Nickel carbonyl vapor deposition (CVD) is a novel metal-forming process used to produce thin-shell molds with a higher level of efficiency and a lower cost than traditional mold making techniques. Not only is the production process highly effective, but the deposited CVD Nickel shell is also both harder (a tensile yield strength of ~584MPa) than conventional polycrystalline Nickel (~100MPa) with a grain size of 10nm and more ductile (a failure strain of ~13%) than nanocrystalline electroformed Nickel (4-7%) with a grain size of 21-30nm. The combination of high strength and high ductility of the CVD Nickel shell is attributed to its unique bi-modal grain structure with large columnar grains embedded in a nanocrystalline matrix. A high density of nano-twins in large columnar grains significantly improve the working hardening capacity and ductility of CVD Nickel, but they are thermodynamically unstable. Most coherent twin boundaries (CTBs) transform into dislocation cells during annealing at temperature higher than 400 degree centigrade. While the driving force for detwinning is well-known, the mechanism by which CTBs disappear is still puzzling. Based on both experimental observation and simulation analysis, we found that although the driving force for detwinning is determined by the density of nano-twins, the thermal stability of each CTB in CVD Nickel is only determined by incoherent twin boundaries (ITBs) and its intrinsic grain boundary dislocations (IGBDs). Driven by the tensile strain originated from grain growth in the nanocrystalline matrix of CVD Nickel, IGBDs detach from CTB planes during detwinning. The emerging dislocations rearrange into sub-grain boundaries which then remove some CTBs during their migration. Twin lamellas terminated by ITBs transform back to the matrix stacking by the migrating ITBs which can also release sessile grain boundary dislocations into the crystal. Our results suggest that replacement of the nanocrystalline matrix in CVD Nickel with nano-twin structure can eliminate the tensile strain originated from grain growth, so nano-twins in CVD Nickel without nanocrystalline matrix might have a much higher level of thermal stability than that in CVD Nickel with bi-model grain size distribution.