Omer Van der Biest

Mechanical properties of Ti-6Al-4V specimens produced by shaped metal deposition

Abstract Shaped metal deposition is a novel technique to build near net-shape components layer by layer by tungsten inert gas welding. Especially for complex shapes and small quantities, this technique can significantly lower the production cost of components by reducing the buy-to-fly ratio and lead time for production, diminishing final machining and preventing scrap. Tensile testing of Ti-6Al-4V components fabricated by shaped metal deposition shows that the mechanical properties are competitive to material fabricated by conventional techniques. The ultimate tensile strength is between 936 and 1014 MPa, depending on the orientation and location. Tensile testing vertical to the deposition layers reveals ductility between 14 and 21%, whereas testing parallel to the layers gives a ductility between 6 and 11%. Ultimate tensile strength and ductility are inversely related. Heat treatment within the α+β phase field does not change the mechanical properties, but heat treatment within the β phase field increases the ultimate tensile strength and decreases the ductility. The differences in ultimate tensile strength and ductility can be related to the α lath size and orientation of the elongated, prior β grains. The micro-hardness and Young’s modulus are similar to conventional Ti-6Al-4V with low oxygen content.

Microstructure–toughness–wear relationship of tetragonal zirconia ceramics

The goal of the present work is to investigate the influence of microstructural variables (grain size, overall yttria content and yttria distribution) and toughness on the tribological behavior of yttria-stabilised tetragonal zirconia (Y-TZP) ceramics. Unlubricated fretting tests were performed on Y-TZP ceramics against commercial hardmetal (WC-Co) ball under ambient conditions of temperature and humidity. The ceramics were processed from commercial yttria-coated and co-precipitated powder as well as the newly formulated powder mixtures with varying overall yttria content and yttria distribution. Microstructural investigation of the worn surfaces was performed and the wear mechanisms were studied. Based on the measured tribological data, the relationships among the friction coefficient, wear, toughness and microstructural variables were elucidated. Within the investigated fretting regime, phase transformation (tetragonal to monoclinic zirconia) induced microcracking and spalling was found to play a major role in the wear of high toughness TZP ceramics. The significant outcome of our research is that a trade-off between the fracture toughness and the wear resistance is achieved in the newly processed Y-TZPs.

Grain Boundary Segregation in High-purity, Yttria-stabilized Tetragonal Zirconia Polycrystals (Y-TZP) -

Abstract In this paper, we use high-resolution transmission electron microscopy and electron energy-loss spectroscopy to study the microstructure of grain boundaries and the segregation of yttrium, respectively, in yttria-stabilized tetragonal polycrystalline zirconia, sintered from different high-purity powders. No amorphous films were observed at the grain boundaries, and only the sample containing the highest amount of silicon impurity showed presence of an amorphous silicate phase in all triple grain junctions. A strong yttrium segregation to the grain boundaries is observed in all samples, despite different grain sizes and impurity levels.

Microstructure of Ti-6Al-4V specimens produced by shaped metal deposition

Abstract Shaped metal deposition is an innovative technology, which creates near-net shaped components by weld deposition and saves time and material due to the absence of further machining. For Ti alloys this is a big advantage, since these alloys are difficult to shape and also very expensive. Ti alloys are very sensitive to the thermal history and shaped metal deposition introduces for each welding step an additional temperature gradient leading to different microstructures in different regions. Regions which were in the single β phase field during the last welding show a fine needle like microstructure, while regions within the α + β phase show coarser lamellae forming a Widmanstätten structure.

Mixed numerical-experimental identification of elastic properties of orthotropic metal plates

Abstract This paper compares results of three different methods to determine the in-plane elastic properties of sheet materials. Results obtained with standard resonant beam and tensile tests are used to assess a mixed numerical–experimental technique developed to determine the in-plane elastic properties of orthotropic plates from the resonance frequencies of rectangular plate samples (the so-called ‘Resonalyser’ technique). Test materials were selected on the basis of an expected low degree of elastic anisotropy in order to put the accuracy and sensitivity of the different techniques to assess anisotropic materials to a test. Therefore, aluminium alloy and stainless steel samples were prepared from hot-rolled plates, deliberately avoiding pronounced cold-rolling textures. The differences between the results obtained with the three experimental approaches are critically evaluated. In the case of very thin plates, the existing mixed numerical–experimental Resonalyser procedure succeeded in accurately identifying the elastic material properties. A slightly adapted procedure is proposed to extend the applicability of the Resonalyser procedure to thicker plates.

