Materials Science Forum Vol. 1016

Abstract: Stainless steels are indispensable materials in many industrial fields. They can be easily shaped and joined by traditional welding methods. Some problematics such as possible decrease in corrosion resistance at the welding bead and in the heat-effected zone, residual stress, crack formation and distortions may take place after welding. Friction Stir Welding (FSW) may be used for joining stainless steels in a single pass and for optimising microstructure and mechanical properties of the processed region. The application of FSW to the widely used AISI304 stainless steel is investigated in food implants. The mechanical properties together with corrosion resistance and surface finishing are characterized. A high energy input is chosen for the welding (2000 rpm tool rotational speed and 50 mm/min advancing speed). The stirred zone (SZ) is characterized by optical microscopy. Vickers microhardness in the SZ results 37% higher than in the base material. Tensile tests highlight elongations up to 40% keeping maximum stress values at 600 MPa. All samples pass accelerated corrosion tests that simulate 20 years of cleaning cycles in a typical food implant.

Abstract: Liquid phase sintering is most widely known in its variant „persistent liquid phase sintering“, in which case the liquid phase is present in constant quantity during the entire isothermal period. There is however also the variant „transient liquid phase“, the melt being present only for a short period in the first stage of sintering and then solidifying through diffusional processes. In this presentation, the preconditions for both variants are presented, in particular with regard to the starting materials. The benefits of transient liquid phases are described, both for sintering – to accelerate material transport, contact formation and microstructural homogenization compared to standard solid state sintering – and for transient liquid phase bonding, a brazing variant which is an attractive method for joining porous powder compacts. Both techniques are highly useful in particular for ferrous powder metallurgy precision components, etc.

Abstract: In this paper the effect of normalizing and tempering (N&T) thermal treatment on mechanical properties of a novel 7.0% Cr steel for forged components is studied. The main innovation is in the increased hardenability following the higher Cr content with respect to the more common 5% Cr steel allowing to lower the content of other chemical elements aimed to achieve the target mechanical properties. Results show that in the case of 7.0% Cr steel, following the high intrinsic hardenability, the austenitic grain size effect on microstructure after cooling is really poor.

Abstract: The effect of thermo-mechanical treatment on the microstructural evolution of low carbon micro-alloyed high strength steel was studied by combining prestrain with tempering (PST) in this paper. It was found that the prestrain causes the dislocation to plug up around the grain boundary and carbide, resulting in carbide boundary fragmentation. Moreover, it breaks the thermo-dynamic equilibrium between the matrix and carbide, induces the dissolution of carbon in the high energy state, and then changes the distribution of carbon in the matrix. In the subsequent tempering process, the precipitation regularity of carbide was changed, which promoted the precipitation carbide at low temperature. The influence of carbide precipitation on dislocation can be divided into two stages: the first stage was precipitation induced creep, which promoted stress relaxation; the second stage was precipitation pinning dislocation, which improved material strength and inhibited stress relaxation.

Abstract: Detonation spraying is used for the binary fuel spraying, C₂H₂/C₃H₈/O₂ of WC/Co and Cr₃C₂/NiCr powder. The particles-in-flight temperature and velocity are calculated and optimized. Spraying distance is varied from 50 to 400 mm and substrate inclination relatively spraying direction is varied up to 60o. Coating properties: microstructure, microhardness, porosity, wear resistance are measured and it is found that binary fuel detonation spraying provides high coating quality. For example, for the WC/Co (88/12 wt%) coatings: porosity is less than 0.7%; microhardness is about 1500 HV₃₀₀; ASTM G65 abrasion wear is 1.17 mm³/1000 rev when using a corundum powder as an abradant. For the Cr₃C₂/NiCr (75/25 wt%) coatings porosity is less than 1.7%, microhardness is about 800 HV₃₀₀, and abrasion wear is 2.7 mm³/1000 rev. It is found that the coating performance does not decrease strongly with the substrate inclination up to 30o. The industrial applications include machinery, aircraft, petrol, gas turbines domains, etc.

Abstract: It is evident that the interface in MMCs plays a crucial role with respect to thermophysical and mechanical properties of the composites. Inert systems like copper/carbon or silver/carbon will have low performance due to a very weak bonding between the constituents, whereas in reactive systems like Al/carbon, Fe(Ni)/WC and WC-Co/diamond the interfacial reaction has to be clearly controlled to avoid uncontrolled reactions. Such reactions may lead to partial or even complete dissolution of the phases and thus can have very detrimental impact on properties. In this contribution, we will summarize and present different approaches and recent results to overcome any interfacial problems. This can be either technological parameters like time, temperature, rate of consolidation, contact time in infiltration or inherent parameters like nominal composition, coatings, and surface terminations. It is of general interest to adjust optimal interfacial conditions for each application to achieve ideal properties in the composites.

