Detection of Ni–Sn phases in temper embrittled Ni–Cr steels
A GENERIC problem in ferritic steels is the reduction in the grain boundary cohesive strength produced by the segregation of trace impurities of Groups IVB and VB of the Periodic Table. Such segregation results in a deterioration in mechanical properties produced, for example, by reversible temper embittlement1 and it also promotes enhanced susceptibility to a range of environmental enhanced cracking processes2–4. It has long been recognised that the embrittlement of steels is enhanced by the presence of certain alloy elements, that is Ni, Cr or Mn, and measurements using Auger electron spectroscopy (AES)5–7 have demonstrated that specific combinations of alloy and impurity elements cosegregate to the boundaries, for example, Ni+Sn, Ni+Sb, Cr+P. These observations can be rationalised using Guttmann's phenomenological theory8 which attributes the increased driving force for segregation to a positive interaction between alloy and impurity solute atoms. Previous investigations1,9–11 have failed to detect the presence of any three dimensional phase using conventional transmission electron microscopy and it is generally considered that the alloy element–trace impurity atoms form two dimensional phases at the boundaries. The only direct evidence for such alloy–impurity interactions and phase formation has been obtained using Mössbauer spectroscopy12,13 but it is not possible using this technique to draw any positive conclusions regarding the location of such compounds. Measurements on a pure Fe-3.5 wt% Ni alloy doped with 119Sn12 showed that Ni atoms associate preferentially with Sn atoms during tempering at 650°C and the marked change in the spectrum observed on ageing at 500°C, within the embrittling temperature range, is consistent with the formation of a Ni–Sn compound (Ni3Sn2 or Ni3Sn4). We report here some initial results from a detailed electron microscopic analysis of the microstructures of a commercial (En30A) Ni–Cr steel and a special laboratory made steel (BE 10) having similar Ni and Cr contents, but doped specifically with tin. The composition of the two steels is presented in Table 1. Both steels were oil quenched from 950°C, tempered for 1 h at 650°C and aged at 500°C for times up to 5,000 h. Carbon extraction replicas, taken from polished surfaces were examined in a Philips EM400 microscope fitted with an energy dispersive X-ray system. Selected individual precipitate particles were analysed using convergent beam microdiffraction and X-ray analysis. Attention was focused on the particles which delineated the lath packet and previous austenite boundaries, that is, high-angle boundaries.
Grain boundary segregation and intergranular fracture in molybdenum
The refractory group VIA metals generally exhibit intergranular brittleness when they are in the recrystallized condition. This causes severe problems in their fabrication and places major limitations on their practical application. The phenomenon, generally referred to as recrystallization embrittlement, results in large increases in the ductile-to-brittle transition temperature and a change in fracture mode in the lower shelf regime from cleavage to intergranular with a significant decrease in ductility. The embrittlement is widely considered to be associated with interstitial impurities but there have been few systematic studies to elucidate their effects. The present paper reports results from a systematic study of segregation and intergranular embrittlement in binary molybdenum-oxygen and ternary molybdenum-oxygen-carbon alloys. The experiments were carried out on ‘bamboo’ specimens containing a series of identical single grain boundaries traversing their cross-sections. Measurements have been made of the activation energy for oxygen segregation to grain boundaries in the binary molybdenum-oxygen alloys. The influence of carbon additions on the level of oxygen segregation has also been determined. In addition, the influence of oxygen segregation on the energy to fracture has been studied and this has involved quantitative measurements of the work of fracture and the contribution made by plastic deformation. Results from metallographic studies are also presented, showing the effects of segregation on fracture surface topography and dislocation structures immediately adjacent to the fracture surfaces. In discussing the results we consider the thermodynamics of oxygen segregation to grain boundaries and the role played by carbon in inhibiting segregation. It is proposed that carbon either increases the effective solubility of oxygen in molybdenum or acts as a trap for oxygen atoms. In either case the effect is to reduce the driving force for segregation. We also consider the influence of segregation on the work of fracture and show that the reduction in oxygen segregation resulting from the addition of carbon produces small increases in fracture energy. This increases the local stress to propagate a crack sufficiently to promote plastic deformation which blunts the crack tip. The consequent change in geometry reduces the stress concentration at the crack tip, thereby resulting in a large increase in the applied fracture stress and the work to fracture.
Submicron polymer structures with X-ray lithography and hot embossing
This article describes the process chain for replication of submicron structures with varying aspect ratios (AR) up to 6 in polymethylmethacrylate (PMMA) by hot embossing to show the capability of the entire LIGA process to fabricate structures with these dimensions. Therefore a 4.7 μm thick layer of MicroChem 950k PMMA A11 resist was spin-coated on a 2.3 μm Ti/TiOx membrane. It was patterned with X-ray lithography at the electron storage ring ANKA (2.5 GeV and λc ≈ 0.4 nm) at a dose of 4 kJ/cm3 using a Si3N4 membrane mask with 2 μm thick gold-absorbers. The samples were developed in GG/BDG and resulted in AR of 6–14. Subsequent nickel plating at 52°C resulted in a 200 μm thick nickel tool of 100 mm diameter, which was used to replicate slit-nozzles and columns in PMMA. Closely packed submicron cavities with AR 6 in the nickel shim were filled to 60% during hot embossing.