Atomic-scale Studies On Dislocation Configurations In Nanocrystalline Alloys Processed By Plastic Deformation

Over the past three decades,nanocrystalline（NC）metals/alloys have attracted considerable attention due to their superior mechanical properties.The research community has been pursing the combination of high strength and ductility in this class of materials.Given the fact that microstrucuture critically affects mechanical properties,superior mechanical properties in NC metals/alloys can be achieved by tailoring microstruture.Concequently,this dissertation performed atomic-scale studies on crystal defects including full dislocations,stacking faults（SFs）,twinning,grain boundaries（GBs）,Lomer–Cottrell（L-C）locks,stacking fault（SF）dipoles,twinning-SF composite structure and 9R structure,in 7075 Al alloy with high stacking fault energy and equiatomic high-entropy alloy（HEA）CoCrFeNi with low stacking fault energy,using high-resolution transmission electron microscopy（HRTEM）.Dislocations are the most important components to identify the aforementioned crystal defects.In an effort to determine the Burgers vector of dislocations based on HRTEM micrographs,a new“matching method”was developed,which comprises two steps:first,measuring the project vector of the Burgers vector on the plane studied by drawing a Burgers circuit;second,calculating projection vectors of all the possible dislocations in FCC materials on the plane studied,and third,comparing the measured projection vector with the calculated ones to determine the Burgers vector of dislocation.The new method is more accurate and reliable than the currently used method,which determines the Burgers vector by speculation based on the measured the project vector of the Burgers vector of the dislocation.Nanocrystalline 7075 Al alloy was produced via high pressure torsion（HPT）of a solid-solution treated coarse-grained 7075 Al alloy at room temperature.In the HPT NC 7075 Al,several crystal defects including full dislocations,SFs,L-C locks,SF dipole and non-equilibrium grain boundary were observed.Unlike other NC metals/alloys,full dislocations with a density as high as 3.8×10¹⁵/m²are evident at grain interiors in the HPT NC 7075 Al alloy,as a result of pinning of full dislocation motion by SFs,L-C locks,SF dipole,as well as solute atoms.SFs enclosed by two Shockley dislocations that are formed by the dissociation of either screw or 60°full dislocations are observed,with the former’s average width being a few times of the latter’s average width.This can be attributed to the different effect of stress on the separation distance of partial dislocations in the two types of SFs,which originates from the fact that the directions of the edge components of the Burgers vector of partial dislocations from screw full dislocation are opposite,whereas those from 60°full dislocation are the same.Interestingly,a new type of SFs enclosed by a Frank dislocation and a Shockley dislocation were observed for the first time in this dissertation.The formation of this type of new SFs may be attributable to the lower interaction energy between a Frank dislocation and solute atoms than that between a Shockley dislocation and solute atoms.This results in a lower total energy in a SF enclosed by a Frank dislocation and a Shockley dislocation than that in a SF enclosed by two Shockley dislocations in the those regions with higher solute concentrations formed as a result of nonunifomity of solute distribution.Not only conventional L-C locks with a stair-rod dislocation but also new L-C locks containing multiple stair-rod dislocations were observed.In addition,SFs dipoles comprising three intersected SFs were observed.L-C locks and SF dipoles are formed by reactions between the partial dislocations at the end of the SFs.The presence of solute atoms delays partial dislocation movement towards GBs,providing sufficient time and enabling these dislocation reactions.High applied stress during HPT generates sufficiently wide SFs in the L-C locks and SF dipoles that can be partially retained by pinning of solute atoms and other dislocations to partial dislocations in the SFs.Non-equilibrium GBs that consist of full dislocations and L-C locks were also observed,and they could be formed by grain rotation during deformation process and thus a reduction in misorientation between two neighboring grains.NC single-phase FCC HEA CoCrFeNi solid solution was processed via cold rolling of cast plus hot-forged material to thickness reduction ratio of 83.7%at room temperature.The microstructure consists of nano-sized grains,lamellar nano-grains and nano-twins.At the lamellar grain interiors,SFs with a density as high as 2.2×10¹⁵/m²,and L-C locks containing a stair-rod dislocation with a density as high as 4×10¹⁴/m² and L-C locks containing multiple stair-rod dislocations with a density as high as6×10¹⁴/m²are observed.SFs are also formed by the dissociation of screw and 60°full dislocations.L-C locks are generated by reactions of Shockley partial dislocations at the ends of the SFs when these SFs in intersected slip planes meet with each other.At the nano-twin interiors,more refined nano-twins with two 3 inconherent twin boundaries（ITBs）and twin-SF composite structure are observed.On the basis of determination of Burgers vectors of those partial dislocations in the 3 ITBs,the more refined nano-twins can be considered to form by overlapping of SFs originating from dissociation of full dislocations on adjacent slip planes.A twin-SF composite structure is formed by the dissociation of a L-C lock into two Shockley dislocations and a Frank dislocation and subsequent slip of the two Shockley dislocations.In addition,9R structure is observed.9R structure is formed by sequential slip of partial dislocations in∑3 ITBs.These dislocation structures can improve dislocation storage at nano-scale grain interiors,studies on them are conducive to design NC metals/alloys with excellent mechanical properties.