Harry Berek

In Situ Observation of Collision between Exogenous and Endogenous Inclusions on Steel Melts for Active Steel Filtration

In a confocal scanning laser microscope, interactions between exogenous and endogenous inclusions on steel melts have been investigated. Higher capillary long-range attraction forces as well as higher acting lengths of the capillary forces between exogenous and endogenous particle pairs have been observed in comparison to endogenous inclusions pairs. The contribution of the roughness of the exogenous inclusions to the capillary attraction forces has been discussed and a surface filter design for a higher filtration efficiency of steel melts has been proposed.

Pitfalls of local and quantitative phase analysis in partially stabilized zirconia

The addition of selected elements into the host structure of ZrO2 stabilizes the tetragonal and cubic phases of zirconia, which are, in their undoped binary form, only stable at high temperatures. From the crystallographic point of view, the increasing amount of the stabilizer causes a continuous transition of the tetragonal zirconia to its cubic modification. In partially stabilized zirconia, local concentration gradients of the stabilizer are frequently present as a consequence of the production process, which results in a coexistence of zirconia domains having different degrees of tetragonality. The presence of the local concentration gradients in such samples and the continuous nature of the phase transformation are features important for many technological applications, but their analysis is not straightforward. Furthermore, these features complicate the quantitative phase analysis in partially stabilized zirconia. For the example of zirconia partially stabilized by magnesium, this contribution illustrates the capabilities and limitations of X-ray and electron backscatter diffraction. In particular, the ability of these experimental methods to reveal the gradual lattice distortion that is associated with the cubic to tetragonal phase transformation in zirconia and the reliability of the quantitative phase analysis are discussed. In this context, it is shown to what extent the choice of the microstructure model influences the result of the phase analysis.

Stress Induced Phase Transformations in TRIP-Steel / Mg-PSZ Composites

Composite materials and micro- and macrostructure designs have been the focus of numerous scientific studies over the past few years according to their crashworthiness [1-3]. Crashworthiness is concerned with the absorption of energy through controlled failure mechanisms and modes that enable a defined load profile during energy absorption [4]. Cellular materials, such as metal foams, are materials which display a unique combination of physical and mechanical properties, e.g. for crash box applications. The defining characteristic of metal foams is a very high porosity, typically in the range of 70 to 90 vol. %. In principle, cellular metals can be manufactured from gas, liquid or solid phases and currently the most advanced methods involve melt-metallurgical processes [5]. Several groups have produced foam structures by using hollow spheres to form the cells of the material [5, 6]. These materials exhibited plateau stresses of 5 MPa and 23 MPa respectively, with volume specific energy absorptions SEA of 2 MJ/m3 and 10 MJ/m3 respectively, up to 50 % strain [6, 7]. By combining ceramics with ductile metals, failure-tolerant metal matrix composites (MMCs) can be created. With regard to application of the MMCs as wear resistant materials in metal forming tools a prolongation of the life time and the resultant reduced equipment downtimes have been achieved by active steel infiltrating of porous zirconia structures with the aid of Ti as activator [8]. A very promising approach concerning zirconia/steel - composite materials with superior mechanical properties has been demonstrated by Guo et al. using a low-alloyed TRIP steel in combination with an Y-PSZ – ceramic [9, 10]. In a previous study honeycomb structures were formed from composites of high-alloyed austenitic stainless TRIP-steel AISI 304 with Mg-PSZ with different mixing proportions due to ceramic extrusion at room temperature and sintering at 1350 °C for 2 h in an 99.9 % Argon atmosphere [11]. One of the most promising manufacturing route to produce open cell composite foams is based on the patent of Schwartzwalder [12] by the replication method using polyurethane sponge as a template. The polymer foam is impregnated in a powder slurry (this first coating contributes as an adhesive porous layer for further coating processes), the ceramic slurry is squeezed out of the functional pores and cold spray coatings are applied in order to eliminate defects out of the squeezing process and reach the critical wall thickness for acceptable mechanical properties. In [13] the authors reported about foams with 90 Vol% high alloyed TRIP-steel and 10 Vol% Mg-PSZ. Up to 50 % compressive strain a remarkable enhancement of the SEA was observed in comparison to comparable structures with TRIP-steel only.

Joining of Zirconia Reinforced Metal-Matrix Composites

Metal-matrix composite materials, based on a metastable austenitic stainless steel reinforced with a magnesia partially stabilised zirconia have been prepared by a ceramics-derived extrusion technology. Using this powder metallurgical method enables the shaping of lightweight cellular structures as well as bulk specimens with a variety of steel/ceramic ratios at room temperature. However, the extrusion of composite structures is limited by the uniform cross section throughout its entire length. Joining of these metal-matrix composite preforms after sintering by conventional welding techniques is a challenging task. The presence of ceramic fractions may lead to several complications and the subsequent heat exposure during joining may initiate phase transformations in both metastable components resulting in a deterioration of the mechanical properties of the composite material. An adapted ceramics-derived joining technology allows the combination of varying TRIP-steel/zirconia composite materials. The main features are the machining and joining of the parts in their dry green state at room temperature before their thermal treatment. Thus, the material’s consolidation and the formation of the joint take place simultaneously. The ability of joining different parts offers the possibility to create structures for complex applications and testing conditions. The key to advanced properties of the joining zone are the base materials, the surface treatment of the parts, and the paste used for joining. The joining process of different base materials, the mechanical properties, and the microstructure of sinter-joint samples are presented.

In Situ Tensile Deformation of TRIP Steel / Mg-PSZ Composites

Composite material on the basis of a TRIP (transformation induced plasticity) steel with zirconia particles as reinforcement was produced by powder metallurgical technology and conventional sinter process. The goal of such type of material is to obtain exceptional mechanical properties like high deformation energy absorption due to the combination of martensitic phase transformations both in steel and ceramic. The steel matrix was made of the commercial steel AISI 304, which shows a deformation-induced martensitic phase transformation from the austenitic phase (fcc) into the α’-martensite (bcc). The zirconia particles were partially stabilized with MgO and show a stress-assisted martensitic phase transformation from the tetragonal to the monocline phase. Flat specimens were tensile deformed in-situ in a scanning electron microscope in order to follow the damage behaviour of the material. Some zirconia particles were characterized before and after tensile testing both by backscattered electron contrast as well as by electron backscatter diffraction (EBSD) in combination with energy dispersive X-ray spectroscopy (EDS).