| Due to excellent low thermal expansion coefficients,Al-Si and Al-Si C composites are widely used in fields such as aerospace and precision electronic equipment.However,for Al matrix composites with low thermal expansion,as the second phase that tunes the coefficient of thermal expansion is positive expansion,the thermal expansion coefficient of the state of the art Al-Si C composite is still above 7×10-6/K,which is significantly higher than 4.5×10-6/K of the Si substrate.This project applies a negative thermal expansion material,Zr W2O8,as the second phase for the Al matrix composite.The preparation,microstructure,thermal expansion coefficient and the corresponding mechanisms of low thermal expansion for the Al/Zr W2O8 composites are studied in detail.In this paper,Al/Zr W2O8 composites were prepared by discharge plasma sintering,the effects of sintering temperature,holding time,and pressurization on the microstructure and properties of Al/Zr W2O8 composites were studied,and Al/Zr W2O8 composites with high relative densities were prepared by optimizing the process.The results show that the relative density of the composites first increases and then decreases with the increase in sintering temperature,and the relative density reaches its maximum value（95.4%）at 550°C.The relative density first increased and then decreased with the extension of the holding time,and the relative density was the highest when the holding time was 5 min.Pressurization during cooling reduces the relative density.The relative density of the composite prepared at a sintering temperature of 525°C with a holding time of 5min and no pressure during cooling was 92.9%,and the content ofγ-Zr W2O8 in the composite was24.2%.On this basis,the effect of Zr W2O8 content on the properties of Al/Zr W2O8 composites was further studied.With the increase of Zr W2O8 content,the average thermal expansion coefficient of the sample decreased almost linearly,and the thermal expansion coefficient of the composite with 70%Zr W2O8 content reached-0.77×10-6/K,and the adjustable interval of thermal expansion was-0.77×10-6/K～7.26×10-6/K.The thermal cycling process of low thermal expansion composites with 70%Zr W2O8 content was analyzed.The annealing temperature selection shows that the service temperature of the composite should be lower than 400°C.Multiple thermal cycles at 25～300℃show that the thermal cycle curves almost coincide,and the maximum difference of thermal expansion coefficient is 4.4×10-9K-1.DSC and in-situ XRD tests show that the second-order phase transition occurs at 157°C during the thermal cycle.The modeling of the aluminum-based zirconium tungstate composite and analysis of the correlation between temperature,stress,and phase change were done using finite element simulation software.The findings demonstrate that when the temperature reaches 120°C,the aluminum matrix frame’s stress inconsistency in the three-dimensional direction surpasses the yield limit,leading to plastic deformation.Since zirconium tungstate at the interface has a compressive stress greater than 200 MPa,the phase change happens when the temperature is lowered to 80°C. |
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