Nanocrystalline Alloys and Nano-grain Size Stability

1. File the high purity low oxygen content bulk materials (Fe, Cr and Hf targets) in the glove box using a reciprocating mechanical filing machine in order to minimize oxygen contamination in the starting powders.
2. Load the powder mixture for a specific alloy (Fe14Cr4Hf wt.% in this study) into a stainless-steel vial along with 440C stainless steel milling balls (Fig. 1). The diameters of the milling balls are 6.4 and 7.9 mm and the ball powder-to-weight ratio is 10:1. The sealed vial needs to be kept under protective atmosphere in the glove box.
3. Carry out high energy ball milling for 20 hours using SPEX 8000M high energy ball mills (Fig. 2).
4. Anneal the ball milled Fe14Cr4Hf for 60 min at temperatures between 500°C and 1200°C, at steps of 100°C.
5. Measure the nanograin size, using X-ray diffractometer and the Scherrer equation. Analyses should be done for as-milled and annealed samples. The grain size can be calculated assuming Lorentzian peak profiles for the four most intense peaks after subtracting the instrumental broadening. For this below steps should be followed: 
   • Run XRD on the heat-treated samples.
   • Measure the width of the peaks at half maximum height.
   • Put the data in the equation 1 and calculate the grain size.
   • These steps should be repeated for all temperatures.
6. Run multiple annealing treatment and X-ray analysis at each of the annealing temperatures of interest in order to establish an accurate grain size and ensure the reproducibility.
7. Employ a 5 mm die and punch with hydraulic press (3 tons) to press the powder for microscopic analysis.
8. Load sample in the Transmission Electron Microscope (TEM) to see the grain size and nanoparticles formations.
9. Compare the grain sizes, resulted from TEM microscope and X-ray diffraction with similar powder with oxygen contamination.

Figure 1: Stainless steel vial with two different sizes of balls.

Figure 2: High energy SPEX 8000M Ball milling.