Prepare Of Austenitic Stainless Steel With Gradient Organization And Rsearch Of The Strengthing Mechanism

Functionally graded material(FGM) is a new composite material with continuously varying characteristics and mechanical properties compared to conventional composite materials. Because of no obvious interface, FGM can alleviate and eliminate the failure due to thermal stress at the interface.Gradient material with refined microstructure can be achieved via severe plastic deformation,for example, equal channel angular processes(ECAP), ultrasonic shot peening (USP), surface mechanical attrition (SMAT) and high pressure torsion (HPT).Among them, HPT is able to acquire greater deformation layer whose thickness depends on the size of the specimen. A larger size of the new material with gradient organization can be obtained during HPT. In this paper, a new material with gradient organizational characteristics has been successfully prepared on AISI304austenitic stainless steel during torsional deformation.In this paper, torsional deformation is applied on AISI304austenitic stainless steel. The feasibility of strain-induced martensitic transformation is analyzed theoretically without considering the effects of alloying elements other than Fe-Cr-Ni. For AISI304austenitic stainless steel, strain-induced martensitic transformation can occur at locations whose distances are greater than0.73mm from the center during the maximum torsional deformation.A new material with gradient organizational characteristics has been successfully prepared during torsional deformation. The volume fraction of martensite increases with the increasing shear strain from center to surface.The maximum volume fraction of the strain-induced martensite is16.8%. From center to surface, the microhardness varies from Hv225to Hv586, higher than the specimen at solid solution whose microhardness is Hv171. With the increasing shear strain, tensile stength increases from691.6Mpa to1298.7Mpa.On contrary, elongation decreases from center to face. Experimental results show that, torsional deformation can achieve gradient strain in the entire cross-section of specimen.Two differrent nucleation mechanism of strain-induced martensitic transformation have been found during the torsional deformation.TEM analysis show that,α’martensite can nucleate at either the junction of s martensite or separate s martensite. Although the nucleation mechanism is different, ε martensitic phase plays a crucial role in the process of martensitic transformation.Large number of ε martensite and twinning have been found in the area with severe deformation,they lay the foundation for martensite nucleation.