Abstract
Several grain sizes were obtained by heat treatment at different temperatures for interstitial-free (IF) and 2.25Cr-1Mo steels. Samples of the steels with different grain sizes were aged at 600 and 680 °C for IF steel and 520 and 560 °C for 2.25Cr-1Mo steel for sufficient time to achieve their equilibrium grain boundary segregation. The grain boundary concentrations of phosphorus were examined using Auger electron spectroscopy. At the same aging temperature, the boundary segregation of phosphorus increased with increasing grain size. The effect of grain size on equilibrium grain boundary segregation thermodynamics was analyzed based on the information of both grain size and phosphorus boundary concentration. The segregation enthalpy increased with increasing grain size and simultaneously the segregation entropy became less negative. Moreover, the segregation entropy (∆S) and enthalpy (∆H) of phosphorus in both IF and 2.25Cr-1Mo steels exhibited a unified linear relationship, being expressed as ∆S = 0.85∆H − 38.06, although it segregated to different types of grain boundaries (ferrite grain boundaries in IF steel and prior austenite grain boundaries in 2.25Cr-1Mo steel). With the aid of the acquired thermodynamic parameters and grain boundary segregation theories, the equilibrium segregation concentrations at different aging temperatures were modeled under different grain sizes for both steels.
Highlights
It is well known that [1,2] grain boundary segregation of impurity elements is the main cause of non-hardening embrittlement in low-alloy steels
Based on the thermodynamic adsorption theory, Mclean [3] firstly proposed the theory of equilibrium grain boundary segregation in binary systems and the thermodynamics of equilibrium segregation can be expressed as 1 − Ceq
The aim of the present work is to explore in detail the effect of grain size on the grain boundary segregation thermodynamics of phosphorus in an IF steel and a 2.25Cr-1Mo steel by virtue of Auger electron spectroscopy along with grain boundary segregation theories
Summary
It is well known that [1,2] grain boundary segregation of impurity elements is the main cause of non-hardening embrittlement in low-alloy steels. The vast majority of technologically useful materials are composed of an aggregate of small crystals. These crystals, or grains, are separated from one another by a network of interfaces, or grain boundaries. The grain boundary is a narrow zone of weakness. Once impurity elements segregate to the grain boundaries in these materials, failure would occur catastrophically by fracture along some grain boundaries. The grain boundary segregation of solute or impurity elements in the alloys has attracted extensive attention in the research community. Based on the thermodynamic adsorption theory, Mclean [3] firstly proposed the theory of equilibrium grain boundary segregation in binary systems and the thermodynamics of equilibrium segregation can be expressed as
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