Abstract: The evolution of microporosity in single-crystal nickel-base superalloy CMSX-4 during hot isostatic pressing has been investigated by high resolution tomography at the European Synchrotron Radiation Facility in Grenoble. The kinetic dependencies of microporosity annihilation in the superalloy in initially as-cast and homogenized conditions were obtained. It was shown that smaller homogenization pores of about 5-10 μm in size are rapidly annihilate during hot isostatic pressing, while annihilation of larger solidification pores of size up to a few hundred micrometer takes a long time. After commercial hot isostatic pressing at 1288 °C, 103 MPa, 4 h only rare pores smaller than 20 μm remain, which are not critical for fatigue strength.

Abstract: The recent revitalization of Ioffe plots (entropy conductivity versus electrical conductivity) reminds us that Isotan (Cu55Ni44Mn1) is an outstanding thermoelectric material with a power factor of up to 60 W cm^-1 K^-2 at a specific electrical conductivity of almost 20,000 S cm^-1 at elevated temperature. Even though, Isotan is widely used in thermoelements for temperature measurement, its high open-circuited thermal conductivity of approximately 70 W cm^-1 K^-2 [1] hindered further research as a promising thermoelectric material. Isotan was chosen as a starting composition. Influence of partial substitution of Cu and Ni with heavy elements (Sn,W) on the thermoelectric properties was studied. The alloys were fabricated by arc-melting and microstructurally characterized for grain size and elemental composition by scanning electron microscope (SEM) combined with energy-dispersive X-ray (EDXS). Lattice symmetry and parameters were estimated by X-ray diffraction (XRD). Functional properties as Seebeck coefficient, electrical conductivity and power factor were used to evaluate the thermoelectric performance.

Abstract: Harmonic structured composites consist of a low fraction metal region like network and a dispersed another major metal region like island. The harmonic structured composites were produced via mechanical milling (MM) followed by spark plasma sintering (SPS), and its mechanical and thermal properties were investigated in detail. Microstructural observation of the MM powders and SPS compacts was achieved using scanning electron microscopy (SEM). The mechanical properties of the harmonic structured composites were evaluated using results of the Vickers hardness and the tensile tests. The thermal properties of a part of the harmonic structured composites were evaluated using results of thermo-mechanical analysis and laser flash method. High speed steel / mild steel harmonic structured composite exhibited high strength and enough ductility in spite of the trade-off relationship between strength and ductility. In addition, the high speed steel / mild steel harmonic structured composite also demonstrates a superior wear properties and low hardness simultaneously. On the other hand, molybdenum / copper harmonic structured composite demonstrate low coefficient of linear expansion and enough thermal conductivity compared to the conventional copper / molybdenum particle dispersed composite. The coefficient of linear expansion and thermal conductivity are the trade-off relation in this composite. In summary, the harmonic structure control is effective for improvement of the trade-off mechanical and thermal properties in the composite.

Abstract: Thermoset adhesives convert from liquid to solid due to chemical reactions. Once cured, these adhesives carry the potential to create strong load-bearing joints, resisting even severe detrimental service conditions. In the progress of curing of a thermoset adhesive the viscoelastic properties of the resin and hardener formulation change as the chemical reaction proceeds. Gelation occurs once a continuous 3-dimensional network of polymer chains has been created. After gelation, the microstructure of the resin is fixed and further cure is affected by diffusion limitations [1]. Mastering of the curing kinetics and the physicochemical changes in the transition from the liquid to the solid-state is essential to reliably process adhesives in industrial applications. Rheological experiments in parallel plate configuration have become a well-established practice in investigating viscoelastic properties in the progress of curing. In practice, it has shown to be challenging to access the full range of viscoelastic parameters of thermoset resins with a low initial viscosity from the very beginning of the curing reaction to the post-cure consolidation of the vitrified polymer. This paper will discuss experimental methods and criteria for the viscoelastic analysis of curing thermoset adhesives and present experimental data of the time-, temperature-, and frequency-dependent viscoelastic properties of a curing thermoset epoxy in relation to the features of its time-temperature-transformation-diagram.