Effects Of Aging Treatment On Properties Of A Training-free Fe-Mn-Si Based Shape Memory Alloy

The FeMnSi-based shape memory alloy (SMA) as a new generation of intelligentengineering materials, has attracted worldwide attention due primarily to theoutstanding processability, excellent weldability，high strength, and high plasticity.Although considerable progresses have been made in the processing technique andproperties of SMA, the engineering application remained limited by the poorshape-memory effect (SME). Aiming at precipitating VN particles in the matrix of thealloy during the process of aging treatment, a training-free FeMnSiCrNi alloycontaining a small amount of V and N was designed. The effects of different agingtime on the shape-memory effect, wear resistance, and corrosion property weresystematically investigated, therefore it is meaningful to determine an optimal agingtime for producing an alloy with the conditions of most favored SME and wearresistance. The phase composition and microstructure, especially the type anddistribution of the precipitates, were detected by employing X-ray diffraction (XRD),scanning electron microscopy (SEM), and transmission electron microscopy (TEM)respectively.The results from XRD, SEM and TEM indicated that a mass of fine VN particleshomogeneously precipitated in the matrix, while a small quantity of VN grainsaggregated as an extremely long strip along the grain boundaries. With the increasingaging time, VN particles continuously clustered and grew up. However, the amount ofparticles wasn’t always increased, but almost saturated when aged for16h. Whenaged for4h and8h, the microhardness of the alloy could reach350MPa, whichimproved by40%over that of solution treatment specimen (250MPa). The alloy agedfor8h showed the most preferable shape memory property, and the recovery ratiocould reach75%(4%deformation at0℃and heated up to300℃), in which agingtreatment for8h may introduce more appropriate nucleation sites for stress-inducedmartensite and strengthen the matrix optimally.The influence of aging time on the wear resistance of the V+N bearing alloy wasinvestigated for the first time. The results showed that the aging treatment for8h givesthe best wear-resisting property, owing to the optimal strengthening effect of fine and uniformly distributed VN particles in the matrix. Moreover, the existence of these VNparticles changed the wear mechanism of the alloy. Adhesive wear was the primaryfactor for the poor wear-resisting property of solution treatment alloy under dryfriction. However, the adhesive wear was effectively restrained after aging treatment,especially aging for8h, since the matrix has been strengthened by the second-hardphase particles. As for oil friction, adhesive wear only occurred in non-agedspecimen.The influence of aging time on corrosion resistance of the alloy was investigatedfor the first time as well. The results of electrochemical potentiodynamic polarizationand electrochemical impedance spectroscopy (EIS) indicated that VN particlesprecipitating from the matrix worsened the corrosion resistance of the alloy. Non-agedspecimen showed the best corrosion resistance compared with the aged specimens.Fine VN particles as micro-cathode, together with austenite matrix around VNparticles as micro-anode, formed numerous micro-battery, leading to the dissolutionof matrix. In addition, V and N elements could not access to V-and N-poor area bylong-range diffusion during short time aging treatment, therefore resulted in thenonuniform distribution of the matrix composition, and the worst corrosion resistancewhen aged for8h and16h.