The linear thermal expansion of bulk nanocrystalline Al and SS304 at low temperature

The linear thermal expansions (LTE) of bulk nanocrystalline (NC) Al, 304 stainless steel (SS304) and their conventional coarse-grained poly-crystalline counterparts (CCPC) were studied by the strain gage method in four directions within rolling plane (for bulk NC SS304 and Al) and one direction (for their CCPC) from liquid nitrogen temperature to 300 K. LTE of bulk NC Al and SS304 in four directions were equal to or less than those of their CCPC. This result was different from the fact that the smaller thermal expansions of many other bulk nanocrystalline materials were larger than those of their CCPC. We had to conclude that the less linear thermal expansions of bulk NC SS304 and Al were attributed to their larger defects at grain boundaries and residual strain. However, the larger linear thermal expansions of many other NC materials resulted from two factors.