Pazhayannur Ramanathan Subramanian

Effect of dopants on grain boundary decohesion of Ni: A first-principles study

First-principles density functional theory (DFT) calculations were used to determine decohesion properties of Σ5(012) grain boundary of Ni with dopants B, C, S, Cr, and Hf. The relative stability of sites was evaluated and cleavage energies were calculated. Electronic structure was used to understand these properties in terms of changes in bonding with addition of dopants. It was found that strengthening of the Ni grain boundary results from Hf, B, and Cr doping. In contrast, the grain boundary weakens with S and C doping. These results should be useful in the design of next-generation nanostructured Ni-based alloys with improved mechanical behavior.

Effect of nitrogen on the magnetic moment of α-Fe and FeCo alloys from first-principle calculations

The effect of nitrogen on the magnetization of α-Fe and Fe1−xCox alloys was studied within the spin-polarized density functional theory. For α-Fe, the average magnetic moment (per atom) μ increases with increasing concentration of interstitial nitrogen. The increase in moment occurs due to inter-atomic charge transfer, which increases the moment on the Fe atoms that are next-nearest neighbor to the N atom. In Fe1−xCox alloys, nitrogen suppresses the peak in μ(x) and shifts it to lower Co concentrations compared to the pure Fe-Co system. The decrease in moment in the FeCo-N system occurs due to competing inter-atomic (promoted by N) and intra-atomic (promoted by Co) charge transfer on the Fe atoms surrounding the N atom. The addition of nitrogen was not found to significantly increase the magnetization of α-Fe and Fe1−xCox alloys.

Characterizing Ultrafine Grained Material using EBSD

Ultrafine grained (UFG) alloys, those with grain sizes significantly below 1 micron, are of general interest from both a property and processing standpoint. Improved mechanical properties have been reported, and many materials are under investigation [1,2]. The nanoscale nature of these materials, however, provides a significant number of characterization challenges. Here we discuss the use of Electron Backscatter Diffraction (EBSD) to characterize the grain size of ultrafine grained Ti-6Al-4V (UFG Ti64). UFG Ti64 was produced by near-isothermal Multi-axis Forging (MAF) [3]. Samples of this material were heat treated to study the static coarsening of the alpha phase, and hot compression tests were performed on UFG Ti64specimens to examine dynamic coarsening behavior. After heat treatment or compression, samples were metallographically prepared in conductive 1.25 inch mounts according to a modified version of a published technique [4]. To understand the coarsening kinetics, it was essential to quantify the average alpha grain size of the material. A high spatial resolution and ability to examine large areas for statistical purposes were critical requirements for this task, making a FEG-SEM with EBSD the instrumentation of choice. Since large EBSD measurements are on the order of hours, even with modern camera speeds, maintaining mechanical and thermal stability of the SEM was critical. Accordingly, the FEGSEM used for UFG grain size measurements was enclosed to isolate it from thermal oscillations inherent to HVAC systems. A chiller with precise PID temperature control was installed to keep the lens cooling stable. These modifications helped keep the short-term temperature fluctuations below 0.5C. To minimize long term mechanical or thermal drift, the sample was positioned for analysis under appropriate beam conditions and then rested for a minimum of 3 hours prior to the EBSD measurements. Long term temperature drift was typically less than 1C. Figure 1 shows the band contrast for a small region of tested UFG Ti64 material. Small maps such as these were used to examine the resolution of the EBSD measurements and get a sense of the grain size. Reasonable grain size statistics, however, were accumulated through EBSD line scans along the tilt axis. For each measurement, the step size within each line was 20nm, and lines were spaced between 1 and 50 microns apart. Scan times were on the order of 14 hours. Initial results of the static coarsening experiments are shown in Figure 2, compared with conventional Ti64 material analyzed optically. The EBSD measurements show that the alpha grain size of the UFG Ti64 is reasonably stable with respect to static coarsening at 650C. It remained submicron after an hour and remained below 3 microns after 77 hours.

Development of Nanostructured and Nanoparticle Dispersion-Reinforced Metallic Systems - Progress and Challenges

Investigations on nanostructured metallic systems have shown that exceptional property enhancements are potentially achievable through structural refinement to the nano-scale. While dramatic property improvements have been reported on these materials in the past, significant technical challenges exist in processing, as well as in retention of microstructural stability and useful properties at elevated temperatures. This presentation will summarize the key challenges in developing nanostructured metallic systems, and highlight our progress and selected successes in the understanding of structure-property relationships in bulk nanostructures as well as nanostructured multilayered systems. Work in nanoparticle dispersion-strengthened metallic systems will also be highlighted in this presentation.