The Effect Of TiC Particle And Pre-strain On The Stability At Elelvated Temperature For Al-Mn-Si Alloy

In recent years,the demand for aluminum alloys in the automotive and aerospace industries has increased,and higher temperature strength and stability have been required at 250 °C – 350 °C.It has been found that the ?-AlMnSi dispersoid in 3 series can be used as a strengthening phase at elevated temperature,Also this phase has excellent stability at high temperature.Therefore,it has a great application prospect.It is generally considered that the 3xxx alloy is a non-heat treatable alloy,which depend on Mn solid solution strengthing and work hardening to improve strength.The traditional processing method to improve high temperature strength is to homogenize the ingot at high temperature for a long time,which has a long production cycle and reduces the solid solution content of Mn element after homogenization heat treatment.Strain can induce the precipitation of a large number of precipitated phases,but for this precipitation process,the precipitation effect,the influence on the microstructure and mechanical properties have not been systematically studied.This thesis systematically studied those aspects.Nano-ceramic particles have an important influence on materials microstructure and properties.In this paper,we not only researched the effect on microstructure and propertiest,but also the effect on the dispersoids.The stability of the precipitated phase determines the stability of the material structure and the stability of the high temperature performance.In this paper,the coarsening of the precipitated phase,the change of the annealed structure and the change of the mechanical properties of the material are discussed by changing the strain.The specific research situation is as follows:This article researched the effect of adding nano sized TiC for the size of dispersoids and the materials mechanical properties through SEM,TEM,XRD and related mechanical properties detected methods.The results showed that the addition of nano sized TiC particle not only refines the grain size,but also refines the dendrite size.With the addition of ceramic particles,the refined effect is more obvious.When TiC particle content is 0.2 wt.%,the yield strength increased from 131 MPa to 155 MPa at the room temperature,an increase of 15.5%.The yield strength and tensile strength of the material at elevated temperature all increased from 63 MPa and 67 MPa to 81 MPa and 83 MPa respectively,the yield strength and tensile strength increased by 29% and 24% respectively.The transmission analysis showed that the addition of nano-ceramic particles promotes the precipitation of dispersoids,the size of dispersoids is smaller,and the coarsening rate of dispersoids is slower during annealing treatment.The effect of pre-strain on the precipitation of the second phase and the microstrucrural stability and mechanical properties during long time holding at high temperature were studied in this paper.The results showed that pre-strain greatly promotes the precipitation of dispersoids.The distribution of precipitates is linear and the size of precipitates is large in ascast samples after annealing.The distribution of strain-induced precipitates is uniform and the size of precipitates is small.The results showed that the strain-induced precipitates have good stability,and the coarsening rate of precipitates is much lower than that of as-cast samples.Strain-induced precipitation promotes a large number of fine dispersed precipitates,which makes the materials have good microstructural stability and mechanical properties stability at high temperature.The material can be used for a long time at 300 °C.In this paper,the precipitation process of large strain sample during isothermal annealing is studied.The experimental results showed that the dispersoids preferentially precipitate at the subgrain boundary,and then precipitate within the subgrain.The sizes of the precipitated phases is larger at the subgrain boundary than that within the subgrain.The growth rate of precipitates at subgrain boundaries is faster at the early stage of precipitation and slower at the later stage.The growth rate of precipitates in subcrystals is slower at the early stage of precipitation and faster at the later stage.The interface between precipitated phase and matrix was studied,the results showed that the orientation relationship between(520)plane of ?-AlMnSi and(111)plane of aluminum matrix is completely coherent.In this paper,the effects of different pre-strain deformation on the coarsening of precipitates,microstructure and mechanical properties during annealing were systematically studied.The results showed that the change of pre-strain has no effect on the volume fraction of dispersoids,which mean different strain has approximately the same volume fraction of precipitated phase at the completion of precipitation.The change of pre-strain effect the size distribution of precipitated phase.With the increase of strain,the size distribution of precipitated phase is different when precipitation is completed,which caused the different of coarsening process,The coarsening of precipitated phase is not directly caused by strain energy,but determined by the size distribution of precipitated phase.Different deformation amount made the microstructure of materals change differently during annealing.When the strain is small,the driving force of recrystallization is small,so the microstructural stability is better during annealing.When the strain is large,although the driving force of recrystallization is larger,the resistance of precipitation to grain boundary migration is larger because of the regular distribution of precipitated phases along sub-grain boundary,which makes the microstructural stability better.For room temperature properties,the tensile strength increases with the increase of strain.When annealing time is 12 h,the strain increases from 20% to 80%,the yield strength increased from 113 MPa to 155 MPa,and the tensile strength increased from 160 MPa to 198 MPa.However,the mechanical properties of large strain specimens decrease greatly during heat preservation,while the yield and tension of small strain specimens keep good stability during heat preservation,and the plastic of the material increases with the increase of deformation.For the high temperature performance,the change of strain has little effect on the high temperature performance,and it has good stability in the process of heat preservation.