| Rare Earth Giant Magnetostrictive Material (RE-GMM) is the most important one of magnetic functional materials which used for producing high-precision servo valve, underwater acoustic transducer, precision stopper and the like devices, it is widely used in military and civilian areas for its larger magnetostriction constant, faster response and better precision than previously possible. But RE-GMM has poor mechanical properties and chemical stability. In the service process, failure or even pulverization has occurred at times, which seriously affects the service reliability of the material and hampers its promotion and application. In this paper, effects of nitriding modification on surface phase structure, morphology, surface mechanical properties, corrosion resistance and magnetostriction of RE-GMM with an typical composition of Tb0.3Dy0.7Fe1.95 were investigated by nitrogen ion implantation.The original surface REFe2 phase breaks down and transformed into REN, a-Fe and Fe8N for a saturated solid solubility at an implantation dose level of 1.0×1017ion·cm-2. When it achieves a higher level, the Fe-N compounds may appear. Nitrogen atom is implanted at a maximum depth of 650nm and the valid depth of nitride layer is 400nm when accelerating voltage achieves 140kV. The maximum of case hardness in nitride layer with a valid depth of 400nm increased by 6GPa and its average increased by 4.5GPa than non-nitrided sample when the implantation dose achieves a higher level of 5.0×1017 ion·cm-2 and accelerating voltage achieves 140kV. In the same conditions, the maximum of modulus in nitride layer increased by 70GPa and its average increased by 27GPa than before. Another important discovery is that the new phases have better corrosion resistance in acidic and chloridion environment, case hardness and modulus than original surface REFe2 phase for the effects of strong ion beam, solution strengthening and the generation of new phases.The corrosion resistance properties of RE-GMM are researched by tested polarization curves in different solutions with different pH value and Cl- ion concentration. We find that the nitrided RE-GMM has higher corrosion potentialφcorr than non-nitrided ones in the same acidic and chloridion environment, and there are some passivations in their polarization curves. The corrosion resistance properties of RE-GMM before and after nitrided are tested in different pH value solutions(20℃,3.5wt%NaCl), and the nitriding technological condition is 5.0×1017 ion·cm-2 and 140kV. The results are as follows, pH=4,△φcorr=0.23413V; pH=7,△φcorr=0.18992V; pH=10,△φcorr=0.01268V. When they are in different Cl- ion concentration solutions (20℃, pH=7), the results are as follows, 2.0wt%NaCl,△φcorr=0.23300V; 3.5wt%NaCl,△φcorr=0.18064V; 5.0wt%NaCl,△φcorr=0.20196V. The results indicate that the corrosion resistance of nitrided RE-GMM in acidic and chloridion environment is better than non-nitrided ones, but it has no significant increase in strong alkaline environment comparatively.Nitrogen ion implantation has nearly no effect on the magnetostriction property of RE-GMM. As nitrided layer on the sample is very thin, and operating temperature is very low, it is impossible for ion implantation to change the grain, microstructure, and preferred orientation of inner the material. The magnetostriction property of RE-GMM is still better after nitriding for this reason. The results prove that nitrogen ion implantation is an effective method for surface modification of RE-GMM.
|
PDF Full Text Request