Effect Of Multiple Elements On The Microstructure And High-Temperature Strength Of Co-Al-W Alloys

In this thesis,Co-9Al-9.5W is used as the base alloy,various contents of Ni and Hf,as well as Re and Ru are added to the alloys.XRD,SEM and TEM were employeed to investigate the effects of the alloying elements on the phase composition,the morphology of theγ’phase,the volume fraction and average size of theγ’phase,the lattice mismatch between theγandγ’phases,and the distribution coefficients of the elements between theγandγ’phases and the interactions between them.The effects of the above alloying elements on the soluvent temperature of theγ’,yield strength and ultimate strength of the alloys were investigated by DSC and high temperature compression tests.The main conclusionsare as follows:Re and Ru are compounded in the base alloy and the alloy is subjected to solid solution treatment at 1230°C/8h and ageing treatment at 900°C/72h.After heat treatment,the 0.5Re1Ru alloy has aγ/γ’dual-phase structure,while the remaining six heats of Ru and Re alloys would precipitate the pin-likeχ-Co₃W phase,the block-likeμ-Co₇W₆phase and the flakyβ-Co Al phase.The volume ofμ-Co₇W₆phase tends to increase with increasing Re contents.The volume ofμ-Co₇W₆phase decreases with increasing Ru contents,while the volume ofχ-Co₃W phase tends to increase.The lattice mismatch between the two phases of the alloyγandγ’tends to increase and then decrease with the increase of Re and Ru elements.The morphology of theγ’phase in the alloys is directly related to the lattice mismatch between theγ/γ’phases.As the 1.5Re1Ru and 1Re1.5Ru alloys both have a low lattice mismatch,0.2%and0.14%respectively,theγ’phase in these alloys is nearly spherical,while the remaining alloys have a lattice mismatch greater than 0.3%,so theγ’phase appears cubic in shape.As the element Re increases,the volume fraction and the average size of theγ’phase show a trend of increasing,then decreasing and then increasing.As the Ru element increases,the volume fraction of theγ’phase increases and then decreases,while the average size of theγ’phase increases,then decreases and then increases.The partition coefficients for both Co and Re are less than 1 for theγphase forming elements.Ru is a stableγ’phase element,but the partition coefficient for Ru in the1.5Re1Ru alloy is less than 1,the partitioning of Ru is influenced by the Re content.Hot compression at 900°C shows that the yield strength and ultimate strength of alloys increase gradually with increasing Re content.As Ru increases,the yield strength and ultimate strength of alloys show a trend of increasing,then decreasing and then increasing.Theγ’phase dissolution temperature of the 1Re1Ru alloy is998°C,while theγ’phase soluvent temperature of the 2Re1Ru alloy is 986°C when the Re element is increased to 2%and 930°C when the Ru element is increased to 2%for the 1Re2Ru alloy,indicating that the Re and Ru elements reduce the phase stability of theγ’phase.When the Hf content is 0.5%,all the alloys with both Ni and Hf added,process aγphase andγ’phase dual-phase structures.When the Hf content is 1%,in addition to the twoγ/γ’phases,the alloy precipitates a lamellar Co₂₃Hf₆phase,blocky Co₂Hf phase in the 20Ni and 30Ni and 40Ni alloys.The addition of Ni effectively reduces the quantity of Co₂₃Hf₆phase.The lattice mismatch of the alloys tends to decrease as Ni and Hf increase.30Ni1Hf and 40Ni1Hf and 40Ni0.5Hf alloys process sphicalγ’phases,which is directly related to their small lattice mismatch.The other five alloys all have larger lattice mismatches,as theirγ’phases present cubic in shape.At 0.5%Hf,the volume fraction of theγ’phase tends to increase,then decrease and then increase as the Ni content increases,and the average size of theγ’phase tends to increase and then decrease as the Ni content increases.The volume fraction of theγ’phase tends to decrease and then increase with increasing Ni when the Hf content is1%.The volume fraction of theγ’phase decreases with increasing Hf content.For the20Ni,30Ni and 40Ni alloys,the average size of theγ’phase decreases with increasing Ni content,and for the 10Ni alloy the average size of theγ’phase increases with increasing Hf content.The partition coefficients for Co all decrease with increasing Hf when the Ni content is 10%,20%and 30%,and the partition coefficients for W and Ni increase with increasing Hf.With the increase of Ni,the yield strength and fracture strength of the alloys show a trend of first increasing and then decreasing.With the increase of Hf,the ultimate strength and yield strength of 10Ni and 20Ni alloys increased significantly,indicating that Hf will significantly increase the strength of the alloy,while Ni will extend the solution limit of Hf in the matrix and then strengthening the alloys.After compression at 700°C,the main deformation mechanism of the 10Ni1Hf alloy is a/3<112>type partial dislocations shearing theγ’phase and also a<010>type super dislocations sliding into the strengthened phase;the deformation mechanism of20Ni0.5Hf alloy is that some of the a/2<110>dislocations enter the strengthening phase to form APB,some of the a/2<110>dislocations decompose at the interface between theγandγ’phases,and the a/3<112>dislocations enter the strengthening phase to form SF,both of which act as pinned dislocations and impede the movement of dislocations;the deformation mechanism of the 1Re2Ru alloy is the slide of paired a/2<110>type full dislocations into theγ’phase and the formation of an anti-phase boundary in the region swept by the two dislocations,accompanied by superdislocation shearing of theγ’phase;after compression at 900°C,the main deformation mechanism of the 30Ni0.5Hf and 30Ni1Hf alloys is that most of the a/2<110>type perfect dislocations slip in theγmatrix channel,and a few a/2<110>type total dislocations decompose at the interface between theγandγ’phases to form a/3<112>type partial dislocations and a/6<112>type partial dislocations,and two partial dislocations of the same Berger vector move towards different surfaces to form Lomer-Cottrell Dislocation,which are fixed dislocations and can impede the movement of the dislocations;the deformation mechanism of the 1Re2Ru alloy is a/2<110>dislocations bypass mechanism.