Study On Microstructure And Properties Of Al-Ce Alloys Based On Composition And Solidification

High-strength heat-resistant aluminum alloys are widely used in key components of automobile engines and aerospace thrusters due to excellent strength properties and heat resistance.With the further lightweight demand for Al alloys,a technical bottleneck is to expand its service temperature（>250℃）.Adding Ce element as the main element in Al alloy is one of important ways to resolve the problem,because of its low solid solubility,large atomic radius and slow diffusion rate.However,due to coarse and irregular morphology and uneven distribution of Al₁₁Ce₃ phases,Al-Ce alloys prepared by conventional cast have poor performance at room temperature and elevated temperature.In this paper,hypereutectic Al-14Ce alloys were studied and the object was to regulate the morphology,distribution and Al/Al₁₁Ce₃ interface.Hypereutectic Al-14Ce alloys containing micrometer,submicron and nanoscale Al₁₁Ce₃ particles were prepared by Sc-Zr composite modification,electromagnetic stirring treatment and selective laser melting forming.The nucleation and growth of primary and eutectic Al₁₁Ce₃ phases were investigated at multiple levels and scales,and the regulation mechanism of Al₁₁Ce₃ phases was systematically revealed.On this basis,the mechanical behavior of modified Al-14Ce alloys at room temperature and elevated temperature was studied.The interfacial relationship between multiscale Al₁₁Ce₃ phases and matrix and its effect on the stability and mechanical properties were expounded.The main results are as follows:（1）The nucleation and growth law of Al₁₁Ce₃ phases and the compound modification mechanism of Sc-Zr addition were obtained.The solidification process of hypereutectic Al-14Ce alloy was observed by the synchrotron radiation X-ray real-time imaging technology.The Al₁₁Ce₃phases in a faceted growth with I-like and V-like pattern exhibit hollow cubic tube shape,groove shape and other solid polyhedron morphology.In order to improve the morphology of the faceted Al₁₁Ce₃ phases,Al-14Ce alloys were modified with Sc-Zr addition.Sc-Zr atoms can be adsorbed on the surfaces of Al₁₁Ce₃ phases and prevent their growth via adsorption effect when the content of Sc is 0.15 wt%and Zr is 0.06 wt%.When the total Sc-Zr content increases,primary DO23-Al3（Zr,Sc）phases preferentially form in the melt and provide substrates for the nucleation of primary Al₁₁Ce₃ phases.Therefore,the modification effect on primary Al₁₁Ce₃phases is based upon the interaction of adsorption effect and heterogeneous nucleation.Compared with Al-14Ce alloy,the average size length of the primary Al₁₁Ce₃ phases is refined from～79μm to～32μm.The UTS,YS and EL of the Al-14Ce-0.45Sc-0.18Zr alloy increase from 117 MPa,75 MPa and 1.4%to 181 MPa,145 MPa and 3.4%,respectively.（2）The effect of electromagnetic stirring on the refining behavior of Al₁₁Ce₃ phase and mechanical properties in Al-14Ce alloy was revealed.After electromagnetic stirring,the coarse dendritic primary Al₁₁Ce₃ phases basically disappeare and a handful of Al₁₁Ce₃ phases are divided into clumps in 1μm size.Eutectic Al₁₁Ce₃ phases change from lamellar to fibrous.The lamellar spacing of eutectic Al₁₁Ce₃ phases is 1.3μm and the fibre diameter is 0.4μm.The effect of electromagnetic stirring in Al-14Ce alloy can be attributed to the combined effect of heredity,dendrite melting and the increase of undercooling degree.Fine primary phase and small eutectic lamellae spacing can improve the load transfer ability and enhance fine grain strengthening effect.The UTS,YS and EL of the Al-14Ce alloy after electromagnetic stirring is 185 MPa,108 MPa and7.1%,respectively.（3）The comprehensive effects of selective laser melting forming and Sc-Zr composite modification on the microstructure and properties of Al-14Ce alloy were investigated.The Al-14Ce alloy mainly shows columnar crystal structure.The primary bulk Al₁₁Ce₃ phases distribute near the melt pool boundary,while the network eutectic Al₁₁Ce₃ phases form inside the melt pool.The average size of the primary Al₁₁Ce₃ phase is about 132 nm,the width of the eutectic Al₁₁Ce₃ network is about 32 nm,and the equivalent circle diameter is about 175 nm.In Al-14Ce-0.45Sc-0.18Zr alloy prepared by selective laser melting,a bimodal grain morphology with coarse columnar crystal and fine equiaxed crystal is obtained.Sc element mainly enriches in Al₁₁Ce₃ eutectic network and a coherent interface between Al₁₁Ce₃ phases andα-Al is obtained.The room temperature strengthening mechanisms of Al-14Ce alloy and Al-14Ce-0.45Sc-0.18Zr alloy prepared by selective laser melting are dislocation strengthening,grain boundary strengthening and nanonetwork boundaries strengthening.The UTS,YS and EL of the Al-14Ce alloy by selective laser melting is449 MPa,353 MPa and 8.1%,respectively.As to Al-14Ce-0.45Sc-0.18Zr alloy,the UTS,YS and EL is 580 MPa、462 MPa and 7.9%.（4）The effect of Al₁₁Ce₃ phases with different scales obtained by different preparation technologies on the elevated temperature mechanical properties was explored.The nano Al₁₁Ce₃networks and coherent interfaces are the key to high elevated temperature mechanical properties.The fracture mechanism of Al-14Ce alloy and Al-14Ce-0.45Sc-0.18Zr alloy formed by selective laser melting mainly is micro-porous aggregation fracture.The main strengthening mechanism is the nanonetwork boundaries strengthening.Al-14Ce alloy formed by selective laser melting with the semi-coherent relationship between Al₁₁Ce₃ phases andα-Al,the tensile strength is 329 MPa,241MPa and 159 MPa at 250℃,300℃and 350℃,respectively.The tensile strength of Al-14Ce-0.45Sc-0.18Zr alloy prepared by selective laser melting,with a coherent interface is 423 MPa,358MPa and 316 MPa at 250℃,300℃and 350℃,respectively.It is superior to similar heat-resistant aluminum alloys reported at home and abroad.