Abstract
The linear thermal expansions (LTE) of bulk nanocrystalline ingot iron (BNII) at six directions on rolling plane and conventional polycrystalline ingot iron (CPII) at one direction were measured from liquid nitrogen temperature to 300 K. Although the volume fraction of grain boundary and residual strain of BNII are larger than those of CPII, LTE of BNII at the six measurement directions were less than those of CPII. This phenomenon could be explained with Morse potential function and the crystalline structure of metals. Our LTE results ruled out that the grain boundary and residual strain of BNII did much contribution to its thermal expansion. The higher interaction potential energy of atoms, the less partial derivative of interaction potential energy with respect to temperature T and the porosity free at the grain boundary of BNII resulted in less LTE in comparison with CPII from liquid nitrogen temperature to 300 K. The higher LTE of many bulk nanocrystalline materials resulted from the porosity at their grain boundaries. However, many authors attributed the higher LTE of many nanocrystalline metal materials to their higher volume fraction of grain boundaries.
Highlights
The thermal properties of materials are important parameters for material applications and they are associated with other physical and chemical properties
Our linear thermal expansions (LTE) results of bulk nanocrystalline ingot iron (BNII) are different from enhanced thermal expansion of other NC materials, BNII has the higher grain boundary volume fraction and higher concentration of defects at grain boundary
Many authors thought that the two factors resulted in the enhanced thermal expansion for NC materials and that LTE of NC materials increased with decreasing grain sizes
Summary
The thermal properties of materials are important parameters for material applications and they are associated with other physical and chemical properties. In order to obtain thermal expansion of materials, one measured the temperature dependence of lattice parameter by X-ray diffraction or neutron powder diffraction [3, 4]. LTE of NC Ni–P alloy and the volume expansion of NC Se synthesised by crystallization of amorphous were 51 and 61% higher than those of their conventional coarse-grained polycrystalline counterparts, respectively [11, 12]. In this work, we investigated LTE of bulk nanocrystalline ingot iron (BNII) at six directions on rolling surface and conventional polycrystalline ingot iron (CPII) at one direction, our LTE results of BNII are different from enhanced LTE of other NC materials. L(T) and L(300) are the lengths of specimen at certain measurement direction at temperatures T and 300 K, respectively Another thermal expansion parameter, the linear thermal expansion coefficient gl(T), was defined as gl ðT Þ
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