Investigation On Microstructures And Properties Of Magnesium Heat Resistant Alloys With Nd Additions

Rare earth elements are the most effective elements on the improvement of the properties of magnesium alloy at high temperature. The as cast magnesium alloys containing rare earth take more than 50% in total magnesium alloys developed in the world, which makes the rare earth magnesium alloys take special place in the heat resistant magnesium alloys. China has the most plentiful magnesium and rare earth resources in the world, which provides advantages in researching and developing high properties rare earth magnesium alloys with the acceptable cost. In the present work, the microstructures, mechanical properties and creep resistance of Mg-Nd (La)binary alloys and Mg-Al-Nd(La) ternary alloys with different rare earth elements La and Nd, or different Nd additions have been investigated systematically by optical microscope (OM), scanning electron microscope (SEM), X-ray diffractometer (XRD), inductively coupled plasma (ICP), Transmitting electron microscope (TEM) etc.The existence form of La was quite different from that of Nd in the as cast Mg-Nd (La) binary alloy. La additions in the Mg-La binary alloy existed mostly in the eutectic phase(Mg+Mg17La2)which distributed along the interdentritic boundaries; however, Nd additions in the Mg-Nd alloy existed partly in the divorced eutectic phase Mg12Nd and partly solid solved in the oversaturatedα-Mg matrix. With the increase of Nd additions, the volume fraction of divorced eutectic phase Mg12Nd increased gradually and the existence form of Mg12Nd also changed from spotted to network formed along grain boundaries.The microstructure of N2 alloy after solid solution (525℃/8h) and aging treatment (250℃) was investigated through SEM and TEM. The results showed that the ribbon-orderedβˊ phase and the small plate phaseβprecipitated fromα-Mg matrix subsequently. Both the phaseβˊ and the phaseβhad coherent relationship withα-Mg matrix, which were described as (110)β'//(0001)Mg and [11 2 ]β'// [1100]Mg,(011)β//(2110)Mg and [100]β//[0001]Mg. The habit planes of the phaseβˊ and the phaseβwere (1100)Mg and (2110)Mg respectively. The metastable phaseβˊ disappeared and the equilibrium phaseβgrown along the direction of [0001] Mg with the aging time increasing. The results of XRD also showed that there was onlyβphase (Mg12Nd) existed in the N2 alloy after 6 hours aging. The curves of hardness with aging time exhibited the hardness of the alloy N2 increased to the peak value and then decreased gradually with the aging time prolonging. The shape of two kinds of precipitatesβˊ&βduring aging was explained through the calculation of the misfit degree of atoms between the precipitatesβˊ&βand matrix on the habit plane and the basal plane ofα-Mg. The results also showed that the two precipitated phases had perfect atoms match withα-Mg matrix on each habit plane , especiallyβˊ phase nearly had no...