Effect Of Surface Gradient Structure On Mechanical Properties Of IN718 Superallo

It is widely used in high temperature and high pressure conditions.IN718 alloy is based on γ austenite phase,and the main precipitated phases are δ phase and γ ′′ phase.Studies have shown that δ has a significant pinning effect on grain boundaries,which can improve the high temperature stability of grain boundaries.The gradient structure,which has been hotly discussed in recent years,improves the mechanical properties of materials by forming an uneven gradient grain structure.However,most of the research on the gradient structure focuses on the room temperature performance,and the influence on the high temperature performance is still unclear,especially the gradient structure containing the precipitated phase gradient.In view of the above problems,this paper explores the effect and mechanism of gradient structure on the high temperature tensile properties of the alloy.By changing the rolling passes,the influence and mechanism of different gradient layer depths on the high temperature tensile properties of IN718 alloy were further explored.In addition,the samples with different grain sizes were treated with the same surface treatment to explore the differences in performance and related mechanisms caused by the difference between the core and the surface grains.(1)The effect of gradient structure with precipitated phase on the high temperature tensile properties of IN718 alloy was studied.High density δ-Ni3 Nb phase and fine recrystallized γ grains were obtained in the surface gradient layer by ultrasonic surface rolling process and subsequent heat treatment.The number of δphase in the gradient layer decreases with the increase of layer depth,while the size of recrystallized γ grains shows an opposite trend.It is worth noting that the gradient structure does not exhibit a microhardness gradient at room temperature,but exhibits excellent strength and plasticity in high temperature tensile tests.This gradient structure promotes the accumulation of high-density geometrically necessary dislocations in the gradient layer during plastic deformation,resulting in additional back stress strengthening.Therefore,the formation of a gradient structure with precipitated phases provides a new way to achieve a good combination of strength and plasticity at high temperatures.(2)The effects of different gradient layer depths(gradient ratio)on high temperature mechanical properties and related mechanisms were explored.The depth of the surface gradient layer is controlled by changing the rolling pass and the related feed depth.After the same heat treatment,the core matrix grains are similar,and the surface gradient structure distribution is different.The results show that with the increase of rolling passes,the thickness of gradient layer and the content of δ phase increase,and the distribution range of recrystallized grains increases.The strength of the rolled sample increases with the increase of the gradient ratio.The gradient ratio has a significant effect on the performance of the sample,and does not show a better product of strength and plasticity than the uniform sample in the low proportion(gradient ratio of 4.8 %)sample.In the higher proportion(gradient ratio of 7.2 %and 8 %),the gradient sample shows a stronger plastic product than the uniform sample.When the gradient layer reaches a certain proportion,the back stress strengthening and back stress hardening produced by the sample with gradient structure can improve the product of strength and plasticity at high temperature.(3)The effect of grain size difference of gradient structure on high temperature mechanical properties and related mechanism were investigated.The samples were solution treated at different temperatures to obtain samples with different grain sizes.The same ultrasonic rolling and subsequent heat treatment were carried out on the sample,and the roughly same surface gradient structure was obtained.The difference is the size difference between the surface grain and the core grain.The results show that the greater the difference in grain size between the surface and the core of the gradient structure,the more significant the high temperature strength increases.The gradient structure with large grain size difference shows significant back stress strengthening and back stress hardening effects.