The Effect Of Microstructure Of Metal-based Multiphase Materials On Fatigue Behavior

For the past few years,metal-based multi-phase materials have been widely developed and applied in high-tech fields.It mainly covers two types of alloys:the first one is with bimetallic phase,such as a dual-phase steel,dual-phase copper and shape memory alloys;the second one is metal com-posite materials doped with reinforcing phases,such as fiber and silicon car-bide.In such kind material,the presence of the second phase has an ef-fect on the evolution of the micro-structure,thereby affecting fatigue perfor-mance of the material and limiting its industrial applications.The purpose of this research is to explore the effect of micro-structure of metal-based multi-phase materials on their fatigue behavior.This is a Sino-French coop-eration research,two typical alloys were selected as research objects:Ti B₂aluminum-based composite materials and the NiTi shape memory alloy.The first part is conducted in Shanghai.Ti B₂aluminum-based Metal Matrix Composites（MMCs）consists of in-situ ceramic particles,with high-strength and high-modulus,and 2024 alloy as a basis.This measurement has greatly improved the strength,hardness,modulus and other properties of the 2024 alloy.In this research,stress-controlled fatigue experiments were performed on Ti B₂/2024 composites specimens and 2024 aluminum alloys specimens,which are manufactured and prepared under the same con-ditions.It was found that aluminum-based materials incorporating second-phase nano-ceramic particles demonstrating a better fatigue behavior.As the response to this phenomenon,microscopic observations of the material were conducted by the means of optical,electron microscopy,and back-scattered electron scanning.It was found that the grain of Ti B₂/2024 is more re-fined,does not show obvious anisotropy.Besides,under cyclic loading,the Ti B₂/2024 alloy has shown a better plasticity property,and longer fatigue lifetime.The second part has been advised in French laboratory.Compared with Ti B₂/2024,shape memory alloys（SMAs）are more special in the context of multi-phase materials.There are two major metallic phases inside the SMAs:martensite and austenite.At a certain temperature,when the alloy is subjected to an external force,the two phases will be transformative between each other,and a pseudoelastic behavior appears in the material.In this dis-sertation,stress-control fatigue experiments were conducted on SMAs,and theoretical and qualitative analyzes would be presented.The stress-induced micro-structure changes are assumed first,and the assumption is been con-firmed by the result of the experiments.Being different from materials like Ti B₂/2024,which are introduced artificially with others phases,the fatigue behavior and lifetime of shape memory alloys,under cyclic loading,depends on the strain level and phase transition.This result is demonstrated within this research,and two innovative parameters have been set up to associate with the fatigue lifetime of SMAs.In this research,two kinds of multi-phase alloys are taken as the ob-jects of study.Through fatigue experiments,the stress-strain behavior of the multi-phase alloys under cyclic loading,as well as the internal micro-structural evolution are analyzed.New approaches and relative information of the multi-phase alloys are provided through this article.