A Current Opinion on Electrophoretic Deposition in Pulsed and Alternating Fields

Electrophoretic deposition (EPD) is a colloidal production process developed in the early 20th century. Industrial scale EPD for the production of electronic components and phosphorescent screens and in the form of cataphoretic painting has known some success. Despite its limited practical applications, the inherent versatility of EPD has never ceased to fuel research into this technique. One of the major drives of this research was to render the method more environmentally friendly by enabling deposition from aqueous suspensions. One particular route, suggested to circumvent the problems caused by the use of water in EPD, is the use of alternating or pulsed fields. Recently, the use of alternating fields in EPD has been investigated for the deposition of biological matter in the form of cells and molecules. With this new avenue of research opening up and coinciding with a rise in biotechnological processes, one can expect a renewed interest in traditional EPD and fundamental research on the use of pulsed and alternating fields in this technique. Hence, this review attempts to summarize a centurys worth of both fundamental and applied research for scientists venturing into the field of EPD.

Thermodynamic prediction of the nonstoichiometric phase Zr1–zCezO2–x in the ZrO2–CeO1.5–CeO2 system

Abstract A thermodynamic estimation of the ZrO 2 –CeO 2 and ZrO 2 –CeO 1.5 systems, as well as the cubic phase in the CeO 1.5 –CeO 2 system has been developed and the complex relation between the nonstoichiometry, y , in Ce z O 2–y and the oxygen partial pressure at different temperatures is evaluated. The behavior of the nonstoichiometry phase Zr 1– z Ce z O 2– x is described based on the thermodynamic estimation in the ZrO 2 –CeO 2 , CeO 1.5 –CeO 2 and ZrO 2 –CeO 1.5 systems. Additionally, the interdependence among miscellaneous factors, which can be used to describe the change in oxidation states of cerium such as the oxygen partial pressure, the CeO 1.5 fraction in CeO 1.5 –CeO 2 in the quasi-ternary system, the nonstoichiometry y and the difference between the activity of CeO 2 and CeO 1.5 are predicted. The calculated results are found to be very useful to explain the influence of pressureless sintering at different O 2 partial pressures on the mechanical properties of CeO 2 -stabilised ZrO 2 ceramics

Assessment of the high temperature elastic and damping properties of silicon nitrides and carbides with the impulse excitation technique

The impulse excitation technique (IET), based on resonant vibration analysis, was used to determine the high temperature elastic and damping properties of hot-pressed silicon nitride (Si3N4) and silicon carbide (SiC) materials with Al- and Y-additives. The Si3N4 materials were doped with 2 wt.% of TiN to suppress the crystallization of intergranular glassy pockets. Near 1000 � C the investigated materials display a characteristic damping peak, which is essentially unaffected by temperature excursions up to 1400 � C. Two existing models which aim at linking elastic and damping properties with microstructural and micromechanical details, are considered. One of the models is used to provide an estimate of the amount of amorphous intergranular pocket phase. This type of information is of particular relevance since the high temperature deformation resistance of silicon based ceramics is severely dependent on the presence and amount of amorphous multiple grain pockets. # 2002 Elsevier Science Ltd. All rights reserved.

Synthesis and characterisation of CeO2-coated ZrO2 powder-based TZP

Abstract A novel technique is described for the preparation of Al2O3-doped CeO2-coated ZrO2 nanopowders from a solution of aluminium nitrate and cerium nitrate in a mixed alcohol/water suspension with monoclinic zirconia (m-ZrO2) powder. Drying of the suspension and subsequent calcination of the powder at 800°C results in alumina-doped CeO2-coated m-ZrO2 nanopowder with a grain size below 100 nm. The as-prepared powder can be pressureless sintered in air at 1450° into dense ceria-stabilised tetragonal polycrystals (Ce-TZP) materials with an excellent fracture toughness of 11 MPa m1/2 and a Vickers hardness of 750 kg/mm2 (HV30). The microstructure and mechanical properties of the alumina-doped CeO2-coated powder-based TZP are compared with that of a Ce-TZP obtained from commercially available co-precipitated powder.

Development of ZrO2-TiB2 composites: role of residual stress and starting powders

Abstract This article reports the processing of fully dense Y-stabilied tetragonal zirconia polycrystal (Y-TZP)-based 30 vol.% TiB 2 composites via hot pressing route using both the commercial co-precipitated and the yttria coated ZrO 2 powders with nominal dopant content of 3 mol% yttria. The indentation toughness of the sintered composites is found to be strongly sensitive to the use of ZrO 2 starting powders. Both the residual stress and the yttria distribution in the ZrO 2 matrix are found to have a significant influence on the achievable toughness.