Investigation of selective doping method for producing stabilized grain structures in metals

Mechanical performance of a polycrystalline metal at high homologous temperature depends in part on grain structure. Grain boundary sliding is a localized plastic deformation and a mechanism for creep and rupture. Failure can occur when grains slide along boundaries transverse to the axis of an applied load. Stabilization of a structure consisting of overlapping grains of high aspect ratio is one approach to inhibiting grain boundary sliding. An overlapping grain structure is expected to offer superior resistance to creep and rupture compared with an equiaxed structure, because weak transverse grain boundaries are bridged by grains in adjacent layers. Selective doping is a method for realizing stabilized overlapping grain structures and consists of inserting discrete arrays of insoluble dopant into a matrix at regular intervals. At elevated temperature, the dopant forms arrays of bubbles of sub-micrometer size. The bubble arrays aid in the formation and stabilization of an overlapping grain structure at high temperature. Two methods of selective doping were investigated. The first approach consisted of doping the near-surface regions of molybdenum foil substrates with potassium by ion implantation and depositing molybdenum onto the doped substrates by physical vapor deposition to form three-layer specimens. The second approach consisted of doping the near-surface regions of tungsten foils with potassium by ion implantation and subsequently diffusion bonding the doped foils to form multi-layer specimens. Doped, multi-layer specimens were annealed for recrystallization and grain growth. The size and spatial distributions of the potassium bubble arrays were documented by observing bubbles on intergranular fracture surfaces using scanning electron microscopy. Results showed that both approaches are viable for producing stabilized grain structures. Selective doping produced a dispersion of potassium bubbles with radii of tens of nanometers. The bubble arrays established overlapping grain structures that remained stable at high homologous temperature. In contrast, undoped and annealed multi-layer specimens developed a bamboo-type grain structure. Bubble coarsening rates appeared consistent with those produced by bubble coalescence due to migration of bubbles caused by diffusion of matrix atoms along the bubble surface.