The Characterization Of Residual Stress In Mg₂B₂O₅ Whiskers Reinforced Magnesium Matrix Composites Using Raman Spectroscopy

Magnesium alloys possess such features as favorable machinability, low density, excellent cast characteristics, and have been widely used in many fields, such as automotive, communication, aerospace and aviation. However, magnesium alloys'corrosion resistance and performance at elevated temperature is not so good, to some extent limit its application. Magnesium matrix composites had solved these problems, Metal matrix composites applied at elevated temperature generated thermal residual stress because of the mismatch of expansion coefficient between the reinforcement and the matrix. Residual stress often caused stress concentration within the local area of the material, make damages on materials performance. Therefore, study the distribution of local residual stress within composite materials is necessary. The traditional experiment methods of mechanical measurement are hard to measure residual stress in micro regions directly. Currently, the main experimental methods used in Micro-area residual stress measure are X-ray diffraction and neutron diffraction, but both the methods are often measure the average stress within the measured area of the material. Raman spectroscopy as a non-destructive, non-contact, high spatial resolution technique has many advantages on measure micro stress. The Raman frequencies induced by the atomic vibrations in the molecules can be shifted by application of external stress or strain, a compressive strain field makes the Raman peaks to shift into higher wavenumbers, while the opposite is happening on application of a tensile strain field. Therefore, the stress state and the surrounding environmental changes can be studied according to the Raman spectra of the materials. In this paper, Raman Spectroscopy was used to characterize the interface residual stress in Mg₂B₂O₅w/AZ91D magnesium matrix composites. The residual stress of the composite material and the distribution of the residual stress in the interface micro-area were studied, the effect of thermal residual stress on the properties of magnesium matrix composite was also analysised. Offer guide for the design and preparation of magnesium matrix composites.A continuous scan of the Raman spectra of borate whisker in Mg₂B₂O₅w/AZ91D from wave numbers 100 cm-1 to 3200cm-1 was performed. In the range of wave numbers 200 1500 cm-1 contained the major Raman peaks of borate whisker. The spectra from the same location of the borate whisker repeated thrice are almost identical, these clearly proved the reproducibility of the major features of the experimental results carried out on the same location of the same borate whisker. Raman spectra from three different locations of the same borate whisker are almost identical within the margin of experimental errors testified the chemical uniformity within the same fiber.Raman spectra of the whiskers in Mg₂B₂O₅w/AZ91D tensile fracture at different temperatures were performed. Raman peaks corresponding to symmetric stretching vibration of magnesium borate whisker is 843.64 cm-1 at room temperature, it changed to 844.69 cm-1 as the tensile temperature is 160℃, it comes to 846.949 cm-1 when the temperature is 250℃. Compared with tensile fraction at room temperature, the Raman peaks shift into higher wavenumbers, the whiskers were on compressive strain field, and the residual stress of tensile fracture at 250℃is larger than that of 160℃.The interface residual stress of the Mg₂B₂O₅w/AZ91D is also analysised with Raman spectroscopy. Compared with the Raman frequency of 846.949 cm-1 at room temperature, the Raman peak in the interface within the Mg₂B₂O₅w/AZ91D tensile fracture at 250℃is 844.69 cm-1, the interface were on tensile strain field. The Raman wavenumber of the interface within specimen was tensile fraction at 250℃is the same with that of room temperature. May be there are reaction products forming at the interface region by various precipitating processes, it also possible that the interface between the matrix material and the reinforcement was debonding, resulting in the load can not be delivered.