| Along with increasing demands of light requirements in industry,the development of light-weight and high-strength components for heat transferring has become a focus of research all round the world.The heat pipes made from LA103Z Mg-Li alloy shows superior performance and has wide application prospect.This paper studied the heat treatment processes of extruded heat pipes such as solid solution,aging and annealing to improve the room temperature properties,high temperature strength and stability.The matrix of the extruded heat pipes is composed of two phases.The ?-Mg phase assumes long strip shape along the extrusion direction and the ?-Li phase keeps near-equiaxed shape.The dispersed particles approximately 1.6 ?m in diameter are composed of AlLi and LiZn,which may weaken the interface combination of matrix and reducing the strength of heat pipes.Mg2Si is also zonally distributed along the extruded direction.Mechanical property test shows that the tensile strength is 170±1 MPa,the yield strength is 137±1 MPa,and the elongation is 49.5±5%at room temperature,while the tensile strength is 90±2 MPa,the yield strength is 59±1 MPa,and the elongation is 65±17%at 90?.The strength and stability of heat pipes should be improved.The transformation from ?-Mg to ?-Li was earlier than the dissolution of the second phase during solid solution treatment.The orientation relationship between these two phases is(0001)?-Mg//(110)?-Li,[1120]?-Mg//[111]?-Li,and the interface tended to semi-coherent when temperature increasing.Meanwhile,the metastable Li2MgAl phase is formed in the matrix with orientation of(011)Li2MgAl//(011)?-Li,[100]Li2MgAl//[100]?-Li,and the interface also tends to semi-coherent.The dispersed particles are completely dissolved at 400?/15 min,which leads to the peak tensile strength of 375.5±2.5 MPa and the elongation of 4.5±1%.The matrix is single ?-Li at 400?/35 min,contributing to tensile strength of 332±4 MPa and the elongation of 16±2%while the plasticity is significantly improved.Microstructure and properties show that the solution strengthening is more effective than coherent strengthening of matrix and precipitation strengthening of second phases.The peak aging temperatures of solid solution heat pipes are 133? and 225?.Low temperature ageing process is 133?/4 h,when ?-Mg is regenerated in needle-like shape from ?-Li and the metastable Li2MgAl phase is retained,causing that solution strengthening is replaced by precipitation strengthening.High temperature ageing process is 225?/4 h,when Li2MgAl is transformed into AlLi,when ?-Mg is spherical in ?-Li grains and widens the boundaries of ?-Li,making the coherent relationship disappear and weakening the effect of precipitation strengthening.The optimum aging process is 100?/4 h,contributing to tensile strength of 245±2 MPa,the yield strength of 202±2 MPa,and elongation of 16±3%.The annealing treatment is carried out on extruded heat pipes directly to obtain excellent comprehensive performance.When annealing temperature increases,the ?-Mg changes into near-equiaxed shape from long strip and the ?-Li grain grows,which causes the extrusion characteristics disappearing at 300?.When annealing time prolongs,a large number of grain boundary precipitations occur at 300?/30 min and 90 min,reducing the strength.When cooling speed is too slow,the second phases increase significantly,while too fast cooling speed causes large thermal stress.Therefore,the optimum annealing process is 300?/60 min and then air cooling,leading to tensile strength of 253±3 MPa,yield strength of 203±2 MPa and elongation of 27±3%,which is close to the optimal performance of solid solution aging samples.But the optimum annealing process is simplified and the thermal stress is less.The feasibility and superiority of this process condition were verified by subsequent high temperature mechanical properties. |
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