Effect Of Ex-Situ Oxide Particles And Warm Rolling On Microstructures And Properties Of Invar Alloys

Conventional Fe-36Ni binary alloy can be used as the precise instruments such as clock pendulum rods,electronic kinescopes and so on,because of its very low coefficient of thermal expansion（CTE）.Recently,high strength is needed for the invar alloy due to rapid development in the fields of long-distance power transmission core lines,liquefied natural gas storage tanks and so on.As the invar alloy always remains single austenite phase in solid,phase transformation strengthening,which is generally used in most of steels,is proved to be noneffective,whereas conventional measures of grain refinement only slightly strengthens the invar alloy because of the austenitic grain size being difficult to refine to a sub-micrometer scale.Under the present industrial technology,combination of solution treatment and aging is main process for strengthening the invar alloy.In this research,introduction of fine oxide particles into the invar alloy,together with warm rolling on this alloy,was tried for the purpose of refining its microstructures.Then good strengthening effect was expected to obtain in this alloy by subsequent solution treatment and aging.We investigated variation in microstructures and mechanical and physical properties before and after introduction of the oxide particles and warm rolling by means of EBSD,XRD,SEM,TEM,positron annihilation lifetime spectra and so on.The following conclusions were obtained:1)The introduced Al₂O₃ and TiO₂ oxide particles were cracked into fine particles in a size of 80～200 nm.Those fine particles act as heterogeneous nuclei to greatly refine microstructure of the invar alloy.The oxide-added invar alloy presents fine austenitic grain size of 77μm,which is 28%lower than that without oxide particles,after solutionizing at1250°C for 3 h and then aging at 700°C for 7 h.The size of V₈C₇ particles precipitated during aging is 44 nm,which is about 40%lower than that without oxide addition.As a result,the oxide-added invar alloy presents the maximum tensile strength of 806 MPa and almost the minimum CTE of 3.01×10^-6/°C in the range of-50 to 100°C.These values increase about 15%and decrease about 16.2%,respectively,than those without oxide particles.2)Warm rolling could increase mechanical strength and decrease CTE of the invar alloy.With an increase in the warm-rolling temperature in the range of 650～800°C,the invar alloy shows a parabolic change in its strength and CTE.The sample warm-rolled at 750°C,followed by solutionizing at 1250°C for 3 h and then aging at 650°C for 7 h,presents the maximum tensile strength of 921 MPa and almost the minimum CTE of 3.2×10^-6/°C in the range of-50 to 100°C.They increase about 21%and decrease about 17%,respectively,than those without warm rolling.Those results should be attributable to increase in concentration of crystal defects such as dislocations,vacancies and twins in the material,decrease in A grain size after solutionizing,which results in dispersed precipitation of the fine secondary phase particles during aging.3)Warm rolling to the oxide-added invar alloy further increases its mechanical strength and decreases its CTE.The oxide-added sample warm-rolled at 750°C presents optimum performance after subsequent solutionizing at 1250°C for 3 h and aging at 700°C for 7 h.Its maximum tensile strength is 1008 MPa and the minimum CTE is 2.6×10^-6/°C in the range of-50 to 100°C.They are about 20%higher and 13%lower,respectively,than those without warm rolling,and about 9%higher and 18%lower,respectively,than those of the warm-rolled sample without oxide addition.The combined effect of fine oxide particles and warming rolling further refines microstructures of the invar alloy,increases its density of vacancies and dislocations,which leads to a much dispersed precipitation of the secondary phase particles during aging.Then an apparent improvement in mechanical and physical properties is obtained in the invar alloy.