Phase formation and properties of vanadium-modified Ni-Cr-B-Si-C laser-deposited coatings

<p>A Ni-Cr-B-Si-C alloy powder was modified by addition of 2 and 5 wt% of vanadium to tackle the high cracking sensitivity of the original composition during laser deposition. The effects of vanadium on microstructure and phases were investigated by Scanning Electron Microscopy, Energy Dispersive Spectroscopy, and Transmission Electron Microscopy (TEM) and the changes in the hardness and cracking tendency of the deposits were evaluated. In comparison to the original composition, V-modified alloys produced deposits with lower hardness and moderately reduced cracking tendencies. Addition of vanadium transformed the nature and the morphology of the boride precipitates and added VC particles to the microstructure but did not induce a significant microstructural refinement. TEM characterizations confirmed that borides phases in the modified deposits consisted of alternating layers of CrB and (Cr1-xVx)B but the VC existed as independent particles which were formed on the boride precipitates. The final phase constitution of the modified alloys was dramatically influenced by the complete solid solubility between CrB and VB and the lack of solubility between Cr7C3 and VC. Addition of vanadium did not provide the phases which could act as nucleation sites to refine the microstructure of the deposits because VB had a tendency to dissolve in CrB and VC was formed at low temperatures on the boride phases. The outcomes of this study can be used to evaluate the effects of adding early transition metals such as vanadium on the microstructure and phase formations of the Ni-Cr-B-Si-C alloys.</p>

Advances in Laser Surface Engineering:Tackling the Cracking Problem in Laser-Deposited Ni-Cr-B-Si-C Alloys

<p>Laser-deposition technologies are being increasingly used for surface modification and three-dimensional manufacturing applications. The biggest technical obstacle to a wider usage of these technologies especially for deposition of hard alloys is cracking of the deposited samples. In this work, the idea of microstructural refinement as a toughening mechanism for Ni-Cr-B-Si-C alloys deposited by laser cladding is evaluated and a new idea for reducing the cracking tendency of these alloys is proposed. The results show that although a significant refinement of the Cr-rich precipitates in these alloys could be induced by a suitable addition of Nb, the cracking susceptibility of the deposits was unchanged. This was so because the continuous network of hard eutectics was still providing an easy route for crack growth. The outcome of this work shows that an effective toughening mechanism for these alloys should include not only a refinement of the hard precipitates but also modification of the eutectic structure.</p>