Crystallographic aspects of deformation microstructures in metals

Abstract

In this thesis, the deformation structures that form in 10-30% channel-die plane strain compressed Gossoriented Ni single crystals and 50-75% cold-rolled polycrystalline interstitial free (IF) steel were characterized by two and three dimensional electron backscatter diffraction (2D- and 3D-EBSD) and transmission electron microscopy (TEM). Emphasis was given to understand the evolution sequence of microbands via a detailed characterization of dislocation structures and crystallographic aspects of their boundaries. In the IF steel, both microband structures and the deformation morphology near grain boundaries were studied. Microbands are well-known deformation features that form in the deformation microstructures of many medium to high stacking fault energy metals and alloys. The crystallographic nature of microband boundaries was investigated in a 30% deformed Ni crystal using the foregoing analysis techniques. When viewed in the three orthogonal sections, microband boundary traces were classically aligned in the normal direction (ND)rolling direction (RD) section at an acute angle from the RD, but appear wavy in the RD-transverse direction (TD) section. The latter observation may lead to the conclusion that microband boundaries are non-crystallographic. The use of 3D-EBSD to reconstruct actual microbands in a volume of deformed material revealed significant new information about their morphology. Here, microband surfaces are largely planar over large distances, but frequently interrupted by local distortions and undulations due to interactions between intersecting, non-coplanar microbands. The overall investigation has revealed that microband boundaries are aligned close to active {111} slip planes (i.e. they are crystallographic) but the bumps and distortions they contain are noncrystallographic in the sense that they deviate away from these slip planes. The non-crystallographic features of microbands (as revealed by their wavy structure in the RD-TD section) may be explained by the crystallographic oscillations of up to ±7.5° towards RD that occur during deformation. Such oscillations result in varying fractions of slip on a given {111} plane, thereby resulting in varying degrees of interaction between the two sets of non-coplanar microbands. These local and intense microband interactions result in the deviation away from their active slip planes. The dislocation structures associated with early microband evolution in a 10% deformed Ni crystal was investigated by 2D-EBSD and TEM. A discrepancy was found between EBSD and TEM measurements in the included angle between two intersecting dense dislocation walls (DDWs), i.e. the EBSD and TEMmeasured angles were 90° and 70°, respectively. After 10% strain, the spacing between DDWs are larger (average >5 micron) than the spacing between microband boundaries (average ~1micron) after 30% strain. The dilocation structures between two DDWs comprise both mash and cell structures. In the former, it is possible to identify individual dislocations whereas, in the latter, dislocations obtain a denser distribution within ~150 nm thick boundaries and their interiors contain very few dislocations. The cells are initially equiaxed but eventually their boundaries orient along the existing DDWs. Subsequently, many such cell boundary segments constitute denser segments of dislocation walls, from which microbands form by splitting into two DDWs at ~1micron spacing. Microbands also form in series on one set of DDWs parallel to a second set of DDWs. These boundaries then propagate into two walls to generate microband channels. Once a DDW forms it may propagate into dislocationfree regions trailing behind a microband channel through splitting events. Microband boundaries are generally regarded as low-angle deformation features that accommodate a small crystallographic rotation in the range 1-4 degree. In this part of the thesis, EBSD/TEM revealed unusual orientation differences of up to 20 degree across microband boundaries in {111}<110>oriented grains in a 50% cold rolled IF steel. Despite their high angle nature, the microband interfaces maintain their wellknown crystallographic characteristics, that is, they are aligned closely with highly stressed slip planes. Rigid body rotations are argued to take place around the interface normal between adjacent microband lattices, thereby generating the unusual microstructure containing alternating microband boundaries that are oriented in equal and opposite relative angles and produce an array of orientation pairs in their spatial distributions. Grain boundaries play an important role in the formation of deformation and recrystallization textures in steels. In this second study using 3D-EBSD, microtexture distributions across a complex boundary between ~{111}<112> and ~{111}<110>oriented grains in a 75% cold rolled IF steel were studied. The coexistence of several deformation features both near to and on the boundaries, such as steep orientation gradients up to 5 degree/micron, high angle boundary networks and numerous thin, elongated blocks on their surface, reveal the complex, irregular and diffuse nature of grain boundaries and why they are excellent sites for nucleation of recrystallization.