Ductility improvement due to martensite α' decomposition in porous Ti-6Al-4V parts produced by selective laser melting for orthopedic implants.

Ti-6Al-4V parts obtained by selective laser melting typically have an acicular α' martensitic microstructure whose ductility is low. Thus, post-heat treatments are useful for increasing ductility. In this work, the effects of sub-ß-transus heat treatments on the mechanical properties of Ti-6Al-4V parts with porous structures are correlated with martensite α' phase decomposition. The precipitation of ß phase and the gradual transformation of α' into α phase by the diffusion of excess vanadium from α' to ß phase are proposed to be the main events of martensite α' phase decomposition in parts fabricated by selective laser melting. The heat treatment performed at 650°C for 1h produced no microstructural changes, but the samples treated for at the same temperature 2h showed a fine precipitation of ß phase along the α' needle boundaries. The heat treatment performed at 800°C for 1 or 2h produced a fine α+ß microstructure, in which ß phase are present as particles fewer in number and larger in size, when compared with the ones present in the sample heat-treated at 650°C for 2h. Heat-treatment of the parts at 800°C for 2h proved to be the best condition, which improved the ductility of the samples while only slightly reducing their strength.

Influence of Si addition on the microstructure and mechanical properties of Ti-35Nb alloy for applications in orthopedic implants.

In the development of new materials for orthopedic implants, special attention has been given to Ti alloys that show biocompatible alloy elements and that are capable of reducing the elastic modulus. Accordingly, Ti-Nb-Si alloys show great potential for application. Thus, this is a study on the microstructures and properties of Ti-35Nb-xSi alloys (x=0, 0.15, 0.35 and 0.55) (wt%) which were thermally treated and cooled under the following conditions: furnace cooling (FC), air cooling (AC), and water quenching (WQ). The results showed that Si addition is effective to reduce the density of omega precipitates making beta more stable, and to produce grain refinement. Silicides, referred as (Ti,Nb)3Si, were formed for alloys containing 0.55% Si, and its formation presumably occurred during the heating at 1000°C. In all cooling conditions, the hardness values increased with the increasing of Si content, as a result from the strong Si solid solution strengthening effect, while the elastic modulus underwent a continuous reduction due to the reduction of omega precipitates in beta matrix. Lower elastic moduli were observed in water-quenched alloys, which concentration of 0.15% Si was more effective in their reduction, with value around 65 GPa. Regarding Ti-35Nb-xSi alloys (x=0, 0.15 and 0.35), the "double yield point" phenomenon, which is typical of alloys with shape memory effect, was observed. The increase in Si concentration also produced an increase from 382 MPa to 540 MPa in the alloys' mechanical strength. Ti-35Nb-0.55Si alloy, however, showed brittle mechanical behavior which was related to the presence of silicides at the grain boundary.

[Diagnostic value of jejunal biopsy]. / Valor diagnóstico de la biopsia yeyunal

Trying to find out the clinical elements that define the precise indication ofr a jejunal biopsy, 28 children with several pathological conditions were studied. They were 12 cases of kwashiorkor, 11 cases with chronic diarrhea, two cases of chronic pancreatitis, 2 cases with ferropenic anemia resistant to the oral treatment with iron and one case of chilous ascitis. It is concluded that only in those cases in which the biopsy is the precise medium for diagnosis is where it would be indicated, such as intestinal lymphangiectasis or in those cases with signs or evidence of malabsorption without diarrhea. The chronic diarrhea per se does not seem to be a formal indication for biopsy.