| 其他摘要 | Surface mechanical attrition treatment (SMAT) can promote the global properties of metallic materials by generating a nanostructured surface layer. Meanwhile this technique provides with ideal specimens to study the underlying mechanism of grain refinement induced by plastic straining. Since most engineering materials are multi-phase alloys, understanding of their intrinsic grain refinement mechanism and chemical compositional changes induced by SMAT is crucial. Low plasticity makes nanocrystalline (nc) materials prone to fracture during wear, and it is helpful for industrial applications of the nc materials to gain an optimized combination of strength and plasticity through proper heat treatments.
In this work, an AISI 52100 steel containing spheroidal cementite particles dispersed in a ferrite matrix was subjected to the SMAT followed by annealing. Strain-induced microstructure evolution, including grain refinement of both phases as well as decomposition of cementite, was systematically investigated by using optical microscope (OM), X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM). The thermal stability of the top surface layer was examined by using XRD and TEM. The relationship between hardness, plasticity and wear resistance of the top surface layer was analyzed by using Vickers hardness tester and ball-on-disk tribometer. The main results are followed:
1. A nanostructured surface layer with thickness of about 60 m was fabricated on the AISI 52100 steel plate by means of SMAT. The average grain size at a depth of 5 m is about 8 nm. Owing to the gradient variation of microstructures, the SMAT surface layer can be subdivided into three sections along the depth, i.e., nanostructured regime, ultrafine-grained regime and deformed regime.
2. The strain-induced grain refinement process manifests the following characteristics:
(i) Dense dislocation walls (DDWs) and dislocation tangles (DTs) were formed between cementite particles subdividing ferrite grains into refined blocks.
(ii) Accumulated DDWs and DTs evolved gradually into subboundaries and highly misoriented grain boundaries (GBs) under further strain, leading to the continuous grain refinement of the ferrite matrix.
(iii) When ferrite grains were refined to sizes smaller than that of cementite particles, plastic deformation occurred in cementite, characterized by dislocation gliding along the {100} and {110} planes. Dislocation gliding in cementite is initiated at intersections of the cementite/ferrite(/) interfaces and GBs or DDWs in ferrite. Accumulated multiple gliding progressively refined cementite into nano-sized cementite particles mixed within ferrite matrix upon straining. Due to a large strain with a high strain rate as well as the multidirectional repetitive loading, equiaxed nanocrystallites with random crystallographic orientations were formed.
(iv) Compared with the strain induced grain refinement in the pure Fe under the same SMAT processing, the refinement process of ferrite in AISI 52100 steel was much facilitated by the presence of dispersed cementite particles, as the / interfaces are effective nucleation sites for dislocations and also barriers for dislocation motions. Meanwhile the increase in interface area due to the fragmentation of cementite further enhanced the nanocrystallization of ferrite.
3. The cementite phase was chemically decomposed in the surface layer of the SMAT steel plate. The result of structural and spatial compositional analyses revealed the intrinsic correlation between dislocation slip and cementite dissolution. During SMAT, slip dislocation crossing cementite resulted in carbon depletion in slip bands and the dissolved carbon atoms may prefer to relocate themselves in the ferrite dislocation cores and/or grain boundaries near the slip steps. With accumulated slips, cementite might be decomposed along the slip bands in a spatially discrete manner. That is to say, dislocation slip is the main cause of cementite decomposition.
4. Finite element analysis showed that a tension stress exists below the step with the size of tension stress field proportion to the step length. It implies that larger tension stress field may favor accommodation of more ferrite dislocations and GBs below the step when more slip dislocations (in cementite) arrive at the interface, which might induce more carbon atoms trapped there. All of these are in a good agreement with the measured results of C-map that the size of carbon atmospheres is larger for larger steps.
5. The nanostructured surface layer of AISI 52100 steel has high thermal stability. The grain coarsening of ferrite is less pronounced with a slight increment in grain sizes when the annealing temperature is below 973 K.
6. The nanostructured surface layer of AISI 52100 steel exhibits a wear resistance comparable to that of the original coarse-grained (CG) steel. After annealing, the wear resistance of the nanostructured steel sample increases when the ferrite grain size (D) increases from 8 nm to 32nm, and decreases with a further increase in D. The highest wear resistance in the sample with D=32 nm, which corresponds to the most significant material transfer on the mating ball, originates from the optimized combination of high hardness and moderate plasticity. The wear volume of the sample with D=32 nm is about 1/4 of that of the CG sample at a load of 70N |
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