Solid State Phase Transformation Of AZ91 Under Strong Magnetic Field

In this essay, topics of primary Mg17Al12 phase solution during homogenization, continuous precipitation (CP) and discontinuous precipitation (DP) in AZ91 magnesium alloys during aging are studied under strong static magnetic field (12T max) and alternative magnetic field (0.8T max). The interphase layer growth kinetics under magnetic field of Mg-Al diffusion couple is also studied. Theoretical study based on solid state physics, plasma dynamics and corresponding phase transformation theory are discussed to understand the influence of magnetic field on the above mentioned issues. The main results are presented as bellow:Theoretical analysis shows that the magnetic field has little influence on the free energy of different phases during solid phase transformation. However, static magnetic field retards the atom diffusion process in metals, while alternative magnetic field accelerates it.Under static magnetic field, the grain boundary (GB) migration speed consists Vc , V p. Converntial GB migration speed decreases due to the retarded atom diffusion. This speed can be expressed as : With the increase of magnetic field (H), Vc decreases.For hexagonal metal like magnesium, it has magnetic induced GB migrantion speed V p, which can be expressed as: P = (1 /2 ) +(τe/e)24υ?GkTm?μ0?2χ(cos2θ1?cos2θ2)with the increase of magnetic field strength, Vp approaches its maxim:Undrer alternative magnetic field, the influence of the field on GB migration driving force is tiny, but alternative magnetic field accelerates atom diffusion, which also increases the GB migration speed.Analysis on the competition between DP and CP indicates that, alternative magnetic field favorites DP and increase the final ratio of DP in the aged microstructure. Static magnetic field can either hinder or favorites DP, which depends on the applied magnetic field strength. Analysis on the influence of T-T and F-C nucleation mechanism under magnetic field indicate that applied alternative magnetic field can accelerate DP nucleation rate based on both mechanism. As for applied static magnetic field, usually, nucleation rate will decrease, however, when the field strength became strong enough, the nucleation rate based on F-C mechanism may recover. Under static magnetic field, the condition for the bowing of grain boundary between two allotrimophs to form DP nucleus can be expressed as: While the static magnetic field is not strong enough, it principly weekens the F-C nucleation mechanism. With the increase of the fiend strength, the nuclearation rate based on F-C mechanism incrase due to ? Fmag increases,. Under the applied alternative magnetic field, nucleation rate based on T-T and F-C mechanism both increases resulting from the accelerated atom diffusion.The growth of interphase layer formed in Mg-Al diffusion couples is studied under different magnetic field strength and temperature. Experiment results show that Mg17Al12 and Al3Mg2 form in the interface after annealing either with or without magnetic field. The thickness data analysis indicates that they all grow in accordance with the parabolic law. As static magnetic field decreases atom diffusion in alloy and alternative magnetic field increases it, the growth speed of these two interphase layers also decrease under static magnetic field while increase under alternative magnetic field. Calculated data indicate that magnetic field has little influence on the interphase layer growth active energy (Q), yet static magnetic field decreases the pre-exponential frequency factor(k0), while alternative magnetic field increases it, which indicate that the atom jumping frequency decrease under static magnetic field and increase under alternative magnetic field.The dissolution of primary Mg17Al12 phase in the cast AZ91 alloy during homogenization at 420℃is studied with and without magnetic field. Experiment results indicate that under static magnetic filed, as solute atom diffusion retards, dissolution speed of the Mg17Al12 phase decreases. Moreover, while almost all second phases dissolute after 24 hours homogenization at 420℃, solute concentration perpendicular to the grain boundary has larger difference for the specimens annealed under static magnetic field. As the dissolution of second phase during homogenization is essentially a diffusion process, these differences should be attributed to the atom diffusion retardation effects of static magnetic field. On the contrary, resulting from the diffusion accelerating effects of alternative magnetic field, specimen annealed under alternative magnetic field has larger second phase dissolution speed and smaller solute concentration difference along direction perpendicular to the grain boundary. Study on the grain growth during homogenization shows shat under static magnetic field, grain growth speed decreases and the annealed specimen has a more even grain size distribution. The reason is that static magnetic field reduces the atom diffusion and the dissolution of second phase, which on one hand reduces the grain boundary migration speed and on the other hand increases the pinning effect of second phase on the grain boundary migration.Microstructures of AZ91 after aging at different temperature with and without magnetic field show that in all cases DP cells appear, and their macro-morphology is roughly the same. With the increase of aging temperature and the elongation of aging time, DP cells grow into the inside of grain, however, strong static magnetic field results in a slower growing speed. Structures inside DP cells are also studied by SEM, the results show that all the DP cells form by Mg17Al12 and Mg lamellar. When static magnetic field is applied, the intermediate distance between Mg17Al12 lamellar decreases and they appear in a more regular parallel form. Statics on the ratio between grain boundary having DP double seam and single seam (N2/(N1+N2)) show that with the applied magnetic field, the value of N2/(N1+N2) decreases, which indicate that compare with T-T nucleation mechanism, F-C nucleation mechanism become relatively more active under static magnetic field. As strong static magnetic field give additional drive energy to the migration of grain boundary, it provides relatively more nucleation sites for DP cells. With image analysis software, the volume ratio of DP cells in the microstructure is studied with the variation of aging temperature, aging time and the strength of static magnetic field. It is proved that the growth dynamics of DP cells can be described by J-M-A equation. With the applied static magnetic field, atom diffusion in the alloy decreases, and thus the DP nucleation rate decreases with the migration speed of the reaction front (RF). However, when the applied static magnetic field strength is larger than 7.5T, DP nucleation rate and RF migration speed increase.Microstructure observation by SEM and TEM indicates that the increase of DP nucleation rate and RF migration speed results from the appearance of continuous precipitation(CP) in the grain interia. With the increase of static magnetic field strength, the nucleation and growth of CP is suppressed too, which keep the area near the reaction RF with a high solute concentration difference, and thus keep a higher drive energy for the DP reaction. Moreover, the reduced CP precipitates decrease their pinning effects on the migration of RF. TEM observation show that under static magnetic field, CP precipitates have larger density and bigger ratio between precipitates length and width, which make the aging specimen reach a bigger hardness. Yet, as static magnetic field make the DP reaction lower at the initial aging process, the hardness of the specimen aged in regular condition has a bigger value than those aged under static magnetic field.Studies on age microstructure under alternative magnetic field show that DP transformation process accelerates under alternative magnetic field. Calculation by J-M-A equation also shows that DP nucleation rate and RF migration speed both increases under alternative magnetic field. However, the value of N1/(N1+N2) is roughly the same under different applied alternative magnetic field. As the strength of the alternative magnetic field is much smaller than the static magnetic field, the effects of magnetic field induced grain boundary migration is quite small, and it has little influence on the DP nucleation mechanism.