Investigation For The Laves Phases Precipitation And Its Effect On Deformation Behaviors And Mechanical Properties Of Thermo Span Alloy

Thermo-Span alloy is a low expansion superalloy with broad development prospects,due to its excellent low expansion property,oxidation and corrosion resistance,and long-term service at 650℃ without protective layer.However,it is still controversial whether the notch sensitivity exists or not in alloy and the reason for notch sensitivity is not clear.More researches have revealed that the precipitation of Laves phases have a significant impact on notch sensitivity of alloys,expecially blocky primary Laves phases precipitating during solidification process.Generally,the primary Laves phase needs to be eliminated,otherwise,they will provide more crack nucleation points in the forging and affect the precipitation of secondary precipitates.However,eliminating primary Laves phases by the traditional homogenization heat treatment needs a long time due to their high stability.It causes serious oxidation loss and reduces plasticity of the alloy.Therefore,in order to balance the above problems,we have systematically investigated the influence mechanism of Laves phases with different sizes and morphologies on crack initiation and propagation,and notch sensitivity.And the effects of multi-channel diffusion homogenization on the dissolution of primary Laves phase and high temperature oxidation behavior are also investigated.On this basis,a new homogenization heat treatment is established to eliminate segregation,reduce oxidation loss and improve the hot workability of the alloy.Meanwhile,notch sensitivity can be effectively eliminated,so that the alloy has high strength and plasticity.The main results are as follows:The effects of multi-channel diffusion homogenization on the primary Laves phase dissolution and oxidation weight are systematically studied.It has been found that multi-channel homogenization treatment,i.e.pre-forging treatment added before homogenization treatment,can break large primary Laves phases up and provide more diffusion channels of elements by reducing grain sizes and increasing the internal defects.Therefore,the diffusion of Nb element and dissolution of primary Laves phases are accelerated,and greatly shorten the homogenization time and reduce oxidation loss.The effect of primary Laves phases dissolution on secondary phases precipitation and notch sensitivity is systematically studied.Meanwhile,the influence mechanisms of Laves phases with different morphologies and sizes on crack initiation and propagation and notch sensitivity are also investigated.The results show that the primary Laves phases,owing to enriching Nb elements,can inhibit secondary Laves phases and γ’ phases precipitation.It results in improvement of intragranular plasticity,and thereby coordinating with the incompatible deformation of neighboring grains,effectively inhibiting intergranular crack initiation/propagation and eliminiating notch sensitivity.Partial homogenization partially dissolves primary Laves phases,resulting in the increase of secondary Laves phases and γ’ phases precipitation,and the increase of strength.However,the grain boundary precipitation is insufficient,the alloy possesses worse plasticity and exhibit notch sensitivity.The multi-channel diffusion homogenization treatment dissolve primary Laves phases,resulting in a sharp increase of secondary Laves phases,and slightly inhibit γ’ phases precipitation.The obvious γ’phase depletion zones around secondary Laves phases at GBs(grain boundaries)can improve the capacity of withstanding geometrically necessary dislocation accumulation.It results in the alloy possessing high strength,high plasticity and notch insensitivity.In addition,Laves phase are found to possess a certain accommodate plastic deformation ability in Thermo-Span alloy for the first time.Small granular secondary Laves phases are more beneficial for the plastic deformation and strength than large blocky primary Laves phases.The precipitation of secondary Laves phases and its effect on notch sensitivity of Thermo-Span alloy are systematically studied.The results show that the morphology of secondary Laves phases can effectively affect the notch sensitivity of the alloy,and the precipitation peak temperature of which is 925℃.Below 925℃,the number of secondary Laves phases at GBs increase with the increase of heat treatment temperature,and the morphology changes from granular to thin film;Above 925℃,the number of secondary Laves phases at GBs decrease with the increase of heat treatment temperature,and the morphology changes from thin film to granular.The granular secondary Laves phases can effectively inhibit the intergranular crack propagation,resulting in notch insensitivity of the alloy.However,the film secondary Laves phases can effectively hinder dislocation movement and promote the stress/strain accumulation at GBs,and thereby results in the crack initiation and propagation along GBs and notch sensitivity.Intermediate heat treatment at 825℃ can effectively improve the secondary Laves phase precipitation at grain boundary and eliminate the notch sensitivity of the alloy.The effect of homogenization treatment on the hot workability of Thermo-Span alloy are systematically studied.The results show that primary Laves phases can promote recrystallization,but the interface between primary Laves phases and matrix is tend to crack.The multi-channel diffusion homogenization treatment can eliminate primary Laves phases and produce more annealing twins to promote the formation of dynamic recrystallization,and thereby expanding the window of alloy hot working.The hot workability of the multi-channel diffusion homogenized alloy is obviously superior to as-cast and partially homogenized alloys.The homogenization treatment has little effect on the shape of true stress-strain curve,peak stress and deformation mechanism.The dynamic softening mechanism is dynamic recovery blow 1000℃,and dynamic recrystallization is less.The dynamic softening mechanism is dynamic recrystallization above 1100℃,and the volume fraction of dynamic recrystallization increases with the increase of strain and strain rate.In addition,the hot working equations of the alloy under different homogenization processes are established.