1988

The expansion of three solid solutions of the ferrite phase, C6A2F, C4AF and C6AF2, in the presence of CaSO4 · 2H2O, Ca(OH)2 and siliceous powder with water/solid ratio of 0.3, has been compared with that of two reference mortars: the first containing C3A, Ca(OH)2 and CaSO4 · 2H2O, the second C3A doped by 4,5% Fe2O3, Ca(OH)2 and CaSO4 · 2H2O. The expansion of mortars, based on the ferrite phase, has been observed to be a function of Al2O3 contents, and is much weaker than that of mortars based on C3A. For these last ones, the expansion decreases from C3A to doped C3A.

The employment of non-dolomitic calcareous aggregates in the composition of concrete may serve to cause degradation if the mother rock contains sufficiently disordered silicates in the form of finely dispersed microinclusions. This paper reports the observation of fissures due to the formation of gel as a result of ASR in aggregates with less than 4% of microinclusions and even when this value was less than 1%.

The investigations were made on alloys OT-4 (Ti-3Al-1. 5 Mn), IMI318 (Ti-6Al-4V) and IMI550 (Ti-4Al-4Mo-2Sn-0. 5Si) of the alpha-beta type. The pressure was fixed at 10 torr and the temperature was made to vary between 800 and 1,000 degree C. A gaseous mixture was chosen, composed of 60 nitrogen and 40 hydrogen vol% and times of 4, 8 and 16 hours were selected for treatment with an ionic discharge. Diffractometric analysis was used for identification of the type of phases present on the hardened surface. It was found that the alloying elements modify the morphology of the surface layer.

Titanium metal and some of its alloys of the type Ti-Al-Me (Me=transition metal) were submitted to plasma nitriding. The resulting compound layer was examined by optical microscopy and by XR diffraction in order to establish both its microstructure and texture degree under different nitriding conditions. To this aim a texture evaluation method was employed, which was formerly devised and applied on surface layers obtained by boriding of steels. In these nitrided layers the Ti2N nitride was clearly textured while the TiN did not show an appreciable degree of preferred orientation of crystals.

A method is proposed to quantitatively evaluate the preferred orientation of FeB and Fe2B crystals, constituting the borided layer obtained on iron. The preferred orientation is expressed by an empirically calculated factor σ, which allows a fair fit of calculated with observed peak intensities by X-ray diffraction analysis of the layer (R value ≤0.15). In a further investigation, this coefficient (σ) was used to quantitatively examine the influence of temperature and reagent composition on the degree of orientation of the phases obtained by boronizing pure iron.

Surface layers having elevated hardness were produced by ion nitriding of titanium and α-β alloys of Ti-6wt.%Al-4wt.%V, Ti-4wt.%Al-2wt.%Sn4wt.%Mo and Ti-4wt.%Al-2wt.%Mn. The treatment was conducted at temperatures between 1073 and 1273 K for times of 4-32 h, using gaseous mixtures of nitrogen and hydrogen containing 20-80 vol.% nitrogen. X-ray diffraction, optical microscopy and microhardness were used to characterize the hardened surfaces.