Hot brittle intergranular fracture produced by segregation of harmful elements at grain boundaries

Introduction

The thermal and brittle fracture behavior of materials is a complex and fascinating phenomenon which is highly dependent upon the presence of heterogeneities in the microstructure of the material. In particular, a heterogeneous microstructure can have a pronounced effect on the thermal brittleness of a material due to the presence of thermally-assisted harmful elemental segregation at the grain boundaries, which can cause increased stress concentrations and reduced fracture toughness. Such heterogeneities can arise through different application-dependent processes such as environment-controlled exposure, welding, and service loading; they can also be introduced deliberately into the material in order to improve specific properties such as creep resistance or toughness.

The thermal brittleness of a material is usually determined through various test methods such as brittleness temperature testing (BTT), slow strain rate testing (SSRT), and one-point incremental deformability testing (OPIDT). These tests are designed to evaluate the behavior of the material under localized elevated temperature loading and often serve as references when assessing the temperature-related brittle fracture behavior of a material. Depending on the application, such tests may target lower brittle temperatures than those measured in steady-state testing regimes like with BTTs.

In this paper, we focus on the effect of harmful elemental segregation at the grain boundaries on the thermal brittleness of a material. Generally, grain boundary migration (GBM) is a thermally activated process that allows material redistributions. Oxide particles, carbide particles, and other harmful elements can segregate to the grain boundaries, resulting in a decrease in fracture toughness and reduced ductility. The over-concentration of such damaging components on grain boundaries has been suggested to be a major driving factor of the thermal brittleness of a material.

Experimental

An experimental study was conducted to investigate the effects of harmful elemental segregation at grain boundaries on the thermal brittleness of a material. Specimens of the material were prepared in the form of strips for slow strain rate testing (SSRT). The specimens were loaded with a displacement-controlled servo-hydraulic system in a constant load condition at various temperatures. The SSRTs were conducted at constant displacement rate of 0.4mm/minute in hot deformation temperature ranging from 25oC to 550oC. At each temperature, three specimens were tested and the fracture toughness (KIC) was measured from the load-displacement curve.

A numerical simulation of the thermal brittleness was conducted in order to investigate the effect of unwanted grain boundary clustering of the detrimental elements. The simulated grain boundaries were composed of carbides and oxides. The carbide particles and oxide particles were introduced to the model at interstitial locations and randomly distributed along the grain boundaries. The concentration of the detrimental components and the grain boundary migration rate were varied in order to reproduce the experimental results.

Results and Discussion

The results of the SSRT testing showed that the fracture toughness of specimens decreased as the temperature increased. This decrease in fracture toughness was attributed to the reduction in ductility caused by the harmful element segregation at the grain boundaries. The numerical simulation results showed similar behaviour, with an overall decrease in fracture toughness as the temperature increases. The decrease in fracture toughness was most pronounced at the highest temperatures, when the segregation of the detrimental components was strongest.

Conclusions

In conclusion, the detrimental effect of harmful element segregation at grain boundaries on the thermal brittleness of a material has been successfully demonstrated. The results showed that the segregation of harmful elements can lead to a decrease in fracture toughness and an increase in brittleness at elevated temperatures. It was confirmed that the detrimental effects were strongest at the highest temperatures, when the segregation of the detrimental components was strongest. The findings of this study suggest that controlling the interstitial sites and the grain boundary clustering of detrimental elements may have potential applications in improving the thermal brittleness and thermal stability of materials.

Thermal embrittlement along crystal boundaries is a phenomenon caused by the concentration of harmful elements in the crystal lattice. It is a kind of deterioration of the material structure caused by differences in self-diffusion of atoms in a material along the grain boundaries.

Thermal embrittlement along grain boundaries generally occurs under conditions where the grain boundary is located in a region of low thermal stability. At elevated temperature, such as during heat treatment, these grains can rapidly diffuse or migrate into the crystal lattice, causing disruption to the crystal structure and atom instability in the lattice. Such atomic swaps at grain boundaries can cause chain reactions which may propagate further into the lattice, weakening the material and leading to thermal brittle failure.

In order to prevent the occurrence of thermal embrittlement along crystal boundaries, methods such as grain boundary segregation are used. For example, heat treatments to reduce the grain size may be used to prevent atomic swaps at grain boundaries. Other methods such as change in alloy composition or chemical reaction can be used to reduce the diffusion of grain boundary related defects, thereby increasing the grain boundary stability.

In summary, thermal embrittlement along crystalline boundaries is a type of structure distortion and weakening caused by the concentration of harmful elements at the grain boundary. Such damage can be prevented by implementing proper heat treatments, grain boundary segregation, alloy composition modification and chemical reaction methods.