Heat-Resistant Phases Architecture And Its Effect On Elevated-Temperature Tensile Strength In Al-Si Multicomponent Alloys

Al alloys have high specific strength,corrosion resistance and good formability.It is an ideal lightweight material for automobiles.However,the strength of Al alloys decreases sharply with increasing temperature,which cannot meet the requirements of modern power equipment.Therefore,there is an urgent need to investigate the thermostability of Al alloys.Introducing heat-resistant phases with good thermal stability into Al alloys is an effective way to increase its elevated-temperature strength,and the strengthening effect of the heat-resistant phases is usually affected by its architecture.Based on this,this article uses alloying,melt treatment and deformation treatment methods to control and optimize the architecture of the heat-resistant phases,and analyzes the strengthening effect of different architectures.On this basis,a new type of Al-Si multicomponent alloy with a network-like reinforced structure was prepared,which effectively increased the elevated-temperature strength of the alloy,and the three-dimensional characterization of the network-like structure constructed by eutectic Si and heat-resistant phases was carried out.From the perspective of architectural interconnectivity,the elevated-temperature strengthening mechanism of the network architecture was studied and revealed.The main research contents are as follows:(1)Optimization of Fe/Ni heat-resistant phases in Al-Si multicomponent alloysTrace Fe can optimize the heat-resistant phases architecture in Al-12Si-4Cu-2.5Ni-1Mg-xFe multicomponent alloys.When the Fe content was 0.15%,the Fe element appeared in the form of needle-like ?-Fe phase,while the fish-bone ?-Al3CuNi phase was distributed discretely on the ?-Al matrix.When the Fe content increases to 0.6%,the Fe element was mainly present in the bulk T-A19FeNi phase,and the main heat-resistant phases were distributed at the a-Al grain boundaries with a continuous or semi-continuous network architecture in the alloy.Further increasing the Fe content to 0.8%,the needle-like ?-Fe phase reappeared,and the T-A19FeNi phase was distributed on the a-Al matrix with a coarse strip-like morphology.After high temperature tensile test,it was found that the heat-resistant phases have a small contribution to the high temperature strength when it were distributed and strip-shaped distribution,while the heat-resistant phases with the network or semi-continuous network distribution have the largest contribution to the high temperature strength,which makes the tensile strength of the tested alloy reach 107 MPa at 350?,22%higher than that of the discrete heat-resistant phases strengthened alloy.However,the first two architectures contribute significantly to the plasticity of the alloy.Since the heat-resistant phases distributed in the network or semi-continuous network can effectively strengthen the grain boundaries at high temperature,which is conducive to improving the high temperature strength of the alloy.The other two architectures have a weaker overall strengthening ability for the grain boundaries,so the strength of the alloy was relatively low when higher plasticity was obtained.Four materials,i.e.Al-12Si-4Cu-1Mg-0.6Fe-xNi,x=2,2.5,3 and 3.5 were prepared by alloying.In the tested alloys,the change of Ni content did not cause a change in the category of heat-resistant phases,but the architecture of the heat-resistant phases changed.The size of the main heat-resistant phases was smaller and the distribution was more discrete in the Al-12Si-4Cu-1Mg-0.6Fe-2Ni.The main heat-resistant phases exhibit a continuous or semi-continuous network architecture in the Al-12Si-4Cu-1Mg-0.6Fe-2.5Ni.Further increasing the Ni content to 3.5%,the Ni-rich phases become coarse and show an aggregate distribution.It found that the network or semi-continuous network architecture was helpful for the heat-resistant phases to exert its own good thermal stability,so that the alloy can achieve the maximum high temperature strengthening effect,and the tensile strength of alloy can reach 103 MPa at 3500C,which was 24%higher than that of the alloy with discrete distribution of the heat-resistant phases.(2)Heat-resistant phases architecture control and its effect on elevated-temperature tensile properties in Al-Si multicomponent alloysOn the basis of Al-Si multicomponent alloys with network architecture.Controlling the heat-resistant phases architecture to further investigate the influence of heat-resistant phases architecture on the mechanical properties of Al alloys.The high temperature strengthening mechanism and the ?-Al matrix deformation behavior were investigated by comparing the strengthening effect of different heat-resistant phases architectures.Based on Al-12Si-4Cu-2.5Ni-lMg-0.6Fe alloy with network architecture,the formation of primary Si was suppressed by the melt treatment and the process of increasing the cooling rate of solidification,which resulting in a large amount of eutectic Si formation and aggregation distribution,and the heat-resistant phases with strip-like morphology were separately distributed on the ?-Al matrix.However,the heat-resistant phases with network architecture reappeared on the ?-Al matrix in Al-4Cu-2.5Ni-1Mg-0.6Fe alloy.The heat-resistant phases changed from network architecture to streamlined distribution after deformation treatment.It was found that the network architecture can significantly improve the high temperature strength of the multicomponent Al alloys,and the tensile strength can reach 103 MPa and 94 MPa at 350?,respectively.Since network architecture can significantly strengthen the grain boundaries and hinder the sliding of the grain boundaries at high temperature,thereby effectively improving the high temperature strength of the alloy.Besides,the network architecture was conducive to the overall deformation of the a-Al matrix,so that the heat-resistant phases can better exert its own high temperature strengthening effect,and further increase the contribution to the elevated-temperature strength for alloy.(3)Network architecture of heat-resistant phases and eutectic Si and its effect on high temperature tensile properties in Al-Si multicomponent alloysThe heat-resistant phases with network architecture showed good high temperature strengthening effect for Al-Si multicomponent alloys.However,the aggregation of primary Si and eutectic Si,the coarsening of the heat-resistant phases will weaken the high temperature strengthening effect of the network architecture.By optimizing the network structure of the Si phases and adjusting the architecture of the heat-resistant phases,a three-dimensional network structure with higher connectivity can be constructed,and further strengthening the Al-Si multicomponent alloys.Based on Al-9Si-6Cu-4Ni-1Mg alloy,Al-9Si-6Cu-4Ni-1Mg-0.6Fe alloy was prepared.The Fe element is involved in the precipitation of many phases during the solidification,which has an influence on the network architecture of the heat-resistant phases.It was found that the addition of Fe element successfully regulated the architecture of the heat-resistant phases,so that the heat-resistant phases showed a continuous or semi-continuous network architecture.Meanwhile,the network structure constructed by the heat-resistant phases and eutectic Si have a very high connectivity rate,increasing from 82.43%to 97.04%,and the tensile strength of alloy was also increased from 98 MPa to 124 MPa at 350?.Therefore,the high-connectivity network architecture can effectively strengthen the a-Al matrix and significantly increase the high temperature strength.The high temperature tensile strength of the alloys were reduced to 73 MPa and 80 MPa,and the elongations were increased from 4%and 2.7%to 5%and 3.8%after thermal exposure at 350? for 200 h.It was found that the thermal exposure treatment reduced the high temperature strengthening effect of the heat-resistant phases,eutectic Si and precipitated phases,resulting in a significant reduction in the high temperature strength of the alloy.The mutual desorption between the heat-resistant phases and the reduced network connectivity of the eutectic Si spheroidization and the coarsening of the precipitated phases together weaken the contribution to the high temperature strength.