Nitrogen-microalloying and partial substitution of Cr for Mn have been employed to enhance the shape memory effect and corrosion resistance of Fe-Mn-Si based alloys. Typically, the tested alloys with nominal composition Fe-25Mn-6Si-5Cr-(0.12-0.14)N in mass% exhibit perfect shape recovery for a 3% pre-strain after only one cycle of thermomechanical training. The related mechanism has been discussed, taking account of the effect of nitrogen on the stacking fault energy (SFE) or the stacking fault probability (P sf ) of the alloy and the strengthening of the austenite matrix. Thermodynamic calculation and P sf measurement showed that the SFE increases with increasing N-content in the concentration range investigated, e.g. less than 0.3 mass%. Thus, the critical stress for the formation of stress-induced martensite increases with N-content. It is believed that the interstitial strengthening of the matrix by nitrogen predominantly contributes to the improvement of shape memory effect. Besides, nitrogen-microalloying remarkably improves the corrosion resistance of the alloys in aqueous solutions containing NaOH and NaCl, but not in HCl solution as indicated by the long-term immersion tests.
A universal theoretical method has been developed in this article to predict and characterize electron diffraction patterns (EDPs) which contain various variants of precipitate(s) with a matrix. The plane and direction transition matrices for three ␥ Љ-phase variants and 12 ␦-phase variants precipitated from a ␥ matrix in the INCONEL 718 alloy were deduced, from which the EDPs for seven low-index zones of ␥ matrix containing ␥ Љ precipitates were predicted by plotting and were found to be consistent with transmission electron microscopy (TEM) observations, showing that some of the results reported by Quist et al. should be corrected. Meanwhile, three variants of ␦ phase, precipitated from any one of four {111} matrices in 12 possible orientational variants, were also predicted and confirmed by EDPs. Different from Paulonis' conclusion, our theoretical calculations indicated that the {1/2 1 0}-type superlattice reflections in the ͗100͘ zone of the ␥ matrix permitted detection of both ␥ Љ-and ␦-phase precipitates, but not of ␥ Љ-phase precipitates. Therefore, the precipitates shown in dark-field images using these superlattice reflections cannot be unambiguously determined to be ␥ Љ phase. A unique approach for identification of ␥ Љ precipitates in the alloy has been proposed.
Compression test results of our research on Al3Ti-base alloys are reported. It is evident that the specimen length-to-width ratio we used for compression testing can significantly reflect the difference in ductility of different alloys. Thus the tests fulfilled the aim of our present research.
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