Materials Science Forum Vols. 825-826

Abstract: A novel and innovative process for full as well as hollow metal-matrix composite bead fabrication using gel-casting process by alginate gelation is investigated. In particular the influence of four different alginates and various solidifying agents have been investigated regarding the formation and stability of full and hollow beads. The suspension contained a metastable austenitic steel powder (16Cr7Mn3Ni) and zirconia particles as well as different alginates and was added dropwise into water containing different solidifying agents for forming beads. With the aid of sodium and potassium alginate dropped in solution with CaCl₂, full metal beads have been obtained. Hollow beads have been produced using potassium alginate and Ca (OH)₂ as solidifying agent and show a hollow space fraction of 65%. At lower strains (up to 15 %), all zirconia reinforced full steel beads obtain higher specific energy absorption (SEA) in comparison to pure steel beads.

Abstract: The paper describes a cost effective and innovative combination of direct pressure sintering and subsequent thixoforming to produce MMC-components in (near-) net shape quality and, thus, to make these interesting materials attractive to mass production.First results of some combinations of aluminium matrix alloys with different ceramic reinforcements, consolidated by fast pressure sintering show the efficiency of this technology.The further processing of the consolidated billets has been performed by thixoforging. It can be shown, that the homogeneous microstructure from the direct pressure sintering stage with uniformly distributed ceramic reinforcements can be maintained over the semi-solid state and a full densification can be achieved. Form filling was complete and surface quality was comparable to forgings from conventional alloys.This new process flow shows advantages regarding the material yield in each of the processing steps. The (near-) net shape quality of thixoforged components allows a reduced effort for machining, which is of special importance for composite materials with a high content of wear resistant hard phases like SiC-particles.

Abstract: Metal Matrix Composites (MMC) based on a TRIP (TRansformation Induced Plasticity)- or TWIP (TWinning Induced Plasticity)-steel matrix reinforced with MgO-partially stabilized zirconia (Mg-PSZ) are an interesting research field as both components exhibit a deformation-induced or stress-assisted martensitic phase transformation and twinning, respectively. The present work deals with the fatigue characteristics of a reinforced CrMnNi-steel as a function of the ceramic particle size. Therefore, the particles were classified into three grades (grade 1: <10 μm; grade 2: 10-30 μm; grade 3: 30-50 μm) whereas the volume fraction concerning the composite material was kept constant at 10 vol.%. The composites were produced using the hot pressing technique. The tests were performed under total strain control in a range of 0.2% ≤ Δε_t ≤ 1.2%. The microstructure of fatigued specimens was examined using scanning electron microscopy.

Abstract: Composite materials, which consist of a metastable austenitic TRIP-steel matrix (CrMnNi TRIPsteel; TRansformation Induced Plasticity) reinforced by alumina particles (25 vol.% ceramic, designated as AT 25/75) and reinforced by alumina and MgO partially stabilized zirconia particles (Mg-PSZ) (35 vol.% ceramic, designated as AT 25/75 + MgPSZ) were synthesized through spark plasma sintering (SPS). In the AT 25/75 + MgPSZ, the steel particles were mainly surrounded by alumina. Hence, mostly steel/alumina and alumina/MgPSZ interfaces existed. The mechanical behavior of the as-sintered samples was characterized by compression tests at room temperature and 40 °C and in a range of strain rates between 10³ s^-1 and 10³ s¹. The influence of the ceramic content, strain rate and temperature on TRIP-effect of the steel matrix was investigated. Due to the increasing ceramic volume fraction, AT 25/75 + MgPSZ exhibits the highest compressive yield strength under all loading conditions and no strain rate sensitivity. This composite showed no measurable TRIP-effect, due to the low fracture strain. The deformation-induced α’martensite within the steel particles in pure steel and AT 25/75 primary depends on the testing temperature and the strain rate. This is attributed to an increase of stacking fault energy with rising temperature. High strain rates cause adiabatic heating, counteracting the martensitic transformation.

Abstract: Composites with interpenetrating metal-ceramic microstructures (IPC, interpenetrating composites) can be tailored for specific applications, such as high thermal conductivity combined with low thermal expansion, e.g. for heat sinks. Heat sinks are required in power electronic devices or in future fusion reactor technology where extreme conditions and high cyclic thermo-mechanical loads appear. Due to its rigid ceramic backbone IPCs are expected to reveal high thermal stability. Pure silicon carbide exhibits high thermal conductivity, low coefficient of thermal expansion, high corrosion and wear resistance. But it is also known as a very brittle material when mechanical loads are applied. Thus a composite of silicon carbide with ductile and highly conductive copper seems to be a promising new material for a number of applications.This paper reports the synthesis of Cu-SiC composites using a unique high temperature squeeze casting process (HTSC). Microstructural design of SiC-preforms with open porosity and its synthesis progress is reported. Influence of preform properties, temperature, pressure and atmosphere during HTSC were investigated. A qualitative and quantitative description of the microstructure of the composites and their composition allows the creation of structure-property correlations that take effect retroactively to the casting process.

