Study on Interface of C_f/Mg Composites Fabricated by Liquid-solid Extrusion Following Vacuum Pressure Infiltration

Author:Li Shao Lin

Supervisor:qi le hua


Degree Year:2018





Carbon fiber reinforced magnesium(Cf/Mg)composites have been attracting much attention for wide application prospects due to their excellent specific strength,good damping performance and machining performance.However,the poor wettability between carbon fiber and magnesium made the fabrication of the composite difficult,while the reaction between carbon fiber and matrix alloy might degrade the composite properties.The above mention disadvantages greatly limited the fabrication and application of Cf/Mg composites.Therefore,the interface was the core issue of Cf/Mg composites.Interface optimization was the key to development and application of Cf/Mg composites.The interface of Cf/Mg composites were affected by fiber coating,matrix alloy and fabrication process.In this dissertation,Cf/Mg composites were fabricated by liquid-solid extrusion following vacuum pressure infiltration technique(LSEVI).The effects of fiber coating,matrix alloy and fabrication process on the interface were studied.The interface characteristics and types were discussed.The main research contents were as follows.Fabrication parameters significantly affected the microstructure and mechanical properties of the Cf/Mg composites.The optimum parameters of the current work were590℃of infiltration temperature,30 MPa of infiltration pressure and 20℃/min of cooling rate.The interface and mechanical properties of the Cf/AZ91 composite could be modified by heat treatment.The number of interfacial precipitates Mg17Al12decreased and the size of the granular precipitates was reduced from about 500 nm to about 200 nm due to recovery.Nanocrystalline ceramic particles MgO with disordered intergranular films and dislocations,resulting from unusual deformation mechanisms and interfacial stress,were observed at the interface.The tensile strength of PyC-Cf/AZ91 composite increased from 400 MPa to 420 MPa after heat treatment.The strengthening of the composite after heat treatment was attributed to recovery,relief of interfacial stress and nano-sized MgO interfacial layer.Fiber coating could protect the fiber and modify the interface effectively.Both TiO2and PyC coatings could modify the interface and improve the mechanical properties of the composites.The ultimate tensile strength of the TiO2-Cf/AZ91D and the PyC-Cf/AZ91D composite were 333 MPa and 400 MPa,which were improved by 41.7%and 70.2%respectively,compared with the untreated-Cf/AZ91D.The microstructure observation revealed that the strengthening of the composites relied on fiber integrity and moderate interfacial bonding.MgO nano-particles were generated at the interface due to the reaction of TiO2 with Mg in the TiO2-Cf/AZ91D composite.The volume expansion resulting from the reaction let to disordered intergranular films and crystal defects at the interface.The fibers were protected and the interfacial reaction was restrained by PyC coating in the PyC-Cf/AZ91D composite.The PyC coating offered greater improvement with a high cost,while the TiO2 coating presented relatively smaller improvement with a low cost.Appropriate coating can be adopted according to the actual conditions,such as existing experimental condition and service environment of the composite.The content of Al element in matrix obviously affected the interface and mechanical properties of the Cf/Mg composites.Compared with the PyC-Cf/AZ31 composite,the amount and size of Mg17Al12 precipitates in the PyC-Cf/AZ91D composite was larger due to the higher Al content.Precipitate free zones(PFZ)were formed at the interface of the PyC-Cf/AZ31 composite due to the low Al content.The mechanical properties of the PyC-Cf/AZ31 composite was higher than the PyC-Cf/AZ91D composite on account of the fine precipitates and PFZ in the PyC-Cf/AZ31 composite.The rare earth element could modify the Cf/Mg composites interface.In PyC-Cf/AE44composites,particle shaped Al2RE and lamellar shaped Al11RE3 precipitated at the interface.The Al-RE phase precipitated preferentially and inhibited the formation of Mg-Al phase.The ultimate tensile strength of PyC-Cf/AE44 composites was 412 MPa,which improved by 127%compared with the AE44 alloy,while Young’s module improved by 74%.The failure mechanism of the composites was fiber bundle pulled out,which indicated the interface bonding was moderate under the combined effect of Al and RE.No interfacial reaction products were detected at the interface,and some block-shaped interfacial precipitates Mg17Al122 were observed instead.The mismatch of thermal expansion between the PyC and AZ91D alloy caused stress field at the interface.The interface of PyC-Cf/AZ91D composites failed at the interface between PyC coating and AZ91D alloy due to the mismatch of thermal expansion and relatively poor bonding.The fracture process of 2D-Cf/AZ91D composites was transverse fiber interface cracking--matrix transferring load--longitudinal fibers bearing load--longitudinal fibers breaking.According to the bonding state,the interface of Cf/Mg composites fell into three types:poor wetting interface,reaction interface and precipitate interface.In the Cf/Mg composites with poor wetting interface,the interfacial bonding was too poor to transfer load,which was adverse to the properties of the composite.In the Cf/Mg composites with reaction interface,the reaction products not only embrittled the interface,but also damage the fiber structure.The properties of Cf/Mg composites with reaction interface were normally rather poor.In the Cf/Mg composites with precipitate interface,the wetting of the matrix and fiber was improved by the interfacial precipitate without interfacial reation.The properties of the composite with precipitate interface were generally excellent.The precipitate interface was the ideal interface for Cf/Mg composites.