Tomáš František Kubatík

The Structure and Mechanical Properties of High-Strength Bulk Ultrafine-Grained Cobalt Prepared Using High-Energy Ball Milling in Combination with Spark Plasma Sintering

In this study, bulk ultrafine-grained and micro-crystalline cobalt was prepared using a combination of high-energy ball milling and subsequent spark plasma sintering. The average grain sizes of the ultrafine-grained and micro-crystalline materials were 200 nm and 1 μm, respectively. Mechanical properties such as the compressive yield strength, the ultimate compressive strength, the maximum compressive deformation and the Vickers hardness were studied and compared with those of a coarse-grained as-cast cobalt reference sample. The bulk ultrafine-grained sample showed an ultra-high compressive yield strength that was greater than 1 GPa, which is discussed with respect to the preparation technique and a structural investigation.

Application of Silicon for a Protection of Titanium against High-Temperature Oxidation

The paper describes a positive influence of silicon on the high-temperature oxidation resistance of titanium. Since silicon additions can be realized both by bulk and by surface alloying, the surface siliconizing techniques, as well as structure of the Si-rich layers, are illustrated. Furthermore, the high-temperature cyclic oxidation resistance of the surface siliconized titanium and of the TiSi2 alloy are compared to that of pure Ti and TiAl6V4 alloy, and the oxidation mechanism is discussed.

High-Strength Ultra-Fine-Grained Hypereutectic Al-Si-Fe-X (X = Cr, Mn) Alloys Prepared by Short-Term Mechanical Alloying and Spark Plasma Sintering

In this work, Al-20Si-10Fe-6Cr and Al-20Si-10Fe-6Mn (wt %) alloys were prepared by a combination of short-term mechanical alloying and spark plasma sintering. The microstructure was composed of homogeneously dispersed intermetallic particles forming composite-like structures. X-ray diffraction analysis and TEM + EDS analysis determined that the α-Al along with α-Al15(Fe,Cr)3Si2 or α-Al15(Fe,Mn)3Si2 phases were present, with dimensions below 130 nm. The highest hardness of 380 ± 7 HV5 was observed for the Al-20Si-10Fe-6Mn alloy, exceeding the hardness of the reference as-cast Al-12Si-1Cu-1 Mg-1Ni alloy (121 ± 2 HV5) by nearly a factor of three. Both of the prepared alloys showed exceptional thermal stability with the hardness remaining almost the same even after 100 h of annealing at 400 °C. Additionally, the compressive strengths of the Al-20Si-10Fe-6Cr and Al-20Si-10Fe-6Mn alloys reached 869 MPa and 887 MPa, respectively, and had virtually the same values of 870 MPa and 865 MPa, respectively, even after 100 h of annealing. More importantly, the alloys showed an increase in ductility at 400 °C, reaching several tens of percent. Thus, both of the investigated alloys showed better mechanical properties, including superior hardness, compressive strength and thermal stability, as compared to the reference Al-10Si-1Cu-1Mg-1Ni alloy, which softened remarkably, reducing its hardness by almost 50% to 63 ± 8 HV5.

Dependence of NiTi Alloy Microstructure on the Conditions of Powder Metallurgy Production

The aim of this work was to describe the dependence of microstructure of NiTi shape memory alloy on the conditions of powder metallurgy processing route. The technology consisted of blending of elemental Ni and Ti powders, uniaxial cold pressing and reactive sintering. The effects of reactive sintering temperature, heating rate, holding duration and particle size were determined. The proposed technology can be used as the alternative production route of NiTi to minimize the contamination of the alloy.

Plasma Spraying of Al-Si and Silicon on Magnesium Alloy under Protective Atmosphere

Magnesium alloy AZ 91 is widely used as a structural material, but its use is limited at higher temperatures and high humidity. Plasma spraying technology allows to prepare protective metallic and nonmetallic coatings on magnesium and its alloys. In this study magnesium alloy AZ 91 was coated using plasma spraying method by AlSi30 wt. % and silicon. The Al-Si was applied to provide a bond-coat for topmost layer of silicon. This study focused on feeding distance (FD) on the quality of the prepared layers. The layers were prepared with a thickness of about 100 μm. The layers were observed with a scanning electron microscope with elemental map analysis. The layers were measured by Electrochemical impedance spectroscopy.

Structure of Nanocrystalline Nickel Prepared by Powder Metallurgy

Nanocrystaline nickel was prepared by selective leaching technology. Consequently, the powder was compacted by spark plasma sintering method. This process is suitable due to its high heating rates, which leads to relatively low thermal exposition of compacted material. The dependence of structure of compacted material on preparation conditions is described in this paper.

Spark Plasma Sintering of Multilayer Ceramics – Case Study of Al2O3-Mg(Ca)TiO3 Sandwich

Spark plasma sintering enables very rapid fabrication of bulk ceramic materials. Suitability of this technique for preparing of multilayer ceramic discs is up to now relatively seldom reported. Our work is focused on a bi-layered disc consisting of thick Al2O3 layer and comparably thick Mg (Ca)TiO3 (MCT) layer sintered in one run. Al2O3 powder was nanometric (less than 40 nm) and composed of the hexagonal α-phase whereas MCT is a solid solution of two orthorombic perovskites. Pre-sintered and crushed MCT powder was coarser than alumina, up to 125 μm. The purpose of this material combination was creating of dielectric sandwich consisting of capacitor MCT material and resistor nanoalumina material. Spark plasma sintering run was done at 1200 °C, using pressure 80 MPa and dwell time only 2 min. Resulting sandwich was subjected to microstructural observations and cross sectional measurement of microhardness. On the border between Al2O3 and MCT the microhardness was higher than in MCT but has markedly higher dispersion than in the individual components. The MCT material exhibits birefringence in the polarized light [1]. We attempted to correlate the microhardness of individual MCT grains to their orientation indicated by birefringence.

High strength Al–Zn–Mg–Cu–Ni–Si alloy with improved casting properties

Abstract The casting properties of high strength Al-7Zn-7Mg-1Cu-3Ni-3Si(wt-%) alloy are described. Compared with common Al-Zn-Mg-Cu alloys, an improvement of casting properties has been achieved by adding elements (Ni, Mg, Si) that form eutectic phases, thus reducing the solidification interval of the alloy. A comparison of thermal cooling curves, castability and hot tearing tendency has been carried out for three alloys: Al-7Zn-2Mg-1Cu (structure consists mainly of solid solution), quasi-ternary eutectic alloy Al-7Zn-7Mg-1Cu-3Ni-3Si and the common casting alloy Al-10Si. In addition, the effect of melt protection against oxidation on castability has been evaluated. It is shown that the casting properties of the protected quasi-ternary eutectic alloy are significantly better than those of the common Al-7Zn-2Mg-1Cu alloy and that they achieve a level close to that of Al-10Si alloy.