Nanocrystalline Alloys and Nano-grain Size Stability

Nanocrystalline alloys are in high demand in industries such as semiconductor, biosensors and aerospace due to their enhanced physical and mechanical properties. Alloys with grain size less than 100 nanometers are known as nanocrystalline alloys.

To produce produce industrial parts with these alloys, nanocrystalline powders are processed using elevated temperature and pressure combined to develop close to 100 percent dense bulk materials. Nanograins however start to grow at high temperatures causing the material to loose it's enhanced properties. To combat this issue high density interparticle bonding with minimum porosity must be obtained at high temperature while minimizing the loss of nanoscale grain size.

This video reveals a new approach to improve the nanograin size stability of Fe14Cr4Hf alloy at elevated temperatures.

Nano materials tend to be unstable causing grain size to increase at elevated temperatures. This results in the material losing it's superior mechanical properties. The instability of nano materials is the result of two factors that cause the material to go far beyond an equilibrium condition. Both grain size and mechanical processing lead to these altered thermodynamic properties. The smaller grains in nano materials have more grain boundary per volume than larger grains and thus a higher gibbs free energy.

Mechanical alloying techniques used to produce these materials also increase the energy available to drive grain growth. The thermodynamic instability caused by these factors drives the movement of grain boundaries especially at elevated temperatures causing grains to grow. To be useful nano materials must be developed that are stable at high temperature. One way of stabilizing grain size is to introduce alloying elements and eliminate oxygen from the solid solution. When oxygen is present, alloying elements form oxides within the grains preventing all of the alloying elements from reaching the grain boundaries. By eliminating oxygen, elements are free to segregate to grain boundaries stabilizing the size of the nanograins.

Studies have shown that if a nonequilibrium stabilizer solute such as hafnium is introduced to a nanocrystalline iron ten chromium alloy it segregates to the grain boundaries at elevated temperatures. This decreases the gibbs free energy of the grain boundaries resulting in a metastable equilibrium state and thus more stable nanocrystalline materials. It has been found that the elimination of oxygen further enhances this stabilization.

To compare nanograin size stability at different temperatures, samples are heat treated over a range of temperatures. Grain size is then analyzed using transmission electron microscopy images and x-ray diffraction. The Scherrer equation is used to calculate grain size based on x-ray diffraction results. Using this equation the size of nanograins is related to the broadening of a peak in the diffraction pattern.

Now that you understand the principles behind the stabilization of nanocrystalline materials, let's see how this method is applied in the laboratory.

Use high purity low oxygen content bulk materials iron, chromium, and hafnium enclosed in a glove box to minimize oxygen contamination. Load 6.4 and 7.9 mm 440c stainless steel milling balls and powder in to a stainless steel vial creating a ball to powder weight ratio of ten to one. The sealed vial needs to be kept under protective atmosphere in the glove box.

Transfer the vial to the high energy specs ball milling machine. Carry out ball milling for 20 hours. Return the vial to the glove box and transfer the milled powder to a small glass vial. Seal the glass vial for annealing. Anneal the ball milled Fe14Cr4Hf for 60 minutes at temperatures between 500 and 1200 degrees celsius at steps of 100 degrees celsius. Run XRD analysis of multiple samples from each annealing temperature as well as samples of the milled material. Employ a five millimeter dye and punch with hydraulic press to press the powder for microscopic analysis.

Now that you appreciate the importance of nanocrystals maintaining their grain size at high temperature let's take a look at some applications where they can be utilized. The life of aircraft can be increased utilizing nanocrystalline materials. Improved fatigue life, strength, and higher operating temperatures lead to a significant increase in aircraft speed and fuel efficiency.

These materials are also perfect candidates for spacecraft components that must work at higher temperatures. For example, onboard igniters on satellites developed from conventional materials may wear out quickly with no possibility of repair. Whereas nano materials will last longer prolonging the life of the mission.

You've just watched Jove's introduction to nano crystal stability. You should now understand the need for maintaining grain size at elevated temperature, ways in which it is accomplished and how grain size is measured.

Thanks for watching.