Papers by Author: Ana Coh O. Hirschmann

Abstract: Ti-6Al-4V is currently used in aeronautic and aerospace industry mainly for applications that require resistance at high temperature such as, blades for aircraft turbines and steam turbine blades. The titanium affinity by oxygen is one of main factors that limit the application of their alloys as structural materials at high temperatures. Notable advances have been observed in the development of titanium alloys with the objective of improving the creep properties. Increased oxygen levels are associated with increased microhardness and decreased ductility in titanium. In spite of this, Ti-6Al-4V containing an (+) structure continues to be the workhorse of the titanium industry due to their high specific strength, corrosion resistance, excellent high temperature properties and metallurgical stability. The objective of this work was to study the influence of equiaxed and Widmanstätten microstructures on oxidation rates and creep behavior of the Ti-6Al-4V alloy. The samples were exposed to different conditions of time and temperature to evaluate the oxidation rates. This influence on the oxidation rates was evaluated in terms of weight gain, -case depth and microhardness profile at 500 and 600 °C. Preliminary results indicated that the equiaxed microstructure with average grain size of 10 m exhibits faster oxygen diffusion. Short-term creep tests were performed under constant load in a stress range from 291 to 472 MPa at 500 °C and in a stress range from 97 to 291 MPa at 600 °C. The stress exponents obtained lie in the range from 4.0 to 11.3. The apparent activation energies for steady-state creep determined in the present work were estimated to be 316 and 415 kJ/mol at 291 MPa for the equiaxed and Widmanstätten microstructures, respectively. On the basis, the creep of Ti-6Al-4V is consistent with the lattice diffusion-controlled dislocation climb process in -Ti, for both microstructures. The creep rates of Widmanstätten microstructure were two orders of magnitude lower than of equiaxed microstructure in both temperatures. Apparently, the higher creep resistance with a Widmanstätten microstructure can be attributed to / interfaces acting as obstacles to dislocation motion and to the average grain size of 395 m, which reduces the grain boundary sliding, dislocations sources and the rate of oxygen diffusion along grain boundaries.

Abstract: Porous ceramics are of great interest due to their numerous potential applications. The objective of the present investigation was to produce porous alumina with 3 mol % yttria-stabilized tetragonal zirconia (Y-TZP). This material will be used in cooling systems of satellites. To obtain the porous ceramics the direct foaming technique was used. This method is based on the preparation of a stable foam to which a slurry of alumina and zirconia is added. The mixture is then vigorously stirred for incorporation of air. The sintered ceramics were characterized by scanning electron microscopy, mercury porosimetry and thermal conductivity. The tests performed with the porous alumina-zirconia ceramic composite obtained by this method, showed low thermal conductivity values, high porosity and uniform microstructure with 20–100 µm open pores. The results show that the alumina-zirconia composites tested in this study have a potential for application in loop heat pipes of cooling systems of satellites.

Abstract: The interest in porous ceramics has increased concurrently with new processes and new applications. This material has been used in several industrial applications such as filters, catalysis and sensors. The objective of the present investigation was to produce porous alumina with 3 % mol yttria stabilized zirconia in tetragonal crystalline structure (Y-TZP). This material will be used in cooling systems of satellites, due to its mechanical properties and chemical inertia. To obtain the porous ceramics was used the direct foaming technique, which is a method based on the preparation of a stable foam slurry and a slurry of alumina and zirconia that are later mixed and blended for incorporation of air in the mixture. The sintered ceramics was characterized by scanning electronic microscopy, mercury porosimetry and permeability measurements. The porous Al2O3–ZrO2 ceramics obtained showed high porosity and uniform microstructure with 20–100 ,m open pores. The results from these alumina zirconia composites showed a potential to apply in heat pipes.

Abstract: Porous materials are of significant interest due to their wide application in catalysis, separation, lightweight structural materials, biomaterials and other areas. Porous ceramics are produced within a wide range of porosities and pore sizes depending on the application intended. Porosity and pore size distribution can be carefully controlled by the choice of organic composite and the amount added. The material may have two types of pores: open and closed pores. The open pores, also called interconnected pores, are those which are in contact with the external surface of the material, being very useful for the manufacture of ceramic filters. A high number of closed pores are important for the manufacturing of materials used in thermal applications. There are many methods for obtaining porous ceramics, in general consisting in adding to the ceramic matrix organic particles, which volatilize during the first heat-up. The objective of this study was to produce ceramic composite nanostructure of alumina and yttria stabilized zirconia (Y-TZP) with micrometric pore sizes. The effects of ZrO2 additions in the mechanical properties of Al2O3 have been intensively investigated, due to the possible increase of the mechanical strength of this material. The organic particles used to create the pores were CMC and PVC. The microstructure of the porous ceramic samples obtained was evaluated considering the degree of sinterization of the nanoparticles, pores formation, porosity, specific surface of the pores and the distribution of the interconnecting pores.