Regulation Of Mo And W Addition On Microstructure And Mechanical Properties In Fe-Ni-Co-Al-Based Alloys

Through the thermoelastic martensitic transformation,Fe-Ni-Co-Al based superelastic alloys possess good superelasticity.However,ordinary Fe-Ni-Co-Al-based alloys do not possess thermoelastic martensitic transformation.Large rolling deformations and ageing precipitation are needed to reduce the transformation heat hysteresis,which can promote the thermoelastic martensitic transformation.However,large rolling volumes are difficult to achieve in the process.In this paper,superelasticity at non-large rolling volumes is obtained by element addition,optimisation of the thermomechanical treatment process and control of the substrate structure.The precipitation mechanisms,martensitic transformation processes and superelasticity of alloys with different compositions,processes and structural conditions are investigated.Firstly,the effect of the addition of W on the precipitation behavior of Fe-Ni-Co-Al-based alloys was investigated,and it is found that both alloys of Fe_43.5-xNi₂₈Co₁₇Al_11.5W_x（x=1,2 at.%）precipitated phase after aging.When the W content is 1 at.%,there is a coarseβPhase.While the W content increase to 2 at.%,βphase is significantly suppressed so that only a small amount of precipitation at the grain boundaries.The elongation of the alloy increases from 3.7%to 23.1%when the brittle phaseβprecipitation content is reduced.Furthermore,the effects of Mo addition on the precipitation behaviors of L1₂ structuredγ′（Ni₃Al）phase and B2 structuredβ（Ni Al）phase and the martensite transformation in Fe_43.5-xNi₂₈Co₁₇Al_11.5Mo_x（x=0,1,2,3 at.%）alloys were investigated.The results show that when Mo content reaches 2 at.%,Mo will be enriched at the grain boundary to form Laves phase of Fe₂Mo.The grain boundary precipitation of Fe2Mo reduces the grain boundary energy,where a Mo content of 2 at.%leads to a reduction in grain boundary energy of 72.2 m J/m²,inhibiting the precipitation and coarsening behavior ofβphase.At the same time,the enrichment of high-sublimation enthalpy Mo element at the grain boundaries also improves the grain boundary cohesion,which leads to an increase of elongation from 18.3%±0.7%at 0 at.%Mo to 51.6%±2.4%at 3 at.%Mo.Asβphase is suppressed by the addition of Mo,the content ofγ′phase in the matrix increases.In addition,with increasing Mo content,the size ofγ′phase decreases from 12.6±0.2 nm at 0 at.%Mo to9.7±0.2 nm at 2 at.%Mo,and its mismatch with the parent decreases from 0.66%±0.04%to0.52%±0.04%.The Mo addition causes a significant decrease in the free energy and equilibrium temperature of alloy,which leads to a decrease in martensitic transformation start temperature from 166 K to 126 K with the increase of Mo content.The alloys with different Mo content have a low thermal hysteresis and exhibit a fully reversible thermoelastic martensitic transformation with temperature changes.Secondly,the effects of different thermomechanical treatment processes on the microstructure and mechanical properties of Fe-Ni-Co-Al-W/Mo alloys were investigated.It is found that increasing the amount of cold rolling improves the overall mechanical properties of Fe-Ni-Co-Al-Mo alloys,including strength,hardness and elongation.The presence of the rolled state also allows the alloy in the rolled-aged state to acquire certain pseudoelastic properties.For solid solution treatment,increasing the solution temperature to 1250°C allows the non-ageing precipitated Ni₃Al second phase to completely solidify into the matrix and increase the grain size.For aging treatment,a W content of 1 at.%inhibits the precipitation of theβphase at the beginning of aging while coarsening of theβphase occurs at the end of aging,which significantly reduces the elongation from 48.5%±2.8%to 3.7%±0.5%.At a Mo content of 3at.%,lots of Fe₂Mo gradually precipitates at the grain boundaries with increasing aging time,which results in a gradual increase of elongation from 43.0%±2.4%to 51.6%±3.5%.Finally,columnar crystal organisation is obtained by directional solidification technique.The superelastic properties of Fe-Ni-Co-Al-Mo alloys with different Mo contents were investigated in tensile and compressive processes.During the directional solidification process,well-oriented columnar crystal zones and fine-grained zones with shifted orientation were found in the matrix.The tensile and compressive properties of the columnar crystal organisation were tested.A maximum of 0.34%superelasticity strain and 0.90%reversible strain were obtained in tensile testing.The superelasticity in tension and compression is inconsistent,with a maximum of 0.60%superelasticity strain and 3.10%reversible strain obtained in compression,which is better than the performance in tension.However,as the Mo content increases,the superelastic and reversible strain decreases in both tension and compression,which is associated with a reduction in matrix strength.