Observations of fracture surface chemistry on Cu-0.5 At. % Sb bicrystals

The present work investigates the effect of grain boundary chemistry and crystallography on creep and on creep damage accumulation in Cu–0.008 wt.% Bi and Cu–0.92 wt.% Sb at stresses ranging from 10 to 20 MPa and temperatures between 773 and 873 K. Small additions of Bi and Sb significantly reduce the rupture strain and rupture time during creep of Cu. High stress exponents (Cu–Bi) and high apparent activation energies for creep (Cu–Bi and Cu–Sb) are obtained. Sb promotes creep cavitation on random high-angle grain boundaries. Bi, on the other hand, causes brittle failure when small crack-like cavities cause decohesion. Both elements suppress dynamic recrystallization, which occurs during creep of Cu at high stresses and temperatures.

In the present work, the influence of small amounts of Bi and Sb on the microstructural evolution of Cu during an ingot metallurgy processing route is investigated. Both elements are known to segregate to grain boundaries in Cu. Cu ingots with an outer diameter of 40 mm containing 0.008 wt.% Bi and 0.92 wt.% Sb, respectively, were vacuum induction melted, cast, and gradually swaged down to a final diameter of 11.7 mm with several intermediate annealing steps. Subsequent annealing treatments were conducted to investigate the microstructural evolution of the swaged bars. Optical microscopy, hardness testing and orientation imaging microscopy were used to characterize the deformation and recrystallization behavior, as well as the evolution of texture in the alloys. The results are then compared to those obtained for pure Cu. It is shown that even small amounts of alloying elements significantly alter the hardening behavior and suppress recrystallization at low temperatures. At higher temperatures, recrystallization in Cu, Cu–Bi and Cu–Sb leads to different textures.

Analytical Electron Microscopy (AEM) has brought significant progress in the study of grain-boundary segregation. Using X-ray energy-dispersive spectrometry (XEDS) in the AEM, elemental segregation information can be related to the crystallographic character of the same boundary via conventional Transmission Electron Microscope (TEM) diffraction techniques. While significant efforts have been made to improve XEDS analysis of sub-nanometer segregation layers, the methods for crystallographic characterization of grain boundaries have remained the same for several decades and labor-intensive processes. Recently, a method termed Automated Crystallography for TEM (ACT) was developed, which automates crystallographic characterization of grains under TEM observation. In the present work, we combine ACT and X-ray mapping via EDS in AEM for the study of Sb grain-boundary segregation in a rapidly solidified Cu–0.08 wt% Sb alloy. In contrast with previous reports, a large degree of anisotropy in Sb segregation level between different boundaries is found. ACT results suggest that one of the several grain boundaries observed with no detectable Sb segregation is very close to a Σ3 coincidence-site lattice structure. The reason for the observed anisotropy in the present alloy is discussed, based upon McLean's theory of segregation.

Cu–2 wt.% Sb alloys in the form of 3-mm rod and 0.10-mm-thick ribbon were prepared and the Sb grain-boundary segregation was studied by using X-ray mapping in the analytical electron microscope. The rod and ribbon were prepared by Cu mold casting and single roller melt spinning, respectively. These two methods correspond to cooling rates of 10² and 10⁶ K/s, respectively. Despite the difference in cooling rates, Sb grain-boundary segregation was revealed in both alloys.

Grain-boundary segregation is a necessary, but not always sufficient, criterion for intergranular embrittlement of many metals and alloys. The reason why certain segregants induce embrittlement while others are harmless remains unclear, but it has been theorized that segregation may be responsible for local changes in the atomic bonding which, in turn, may produce brittle failure. Recent developments in electron energy-loss spectrometry in the analytical electron microscope permit correlation to be made between the presence of certain segregants and changes in the bonding of the atoms on the grain boundary. Because the effects in the energy-loss spectrum are small, the spatial-difference method (equivalent to a first-difference spectrum) is sometimes used to discern the spectral intensity changes. It is important to ensure that this method of processing the data does not induce the changes in the spectrum that are usually associated with changes in bonding, and this can be demonstrated by experimental comparison of energy-difference and spatial-difference spectra. Using direct measurement, as well as spatial-difference techniques, it is demonstrated that Bi and Sb, well-known embrittlers of Cu, induce consistent changes in the bonding of Cu, while Ag, a similar segregant which does not embrittle Cu, produces no detectable changes in the bonding.

The chemical composition of several grain boundaries in well-defined bicrystals of an Fe-6at%Si alloy containing phosphorus was studied by means of scanning Auger microscopy on in-situ fractured samples. The observed segregation of Si and P shows a strong dependence on the grain boundary type. Whereas P. segregates to all grain boundaries, Si segregation is only observed at Σ3 grain boundaries. In the case of both symmetrical and asymmetrical Σ5 grain boundaries a depletion of Si is detected. Some quantitative differences also exist in the segregation at individual grain boundaries corresponding to the same orientational relationship 37 ° [100]. The anisotropy of the segregation behaviour is explained on the basis of both the different atomic structures of individual grain boundaries and the mutual interaction between P and Si atoms. The main topographical features observed at the fracture surfaces are also classified and their effect on the measured segregation data is demonstrated.