Development of surface mechanical attrition treatment (SMAT) and electrodeposition process for generating nanostructured materials and study of their tensile properties

Abstract

This work systematically investigates two of the most promising synthesis methods for producing nanostructured (NS) materials: surface mechanical attrition treatment (SMAT) and the electrodeposition (ED) process, and obtains the proper conditions for fabricating NS materials in bulk form and studies the properties of these materials. The development of the fabrication process for NS materials begins with the discovery of the superior mechanical properties of the materials with nano-sized grains on the laboratory scale. The unique properties of NS materials and the ways to produce them have drawn much attention from materials researchers. Recently, one of the research directions for nanotechnology has been extended to the development of the fabrication methods for generating NS materials in bulk form. The SMAT process is one of the promising synthesis methods to fabricate bulk NS materials. Exploiting the actuation of spheres at high velocity which results in high strain-rate, SMAT facilitates the formation of nano-twins which render high strength as well as high ductility. This project first examined the key parameters that influence the effect of SMAT. The effects of sizes, materials and numbers of balls on the ball velocity were evaluated using a high-speed camera. The factors affecting the ball velocity were examined by using images taken by the high-speed camera. The kinetic energy transmitted from the balls to the treated specimens was computed and hence correlated with the tensile properties of the SMATed samples treated by different numbers of balls; eventually, the relationship between the treatment efficiency and the number of balls was determined. Transmission Electron Microscopy (TEM) was adopted to examine the microstructures at different depths from the surface of the treated samples. Using the measured velocity, a theoretical model was then established to predict the strain-rate reached by a ball at different depths of the treated materials. Simulation using a finite element method (FEM) was also performed to reveal the plastic strain history of a ball on the treated materials. By associating the strain-rate obtained from calculation and plastic strains from simulation with the microstructures from TEM observations, the least requirement of the strain-rate for the formation of twins with nanoscale spacing was evaluated. Materials subjected to SMAT have a gradient structure with layers composed of nano-grains having a thickness of about ~30 - 100 {471}m. To obtain NS materials with favorable strength as well as desirable ductility, the selection of base materials is crucial. One promising method for producing matrix materials in bulk form is the ED process. It has the great potential to be applied to many different materials, and can tailor the metallurgical structure and hence the properties of the electrodeposits when the proper process conditions are chosen. In this study, electrodeposits of thickness ranging from ~160 {471}m to 1 mm were examined. In the general conditions for small-mass production scale, the effect of process parameters, including the current type, the current density and the deposition duration was studied. Tensile property tests were carried out, both in situ and in vitro, to reveal the microstructure-property relationship. Scanning Electron Microscopy (SEM) was used to observe the fracture mechanism and the morphology of the electrodeposits. Focus ion beam (FIB) and electron backscattered diffraction (EBSD) techniques were employed to exhibit the structures and grain orientations, respectively. TEM observations were carried out to statistically summarize the grain size and twins thickness distribution of the electrodeposits obtained in various durations. The relationship between the ED duration, electrodeposits thickness, and grain size and twins thickness was revealed. Texture characterization was performed by X-ray diffraction (XRD), which showed the strong relation of texture of the electrodeposits and the process duration. EBSD was further used to demonstrate the relation of the texture and the structure of the electrodeposits. An analysis from the point of view of Hall-Petch relation and strain hardening exponent showing the comparable properties of this study with the other literatures, is presented. The process conditions used in this study were applicable to produce NS materials with desirable strength, ductility and sufficient thickness. The effect of SMAT on the electrodeposits was studied. Tensile properties, microstructures and textures of the SMATed electrodeposits were examined. The results demonstrated that the NS matrix obtained by the ED process with sufficient thickness retained desirable ductility after employing SMAT technology, and the SMAT process further enhanced the strength of the electrodeposits.