Thermal expansion and boundary excess free volume of electroformed nanocrystalline Ni-Co from dilatometer and digital caliper measurements

The coefficient of thermal expansion (CTE) as measured by dilatometry of electroformed nanocrystalline Ni-32at %Co macro-defect free sheet metal up to 500 degrees C can be separated into three regions. Below 185 degrees C, the nanocrystalline material's CTE is similar to its polycrystalline counterpart. Between 185 and 310 degrees C, the CTE of the nanocrystalline material plateaus, then drops at temperatures above 310 degrees C due to volume shrinkage induced by different grain growth stages. The grain boundary excess volume (BEV) was determined to be between 0.20-0.24 nm. In addition, it is shown that simple digital caliper measurements are a reliable technique to estimate BEV for specimens with grain boundaries being the main defect.

Automated summary

The coefficient of thermal expansion (CTE) of electroformed nanocrystalline Ni-32at%Co macro-defect free sheet metal can be divided into three regions up to 500 degrees C. Below 185 degrees C, the CTE is similar to its polycrystalline counterpart. Between 185 and 310 degrees C, the CTE plateaus and then decreases above 310 degrees C due to volume shrinkage induced by different grain growth stages. The grain boundary excess volume (BEV) was determined to be between 0.20-0.24 nm. Moreover, digital caliper measurements were proven to be a reliable technique for estimating BEV in specimens with grain boundaries as the main defect.