Abstract: Two different systems, the non-reactive Ag–diamond and the reactive Al–diamond system, were assessed by their thermal conductivity behaviour, both were fabricated by gas pressure assisted infiltration of densely packed diamond bulks with aluminium or silver and different Si-concentration and diamonds of varying particle sizes. The effect of Si-concentration on the interface thermal conductance h between Al, Ag and diamonds was investigated in dependence of temperature by measuring thermal conductivity of composites with different sized diamond particles in the temperature range from 4 K up to ambient. Composite thermal conductivities κ_c(T) can be as high as 860 W m^-1 K^-1 at roughly 100 K for Al/diamond and 1100 W m^-1 K^-1 for Ag–Si/diamond at approx. 150 K. Although the Si concentration in the matrix plays an eminent role for κ_c(T), i.e. the lower the Si concentration, the higher κ_c(T), interface thermal conductance is almost unaffected in the reactive Al-diamond system. Furthermore, they are close to values determined on clean model systems, i.e. sputtered and evaporated metal layers on diamond monocrystals. For Ag–diamond composites, the matrix composition of Ag–1Si seems to reflect an optimal composition, as the highest thermal conductivity κ_c(T) and an extraordinary higher interface conductance was achieved compared to Ag–3Si/diamond composites.

Abstract: The integration of functions in lightweight structures features great potential for future applications in diagnosis and control. The combination of shape memory wires or ribbons made of NiTi embedded in aluminium and manufactured by composite extrusion offers the possibility to produce a composite actuator material in a single production step. The extrusion process allows a wide range of shapes and provides higher versatility than actuators made of bi-metals. The transformation temperature of NiTi varies depending on the composition of the alloy, between -100 °C and 100 °C. However, NiTi can also transform stress-induced. In the designated application, a force is applied via the interface onto the matrix material to deform it. Due to the resulting stress, the transformation temperature rises to temperatures higher than those of the unloaded material. Furthermore the production of composite extrusions leads to a significant heat input on the shape memory alloys followed by another increase of the transformation temperature.Therefore it is essential to reproduce the heat treatment and the stress-induced transformation to predict the transformation temperature in the resulting composite influenced by the interface. For that purpose, the wire gets annealed in a furnace with different durations at a temperature similar to that of the bar extrusion process. After this, the transformation temperatures can be observed at various stresses to evaluate their applicability for aluminium composite actuators.

Abstract: This article focuses on the development of phenolic resin moulding materials for the production of new carbon fibre-reinforced ceramic composite materials based on C/C-SiC by utilising the LSI (liquid silicon infiltration) production method. The production of these moulding materials is being accomplished by combining phenolic resin and carbon fibres with the addition of a few selected parts of processing aids, during which the influence of the used lubricants on the processability of the moulding materials is examined. The starting materials, microstructures and mechanical properties of the materials were characterised at every step of the entire process (CFRP and C/C composites) as well as the end of the whole production (C/C-SiC composites). During this investigation a link between the portions of the lubricant used, the forming of the porosity and the impact on the mechanical properties was discovered. In regards to the optimisation of the process the involved parties were able to determine an optimal lubricant ratio.

Abstract: SiC/SiC ceramics consist of silicon carbide fibres embedded in a silicon carbide matrix. As an alternative to classic CVI and PIP routes, Liquid Silicon Infiltration (LSI) was chosen as a technique with short process times to obtain composites with low porosity. Silicon carbide composites show good thermal shock resistance, a low coefficient of thermal expansion and excellent physical and chemical stability at elevated temperatures and are therefore regarded as promising candidates for various applications in jet engines and in power engineering. To build up the matrix, different phenolic resin based carbon precursors were infiltrated in fibre preforms and thermally cured, pyrolysed and siliconized. The aim is to obtain a high carbon yield during pyrolysis and to control the pore morphology in a way that the following liquid silicon infiltration leads to a complete reaction of the carbon matrix with silicon to SiC. The siliconization behaviour and conversion into SiC in dependence of pore morphology and chosen precursor is analysed.At the same time a functional fibre coating has to be developed which protects the fibres from liquid silicon and simultaneously provides a weak fibre matrix bonding. A LPCVD-SiN_x fibre coating has been chosen and is investigated in fibre composites especially in terms of protection and reactivity in different atmospheres during pyrolysis and siliconization.

Abstract: C/C-SiC composites, fabricated by the Liquid Silicon Infiltration process (LSI), typically use phenolic resin-based thermosets as carbon precursors. In contrast to this, two different thermoplastics (Polyetheretherketone PEEK and Polyetherimide PEI) were examined for their suitability as carbon precursors for C/C-SiC composites. The carbon fiber surfaces were pretreated between 400 °C and 800 °C in nitrogen atmosphere to modify the fiber/matrix bonding. The microstructures of the materials show an increasing SiC content with increasing fiber pretreatment temperature. The flexural strength of the resulting material was determined by 4-point-bending tests.