Investigation Of Microstructure And Properties On Ho Modified AZ91 Magnesium Alloy

Magnesium alloys are widely used in aviation, automobile, electron and other fields due to their light density, good dimensional stability, excellent mechanical property and shielding electromagnetic interference property. Application in engineering is limited due to its inherent drawbacks, however, such as low elastic modulus, poor plasticity and corrosion resistant properties and so on. Poor corrosion resistant property is the most important factor that restricts to its application. Therefore, it is important to study the anticorrosion properties and push the development of magnesium alloys applications.Nowadays, most of the anticorrosion researches are focused on the surface protection magnesium alloy and so in the state of passivity. Surface anticorrosion technology is just for some specifically condition or accessory. Such one-to-one corrosion control method can not meet apparently the enlarging of magnesium alloys application fields.Alloying addition agent can modify the phase structure and matrix corrosion potential of Magnesium alloy and thus it is an important and essential way to improve the corrosion resistant properties of Magnesium alloys. Consequently, this paper adopted alloying method and added rare earth element Ho into Magnesium alloys in order to improve their anticorrosion properties. Five kinds of heterogeneous Magnesium alloys are AZ91,AZ91+0.1wt.%Ho,AZ91+1.0wt.%Ho,AZ91+2.0wt.%Ho and AZ91+3.0wt.% Ho respectively.The microstructure and phase of magnesium alloys have been analyzed by scanning electronic microscope (SEM) and X-ray diffraction (XRD). The results indicated that there wereαphase,β(Mg17Al12)phase and Al2Ho phase coexisted in Magnesium alloys added with rare earth element Ho. With the addition of rare earth element Ho, the microstructure refined into grain state from bulky bones state and the size of crystal grain decreased; discontinuous reticularβphases distribute over the grain boundary ofα-Mg phase, its eutectic mode have changed into fully-divorced eutectic from partially-divorced eutectic. The formation of second separationβphases is restricted. Tensile and hardness tests showed that rare earth element Ho improved the tensile strength, yield strength and hardness of AZ91 magnesium alloys. The fracture mechanism changed from cleavage fracture to quasi-cleavage fracture. The AZ91 magnesium alloy with 2.0wt.% rare earth element Ho has the best microstructure and mechanical properties in this test.