Search results
Show moreThe focus of this research was to determine a milling procedure to help facilitate the Cr2Nb transformation. The metric to evaluate a facilitated phase transformation was a reduction in endothermic onset temperature during Differential Scanning Calorimetry (DSC). The transformation of the Cr+Nb powder to Cr2Nb alloy is diffusion dependent, therefore by creating more intimate contact the elements will be closer together. A reduction in spatial distance for a diffusion-based reaction will lead to a reduction in total time for the reaction to complete. The design of a multi-stage milling process allows the refinement of the powder particle size so that diffusion can more easily occur at a lower temperature. The effectiveness in powder size reduction was tested for two bearing sizes to determine the required milling time for each stage. It was determined that dry milling elemental Cr powder for 5 minutes with 4.88mm bearings and then wet milling for 3 minutes with 2.47mm bearings produced a size distribution with a D90 less than 25𝜇m. After determining this 2-step milling procedure, a mixture of Cr+Nb powder was milled with the 2-step process and compared with Cr+Nb powder that had been milled with a 1-step process. X-Ray Diffraction (XRD) was done on the mixtures before DSC and the results showed the presence of elemental Cr and Nb. These two mixtures were then exposed to a controlled heating rate of 20 degrees C per minute up to 1400C in a Netzsch Pegasus 404 F1 DSC and then cooled. Post-DSC, the XRD results showed a significant increase in the presence of Cr2Nb alloy. Therefore, the endothermic reaction visible in the DSC results must have been the phase transformation from elemental Cr and Nb to Cr2Nb alloy. In comparing the 1-step and 2-step milling processes, the DSC results show that the transition temperature of the Cr+Nb powder was reduced from 1229C to 1089C when the milling was changed from 1-step to 2-step. Future research will investigate the effects of such milling on the in-situ formation of Cr2Nbduring additive manufacturing processes.
Show less