Investigation On Microstructural Parameters, Thermal Stability And Reversal Martensitic Transformation Of Nanocrystalline Metallic Materials

Surface mechanical attrition treatment (SMAT), as a new technique producing nanocrystalline materials by severe plastic deformation, has been used to produce nanocrystalline surface layer in a variety of metallic materials, aiming at acquiring the properties and behaviors different from their coarse-grained counterparts. By means of SMAT, the different nanocrystalline surface layers of Fe-30wt.%Ni alloy(fcc), pure Ni(fcc), Fe(bcc) and Co(hcp) metals are obtained. By using optical microscopy (OM), X-ray diffraction (XRD), transmission electron microscopy (TEM) attached energy dispersive spectrometer (EDS), scanning electron microscopy (SEM) attached EDS and differential scanning calorimetry (DSC), the microstructural parameters, thermal stability and reversal martensitic transformation of nanocrystalline materials are investigated, which provides the theoretical instruction for the application of nanocrystalline materials produced by SMAT and enriches the research on reversal martensitic transformation of nanocrystalline materials.During SMAT, severe plastic deformation occurs in the surface layer of Fe-30wt.%Ni alloy, pure Ni, Fe and Co metals. The grain size and microstrain present the gradient distribution with the increase of the depth. Nanocrystalline surface layers are determined as 10～20μm depth. Based on the TEM observation, the average grain sizes in the top layer decrease to less than 10nm. By means of XRD single-peak fourier analysis, the microstructural parameters of Fe-30wt.%Ni alloy, pure Ni and Fe metals are quantitatively measured. The results show that the grain size drops rapidly into the nanometer scale, and high values of root mean square (r.m.s.) microstrain, dislocation density and stored elastic energy are gained after SMAT. For example, the orders of magnitude for dislocation density and stored elastic energy are as high as 1015-1016m-2 and 106-107 Jm-3, which both exceed by one order of magnitude value in the tensile-deformed counterparts. The high resolution TEM (HRTEM) images of nanocrystalline Fe-30wt.%Ni alloy show that nanosized grain consists of some subgrains separated by low-angle grain boundary, and a large number of dislocations distribute at grain and subgrain boundaries, while few dislocations distribute in the subgrain interior, which makes grain and subgrain boundaries be in high-energy and non-equilibrium state.The thermal stability of nanocrystalline Fe-30wt.%Ni alloy and pure Ni are studied by annealed at different temperatures for different times. By using the the single line approximation analysis, the grain size and microstrain of different samples are determined. The results show that two kinds of nanocrystalline materials have common features of grain growth as follows. The grain size increases rapidly within the early stage of annealing (～15min), while it becomes slow during sequent annealing time (15min～120min). The higher the annealing temperature is, the faster grains grow at the early stage of annealing. The microstrain decreases rapidly within the first 15nm of annealing and decreases slowly during sequent annealing time. The higher the annealing temperature is, the lower the value of microstrain drops. Through the in-situ TEM observation, the incorporation of subgrains may be the main reason for the initially rapid grain growth. By the measurement of grain growth kinetics parameters of nanocrystalline Fe-30wt%Ni alloy and pure Ni, the value of time exponent, n, of Fe-30wt%Ni alloy and pure Ni are 0.1 and 0.14, respectively, indicating that the grain growth rate of Fe-30wt%Ni alloy is slower than that of pure Ni. When annealed in the low temperature (pure Ni:～250℃, Fe-30wt.%Ni alloy:～350℃), the activation energy, Q, of Fe-30wt%Ni alloy and pure Ni are both 30～40kJ/mol, suggesting that the grain growth is governed by incorporation of subgrains undergoing the rearrangement of the grain boundaries. When annealed in the comparatively high temperature (pure Ni: 250℃～450℃, Fe-30wt.%Ni alloy: 350 ℃～550℃), the activation energy, Q, of Fe-30wt%Ni alloy and pure Ni are 176.8kJ/mol and 121.3kJ/mol, suggesting that the grain growth is governed by the grain boundary diffusion. Abnormal grain growth are both observed during annealing of Fe-30wt%Ni alloy and pure Ni, which can be attributed to the non-uniformity of the grain size distribution.Based on the experimental and theoretical analysis on reversal martensitic transformation of nanocrystalline materials reported in the literatures, some errors are corrected in the explanation of the reversal martensitic transformation of nanocrystalline Fe-30wt.%Ni alloy and Co previously suggested by our research group. Considering the difference of surface energies of martensite and austenite, a thermodynamic expression of the reversal martensitic transformation in nanocrystalline materials is established and is used for SMAT nanocrystalline Fe-30wt.%Ni alloy and Co by the addition of the store energy term. The theoretical calculation and analysis predict that the start temperatures of reversal martensitic transformation, As, of nanocrystalline Fe-30wt.%Ni alloy and Co are both lower than or close to those of their coarse-grained counterparts. The experimental results from DSC show that As of nanocrystalline Fe-30wt.%Ni alloy is higher than that of conventional coarse-grained alloy, while As of nanocrystalline Co metal is lower than that of coarse-grained Co, which are consistent with previous experimental results in our group. However, the experimental result of Fe-30wt.%Ni alloy is contrary to the prediction from the thermodynamic expression in this paper. The theoretical analysis suggests that the chemical free energy change resulting from the composition deviation during SMAT may be responsible for the increase of As. The further experimental results show that alloying elements do diffuse from steel balls into the Fe-30wt.%Ni alloy and Co metal during SMAT, leading to the composition deviation of surface layer from their original compositions, in turn resulting in the different As from the conventional coarse-grained samples. By removing the surface layer of nanocrystalline Fe-30wt.%Ni alloy and Co with 5μm thickness, the effect of diffusion of alloying elements on As are eliminated. The DSC results show that As temperatures of nanocrystalline Fe-30wt.%Ni alloy and Co are very close to those of their coarse-grained samples, which agree with the predicted results from the thermodynamic expressions.The diffusion of alloying elements can occur between steel balls and the treated sample during SMAT. The diffusion of alloying elements leads to the remarkably different saturation magnetization and Curie temperature of nanocrystalline Fe-30wt.%Ni alloy and Co from their conventional coarse-grained counterparts, suggesting that SMAT is not only a surface nanocrystallization technology, but also an effective alloying method for surface modification and thus has potential application in practice.