Preparation And Properties Of Nanocrystalline Steel With High Strength And Thermal Stability

Compared with conventional coarse-grained materials,nanocrystalline materials have a series of excellent physical and mechanical properties,which are mainly manifested in extremely high hardness,strength and diffusibility.As a new type of high performance materials,nanocrystalline materials have shown many application potentials.However,the excellent properties of nanocrystalline materials will be lost because their poor thermodynamic stability that leading to grain growth.Therefore,the preparation of nanocrystalline materials with high strength and thermal stability has been chosen as a research topic.In this dissertation,the nanostructured 304 stainless steel samples with/without 1 at.%La addition are fabricated by a high energy ball-milling method and a high-temperature/high-pressure technique.The change of their microstructure and mechanical properties are studied and summarized below.1.The samples including coarse-grain 304?CG304?,nanocrystalline 304?NC304?and nanocrystalline 304-1 at.%La?NC304-La?are annealed at various temperatures?between room temperature and 1200 ^??for one hour.It has been found that the grain size and hardness of NC304-La samples can be stable up to 1000 ^?,leading to an initial grain size of 44.6±24.2 nm,and a hardness value up to 6.4±0.2 GPa.The initial grain size?118.7±39.7 nm?and hardness?5.7±0.2 GPa?of NC304 do not change until 600 ^?.Based on the Hall-Petch relationship,the data in the grain growth stage are linearly fitted and give a slope?K_H?of 57±7 GPa nm^1/2 for NC304 samples and 35.7±2.3 GPa nm^1/2for NC304-La samples.It is observed that the addition of La makes samples have a higher thermal stability of microstructure and mechanical properties,finer grains and higher hardness.The NC304-La samples consolidated under 4 GPa at 1000 ^? are annealed at different annealing times?0¹80 h?at selected temperatures?500 ^?,800 ^? and 1000^??.The hardness?6.5 GPa?and grain size?⁴2 nm?of samples remain constant at an annealing temperature between 500 and 800 ^?.The hardness and grain size do not change after annealing at 1000 ^? for 50 h.The hardness can still be as high as 5.4±0.2GPa,even when annealing time reaches 180 h.The enhanced thermal stability of NC304-La is mainly due to the thermodynamic effect?reduction of grain boundary energy?after the addition of La.2.The mechanical properties of NC304 and NC304-La samples conslildated at different high-temperatures?900 ^?,1000 ^? and 1100 ^??are studied.When sintering at1000 ^?,NC304-La has excellent mechanical properties with a high compressive yield strength of 2720 MPa and a fracture strain of 46.7%.After annealing at 1000 ^? for 1 h,NC304-La still has a high compressive yield strength of 2453 MPa and a fracture strain of62%.However,the compressive strength of NC304 samples dropped from 1671 MPa to666 MPa after annealing at 1000 ^?.The microstructure of the material is homogenized and the fracture strain is improved after annealing.The addition of La makes NC304-La samples have a higher thermal stability of mechanical properties under high temperature annealing.