Research On The Preparation And Mechanical Properties Of Multi-level Nanostructured Steel

One of the main motivations for the synthesis of nanocrystalline(NC)and ultrafine-grained(UFG)materials originates from the expectation to attain high strength and high ductility simultaneously.Unfortunately these properties are generally mutually exclusive in such materials.Single-hierarchical NC and UFG materials usually exhibit a rather low tensile ductility,in particular uniform elongation,and this places a severe limitation on their practical application.Recently,numerous efforts have been paid to improve the low ductility of NC/UFG metallic materials by transforming their single-hierarchy structures into the multi-hierarchy,multi-phase and/or multi-scale forms.Accordingly,several effective strategies that are available to achieve both high strength and substantial ductility in the resultant structures have been reported and hence,attracted a rapidly growing research interest.However,these promising strategies have so far not been available to steels because some difficulties still exist in controlling their microstructures at the submicron-or even nano-scale.Drawing inspiration from these strategies,two typical multi-hierarchy and multi-scale NC/UFG steels were synthesized and investigated in this stuty.An age-hardenable martensitic steel and a single-phase ferritic stainless steel were respectively chosen as the experimental materials according to their own specific microstructural features.Meanwhile,equal-channel angle pressing(ECAP)technique was employed to prepare the NC martensite matrix of the former and UFG ferrite matrix of the latter respectively,and then the proper heat treatment was conducted to produce their own multi-hierarchy and multi-scale structural architectures,respectively.Microstructural evolution and formation mechanisms,microstructural control factors,corresponding mechanical properties and the associated strengthening/toughening mechanisms of the resultant multiscale hierarchical NC/UFG microstructures in the tow kinds of experimental steels were meticulously analyzed and systematically discussed.The primary results of this study are summarized as follows:(1)An overall lamellar-type nanostructure was successfully obtained in maraging steel subjected to equal-channel angular pressing(ECAP)for more than 8 passes at room temperature.Microstructural observations indicated that the microstructural evolution of lath martensite was always characterized by the lamellar morphology during ECAP processing.Increasing the number of ECAP passes resulted in the average lamiellar spacing(LS)decreasing while the volume fraction of high-angle lamellar boundaries(HALBs)increasing.After ECAP for 8 and even more passes,a full lamellar-type NC microstructure was completely accomplished(LS<100 nm,Vol.%HALBs>70%).ECAP deformation refines martensite lamellae dominated by the unidirectional dislocation-subdivision mechanism.i.e.,the formation of lamellar boundaries depended upon dislocation accumulation along the shear direction under the control of dominant slip system(111)a’/{110}a’.More significantly,The relationship between the yield strength(YS)of the lamellar-type nanostructured maraging steel and lamellar spacing(d)can be correlated by a Hall-Petch type model,i.e.YS = 507 + 230·d-1/2,implying the lamellar spacing(the generalized lath width)is the primary microstructural unit controlling the yield stress of maraging steel.(2)Subsequent aging treatment provided a multi-hierarchy NC structure in the multi-pass(>8 passes)ECAPed maraging steel where the dense,nano-rodded precipitates dispersedly emerged inside the interior of NC martensite lamellae.TEM and electron diffraction examinations showed that the aging structure was in fact a three-dimensional truss structure in which the nano-rodded δ-Ni3Mo precipitates was parallel to the different orientations of the matrix.A close examination of lamellar interior showed that the δ-Ni3Mo precipitates preferentially nucleated at dislocations,suggesting the high dislocation density promotes the diffusion kinetics and the distribution of precipitates significantly.ECAP deformation had significant influence on the size of rod-shaped δ-Ni3Mo precipitates,resulting in a swift and enhanced strengthening effect during initial aging stage;whereas a decomposition of the δ-Ni3Mo precipitates was accelerated after the peak ageing time,resulting in a decrease in the strength.The tensile strength could be enhanced significantly up to 2400-2700 MPa by ECAP for 12 passes and subsequent aging treatment.The finer martensite lamellar spacing and the profuse nano-precipitates were found to be mainly responsible for the ultrahigh tensile.(3)A new thermo-mechanical approach,involving processing by severe plastic deformation and proper annealing treatment,was adopted for to produce a multi-scale UFG microstructure in steels.An solution-treated ferritic stainless steel(OCr 13)was subjected to ECAP at room temperature and subsequent annealing treatment.Microstructural observations showed that a homogeneous ultrafine-grained structure was well developed in the 4-pass ECAP sample,and the corresponding average grain size was about 349 nm.After subsequent annealing,partial recrystallization occurred and the island-like UFG areas still survived with a uniform distribution.The annealed ECAPed samples exhibited a bimodal grain size distribution including relatively coarse recrystallized grains(CRGs)and remaining ultrafine grains(UFGs).With the increase of the annealing temperature,the average grain size was increase in both of CRGs and UFGs,while the volume fraction was increase in CRGs but decrease in UFGs;With the increase of the number of ECAP passes,average grain size was decrease in CRGs but slightly increase in UFGs,while the volume fraction was increase in CRGs but decrease in UFGs.It is intended that the multi-scale UFG microstructure of 0Crl3 steel could be further optimized through manipulating the processing parameters.(4)The combined effects of severe plastic deformation and partially recrystallization on the mechanical properties of the multi-scale UFG 0Cr13 steel were systematically investigated.a significant enhancement in strength of the 0Cr13 ferritic stainless steel could be obtained by means of ECAP deformation,while the subsequent annealing led to a significant improvement in its ductility with a relatively little loss in strength.it is generally believed that the presence of CRGs suppress crack propagation and provide the strain hardening required for a combination of superior ductility and good toughness whereas the UFGs provide high strength,and hence the comprehensive mechanical properties are enhanced in the multi-scale UFG 0Cr13 sample.The finer structures and higher work-hardening capacity were responsible for the improved mechanical properties of the multi-scale UFG ferritic stainless steel.