempering treatment.To further ameliorate the joint inhomogeneity,two-step heat treatment processes were imposed to the joints and optimized.δ-ferrite in the layered structure transformed intoγ-phase in the first-step normalizing and remained stable during cooling.In the second-step of tempering,tempered martensite was obtained in the HAZ of the RAFM steel,while the microstructure of 310S-WM was not affected.Thus,the optimized properties for HAZ and 310S-WM in dissimilar welded joint was both obtained by a two-step heat treatment.The creep failure position of two dissimilar joints both occurred in CLAM-BM.

Impact of friction stir welding on recrystallization of oxide dispersion strengthened ferritic steel

Oxide dispersion strengthened （ODS） steels can be used as the structural materials in the future fusion reactors and the fuel cladding materials in the advanced fission reactors. However, the weldability of ODS steels is a severe problem. In the present study, defect-free joints of the 15Cr-ODS ferritic steel were achieved by friction stir welding at different rotation speeds. The recrystallization, hardness and tensile properties are highly related with the rotation speed of the stir tool. The higher rotation speed results in coarser grains in the top SZ, while the grain size exhibits more complicated relation with the rotation speed in the SZ center. The joint welded at 250 rpm exhibits a maximum tensile strength of 974 MPa that reaches about 84% of that of the base metal.

Effect of Deuterium Ion Implantation on Microstructures of Fe-M (M=V, W, Ta) Alloys

To study the effect of tungsten, vanadium and tantalum on the microstructures in CLAM （China Low Activation Martensitic） steel after irradiation respectively, the microstructures of Fe-M （M= V ,W, Ta） model alloys were investigated after implanted deuterium ions using an ion accelerator at 773 K. After implanted deuterium ion, TEM （Transmission Electron Microscope） observation and EDX （Energy Dispersive X-ray Spectrom） analysis have been carried out. The result showed that tiny voids were observed in all model alloys after implanted the same dose of deuterium ions. The swelling rate in FeTa alloy was the smallest among the three alloys. Unlike FeW and FeV alloys, there was the segregation in FeTa alloy under a fluence of 5×1017D＋ /cm2 at 773 K. A theoretical analysis showed that the void growth in FeTa alloy slowed down due to tantalum segregation near voids. It indicates that tantalum plays an important role in the improved irradiation resistance of CLAM steel.