Thermoelastic Martensitic Transformation Of The Aged Fe-Ni-Co-Al-Ta-B Alloys

Both the shape memory effect(SME) and superelasticity(SE) result from thermoelastic martenstic transformation in shape memory alloys(SMAs) such as Ni-Ti and Cu-based SMAs. Thermoelastic martensitic transformation is generally characterized by small hysteresis(defined as Th = Af- Ms, Ms and Af are martensitic start and reverse transformation finish temperatures, respectively) and a movable A/M interface, namely, austenite and martensite interface. However, most of the ferrous alloys, such as Fe-Ni-C, Fe-Ni-Co-Al and Fe-Mn-Al, do not show shape memory effect and superelasticity owing to their non-thermoelastic martenstic transformation. To date, some kinds of heat treatments, including ausforming, austenite ordering and ausaging, can effectively modify those alloys with a FCC�'BCT martensitic transformation as shape memory alloys. Those alloys include Fe-Ni-C, Fe-Pt, Fe-Ni-Co-Ti, Fe-Ni-Nb and so on. Most recently, ordered coherent precipitates introduced by ausaging were successfully employed to tailor the Fe-Ni-Co-Al-X(X=Ta, Nb, Ti) alloys as SMAs with large superelasticity. Especially, the Fe-Ni-Co-Al-Ta-B shape memory alloy with a huge superelasticity of more than 13%, large tensile strength(more than 1GPa), large damping capacity and reversible magnetic strain. In order to realize the superelasticity in polycrystalline Fe-Ni-Co-Al-Ta-B, a certain {035} <100> recrystallization texture should be formed after large cold deformation followed by solution and aging heat treatments.In fact, aging heat treatment dominates the thermoelastic nature of martensite in Fe-Ni-Co-Al-Ta-B alloys. The solution treated and longer time aged alloys show nonthermoelastic martensitic transformation and hence no shape memory effect and superelasticity. Only the properly aged alloy shows thermoelastic martensitic transformation. Thus, the microstructure evolution during aging treatment should be confirmed firstly in order to reveal the origin of thermoelastic martensitic transformation in Fe-Ni-Co-Al-based shape memory alloys. In addition, as the samples aged for longer time, the transformation thermal hysteresis increases. The reversible microstructure evolution should be revealed in the aged samples after martensitic transformation. Finally, the origin of martensitic transformation was analyzed in the viewpoint of coherency of precipitate with matrix.The martensitic transformations were characterized by electrical resistivity measurements. The electrical resistivity remarkably changed accompanying martensitic and reverse transformations during cooling and heating, respectively. The precipitate composition and morphology were analyzed by three-dimensional atom probe microscopy(3DAP). The microstructure evolution was observed by transmission electron microscope(TEM). The coherency of precipitate was characterized by highangle annular dark-field scanning transmission electron microscopy(HAADF-STEM).Our conclusions are listed as follows:1) The microstructure and composition evolution of the aged Fe-Ni-Co-Al-Ta-B alloy have been confirmed. The solution-treated alloy is composed of only γ matrix(A1) with grain size of 300~500μm. The aged alloys consist of γ matrix(A1), γ’ precipitate(L12) and β precipitate(B2). The main composition of γ’ precipitate Ni, Al and Ta, while the γ matrix is comprised of Fe and Co. 2) The martensitic transformation of Fe-Ni-Co-Al-Ta-B alloy is γ(FCC) �' α’(BCC/BCT). It could change from thermoelastic to non-thermoelastic when the samples were aged. Firstly, both the solution-treated and short time aged alloys do not show martensitic transformation owing to the high volume fraction of quenched-defects, such as vacancy and dislocation. Secondly, the alloys have a thermoelastic martensitic transformation when they were aged at 600℃ for ~72h-~240h or at 700℃ for ~5h- ~48h. Thirdly, as the samples were aged for longer time, the martensitic transformation is a thermoelastic one but with irreversibility resulting from dislocations and stacking faults. Lastly, when the samples were aged at 700 ℃ for t > 48 h, the alloy shows a non-thermoelastic martensitic transformation, and no reverse transformation was observed during heating to higher than 500℃. 3) At the early aging stage, the growth behavior of γ’ precipitate could be described as Ostwald ripening function, namely, 3 30r-r(28)k ×t, where r and r0 are precipitate size(nm) and Ostwald ripening starting size(nm), respectively, while k is a constant depending on the aging temperature and t is aging time(h). As aged at 600℃, it is 3 3r-3.9(28)0.0005t(72h < t < 240h), while aged at 700, it is 3 3r-9.1(28)0.06t(5h< t < 48h). 4) The irreversible microstructure, after the reverse transformation, changes from dislocations and stacking fault, twinning martensite, lath martensite with twinning martensite as substructure to lath martensite in the aged samples when the aging time increases, accompanying the thermal hysteresis increasing. As the alloy samples showing a fully recovered microstructure in the electrical resistivity ρ(T) curves, only dislocations are found at the austenite/martensite(A/M) interface. As the irreversibility is shown in the ρ(T) curves, abundant of stacking faults lies in the matrix. Moreover, as the non-thermoelastic characters are observed in the ρ(T) curves, the stable martensite remains in the matrix even heating to higher temperature(> 500 °C). 5) The irreversibility is attributed to the interaction between precipitate and matrix, and is dependent on the amount of transformed martensite or the proceeding of martensitic transformation. There is no irreversibility in the samples, as martensitic transformation is not completely proceeding. The irreversibility increases with martensitic transformation proceeding owing to the hamper effects of γ’ precipitates with large size. 6) One of the necessary conditions of thermoelastic martensitic transformation in Fe Ni Co Al Ta B alloy is the coherency of precipitates with both austenite and martensite. The elastic strain originating from the crystal misfit forces the precipitate to shear with austenite to martensite and play a vital role on reverse transformation.