Microstructure And Properties Of TiB_2(P)/AZ91D Composites Fabricated Under High-energy Ultrasonic Field

The AZ91D magnesium matrix composites reinforced by TiB₂ particulates are successfully fabricated under high-energy ultrasound field from Al-TiB₂ master alloy. The phase compositions,microstructures,mechanical properties of the composites and the sizes,morphologies,distributions of the in-situ particles in matrix are investigated with the helps of modern analysis methods,such as OM,SEM,EDS, XRD and so on.To avoid being oxidized severely during melting and pouring process because of high activity of magnesium element,the Ca,RE,Sr alloying elements are employed and the effects of the Ca,RE,Sr alloying elements on the ignition-proof and microstructures were investigated.The experimental results show that the ignition of AZ91D magnesium alloy reaches to 800℃after micro-alloying processing.The microstructures are finer and primaryα-Mg phase refine,the second phaseβ-Mg₁₇Al₁₂ become intermittent by the chain-like.In additional,the as-cast microstructures are refined greatly by adding three elements of Ca,La,Sr comparing with that of adding two elements of Ca,La or Ca,Sr process.The Al-TiB₂ master alloy has been successfully fabricated from Al-K₂TiF₆-KBF₄ system.The morphologies of the TiB₂ particles are in diamond shape,the sizes of TiB₂ particles are about 0.2～1.0μm,the distributions are uniform in the aluminum matrix and the volume fraction is more than 20 percent.TiB_2p/AZ91D magnesium matrix composites have been fabricated successfully by master alloy technology.The TiB₂ particles in TiB_2p/AZ91D magnesium matrix composites fabricated without high-energy ultrasonic assistance are seriously agglomerated.The sizes and morphologies of TiB_2p in TiB_2p/AZ91D magnesium matrix show no obvious variations comparing with that in Al-TiB₂ master alloy.The infaces between the matrix and TiB₂ particles are net and no interface reactants are observed.With high-energy ultrasonic assistance,the distributions are more unform, the morphologies are in square-shape or quadrangle-shape.With the high-energy ultrasonic power and the applying time increasing,the distributions are becoming more unform.These are due to ultrasonic cavitations effects and ultrasonic stream effects.In additional,the effects of degassing and deslage to the melt are contributed to the increasing properties of the composites.The mechanical properties test results show that the tensile strengthσ_b and elongation of the compositesδreach to 201MPa and 3.61%after micro-alloying AZ91D magnesium alloy.The tensile strengthσ_b of TiB_2p/AZ91D composite are improved,and the elongationδis reduced without high-energy ultrasound.With the increase of ultrasonic power,the tensile strengthσ_b of composite are significantly increased.When the ultrasonic power is 0.8KW,the tensile strengthσ_b and elongation of the compositesδreach to 256.2MPa and 33.4%.The strengthening mechanisms of the composites are:fine grain strengthening,solid solution strengthening and dislocation strengthening.