The hardening mechanism of high strength Co-based alloys due to cold work was not fully understood because of the microstructural complexity and the combination of several hardening mechanisms. The present study was focused on HAVAR, Co based alloy and the correlation between the mechanical properties and the hardening mechanisms. The microstructure investigation included XRD, SEM, TEM and the mechanical properties were determined by tensile tests. TEM images and electron diffraction observations showed that the annealed microstructure was dramatically changed to faulted structure due to Cold Rolled (CR) process. It was found that CR microstructure contains very high dislocation density, sub-grains, twins, two coexisting crystallographic phases: Matrix, () FCC and () HCP phase, stacking faults and cubic M23C6 carbides. The CR state has typical microstructure contained alternating wide parallel bands each band is a different phase and . The cold rolled process forced some grains in the matrix to create the hexagonal phase by Strain Induced Transformation (SIT) mechanism. These bands are deformation bands and their width is between 100-500 nm. The bands have a uniform internal texture while the bands contain very fine parallel striations have width less than 1 nm. HRTEM revealed that these striations are intrinsic stacking faults. The overall observations lead us to conclude that variety of planar defects and their high concentration due to cold work is the primary hardening mechanisms. Therefore, it can explain the increased of yield and ultimate tensile stresses by more than four times.
|State||Published - 26 Feb 2017|