The High-temperature Thermal Stability Of Nanocrystalline Steel

Nanocrystalline materials exhibit exceptional physical and mechanical properties due to their small grain size(<100 nm). For instance, they tend to possess extremely high strength/hardness, improved toughness, reduced elastic modulus and ductility in comparison with conventional polycrystalline materials. In spite of these outstanding properties, nanocrystalline materials typically show relatively poor thermal stability, which severely limits their applications at higher temperatures. As a result, the growth and controlling of nanograins has led to great interests in the study of nanoscience and nanotechnology. In this dissertation, we use the nanostructured 304 austenitic steel powder with/without 1 at.% Y addition prepared by a high energy ball-milling technique to investigate the changes of microstructure and mechanical properties during annealing at various temperatures for the same time(1 h). Also we study the influence of 1 at.% Y addition on stabilizing nanocrystalline grain size at higher temperatures and try to find the mechanism of thermal stability.The investigation has shown that adding 1 at.% Y can stabilize grain size in the nano-scale range up to near 1000℃(0.71 Tm), whereas the average grain size of nanostructured 304 austenitic stainless steel has grown up to 680 nm after annealing at 1000℃. When the annealing temperature increase to 1200℃, the 304 austenitic stainless steel with 1 at.% Y addition has a average grain size of 205 nm. In comparison, the 304 austenitic stainless steel exhibits a rapid increase in grain size, resulting in micrometer-scale grains after during annealing at 1200℃. At lower temperatures(<600℃), the grain growth for both 304 austenitic stainless steel and 304 austenitic stainless steel with 1 at.% Y addition occurs, but is significantly retarded when compared to pure Fe. This is attributed to the reduced mobility of grain boundaries due to solute drag. At higher temperatures, the super stabilization behavior of nanostructured 304 austenitic stainless steel with 1 at.% Y addition is attributed to segregation of Y solute to the grain boundary, which lowers the grain boundary energy.As a result of the grain size stabilization effect, the 304 austenitic stainless with 1 at.% Y addition can maintain a higher hardness at higher temperatures when compared to the stainless without Y addition. After annealing at 1000℃, the 304 austenitic stainless with 1 at.% Y addition exhibits an extremely high yield strength up to 2649 MPa, about 2 times that of 304 austenitic stainless without 1 at.% Y addition and 6 times that of coarse grain sample(380 MPa). The ultimate compressive strength(UCS) is 2844 MPa, about 55% higher than the UCS of 304 austenitic stainless without 1 at.% Y addition.Through above-mentioned studies and experiments, we have prepared nanostructured stainless steel alloys with a high thermal stability and high yield strength, which provides a new approach for the design of a new generation of high thermal stability, high strength nanocrystalline steels.