Materials Science Forum Vol. 762

Abstract: This paper presents the research on the flow characteristics of the Ti-6V-4Al alloy in wide ranges of temperature (725 ‑ 950 °C) and strain rate (10^-5 ‑ 10^-2 s^-1). The material processing maps were constructed based on the basis of dynamic materials model (DMM) developed by Prassad and modified by Narayana Murty. For the construction of such maps the data of the material flow stress at different temperatures and strain rates is necessary. To obtain such data the stepped tensile tests which allow obtaining the stress - strain rate dependence at a given temperature are ideal. The experiments conducted consist of the tensile test series at various temperatures with stepped change of the deformation rate. By the results of these tests the constitutive equations, which describe relationship between stress and strain rate for each temperature, were obtained. The data was analyzed in terms of the two different approaches proposed by Prassad and Narayana Murty to assess the impact of deformation conditions on the formability and flow stability of the material. Based on these approaches, the processing maps, which enable identifying the conditions of the Ti-6V-4Al alloy superplasticity, were constructed.

Abstract: The applicability of a physical model to describe the austenite microstructure evolution after hot deformation in High-Mn steels was investigated. Double-hit torsion tests were carried out to determine the softening behaviour of two High-Mn steels, one of them microalloyed with 0.11 wt% Nb. The values of the unknown parameters included in the model were determined by fitting experimental results. The model describes adequately the softening evolution of the steels. At high temperatures recovery and recrystallization contribute to mechanical softening, the latter having the main contribution. In contrast, when strain-induced precipitation occurs recovery has a larger effect.

Abstract: The highly interesting properties of high manganese steels can be further improved by microalloying. The introduction of carbon-nitride precipitates improves the yield strength and the microstructural control during the production process. Due to the high manganese content in these austenitic steels significant changes in the precipitation behaviour have to be expected in comparison to conventional carbon-manganese steels. However, although crucial for steel design, this has not been systematically described before. Preliminary results showing the effect of Nb and V are presented. Namely the softening behaviour is related to the precipitation state. In summary this allows describing the precipitation-time-temperature evolution and provides the necessary background for the alloy and process design.

Abstract: The fatigue cracking mechanisms of two high Mn TWinning-Induced Plasticity (TWIP) steels are investigated in detail using electron channelling contrast imaging (ECCI) and electron backscatter diffraction (EBSD). Furthermore, the fracture surfaces of the fatigued steels have been studied by employing a field emission gun scanning electron microscope (FEG-SEM). The fine details of the fatigued surface topography are verified using an atomic force microscope (AFM). The results indicate that the fatigue crack embryos nucleate at an early stage of the fatigue life as a result of local straining at grain and annealing twin boundaries at sites, where persistent slip bands create dislocation piled-ups that impinge on boundaries. The EBSD measurements showed that unlike in monotonic straining, the formation of deformation twins is not observed under cyclic straining.

Abstract: The heat treatment process of 31Mn2SiREB cast steel used in crawler shoes is directly lifted from the heat treatment process of Mn13 high-manganese cast steel, i. e., quenching at 1050 oC after casting. The reasonableness of the process needs to be surveyed. In this paper, the effects of quenching temperature and diffusion annealing pre-treatment on mechanical properties, micro-area composition uniformity and micro-hardness uniformity were investigated. For quenching after casting, the tensile strength and impact energy increase observably with the elevation of quenching temperature, but the impact energy at higher quenching temperature is still very small. The fluctuation of micro-hardness and chemical composition at different micro-areas becomes obviously small with the increase of quenching temperature. For quenching after a diffusion annealing pre-treatment, the impact energy is very high and up to 36.3 J.

Abstract: The influences of the heating rate and annealing duration on martensite formation and its reversion to austenite have been investigated in a 60% cold-rolled metastable high-manganese austenitic Type 204Cu stainless steel. A Gleeble 3800 thermomechanical simulator was used for dilatometric measurements. Cold-rolled steel pieces were either heated up to 1000 °C at various heating rates between 5 °C/s and 150 °C/s followed by quenching to room temperature, or heated and held at temperatures in the range of 450 620 °C for different durations between 0.1 600 s. In heating experiments, dilatation curves revealed an expansion of a specimen starting around 550 °C followed by contraction, both processes depending on the heating rate. These dimensional changes could be correlated to the formation and reversion of a ferromagnetic phase, α-martensite. Some martensite was also formed during isothermal holding in connection with tempering of the pre-existing α-martensite before the following reversion, as established by magnetic measurements. Tempering of martensite was revealed by microhardness behaviour, X-ray diffraction analysis and transmission electron microscopy.

Abstract: The tensile curves of AISI 316L deformed at temperatures between 700 and 1000 °C in the strain rates range 10^-5-10^-2 s^-1 are modelled with the Voce equation, starting from strain hardening analysis. The parameters, needed to draw the Voce equation, are the saturation stress σ_V, the critical strain ε_C and the stress σ_o, that respectively define the height of the flow curve, the velocity to achieve σ_V and namely the back-extrapolated flow stress to zero strain. A two-parameter model of strain hardening recently proposed [ is used to analyze the strain hardening rate, dσ/dε, vs. the flow stress, σ. Through this analysis, σ_V, ε_C and the thermal activation of plastic flow s are obtained. In fact, the two-parameter model assumes that s and the total dislocation density ρ are the only two parameters needed to describe strain hardening. It has been reported [ that the parameter s can be parameterised in terms of strain rate and temperature and, furthermore, relationships between σ_V, σ_o, ε_C and s can be established. At this stage, the Voce equation can reproduce the experimental tensile curves at the explored temperatures and strain rates. However, the obtained Voce equations can well describe the tensile curves at large strains, while significant discrepancy occurs at small strains [. Preliminary results of an improved model based on two coupled differential equations with physical meaning are reported to correct this discrepancy at low strains.

Abstract: Smart-Cut^® is an innovative and highly efficient technology to fabricate high quality Silicon-on-Insulator (SOI) wafers, especially when the top film of SOI wafers is very thin. In the present paper, a fracture mechanics model is established to examine the effect of bonding flaws on defect growth in the Smart-Cut process. It is found that although defect growth can occur in a practical Smart-Cut process, large bonding flaws are inclined resulting in severe deviation of the direction of defect propagation, leading to a non-transferred area of thin film when splitting. Moreover, at the expense of low defect growth, increasing the internal pressure of bonding flaws decreases the defect growth deviation and thus benefits to improve the quality of final SOI wafer. The mechanism of relaxation of stiffener constraint is proposed to clarify the effect of bonding flaws. Finally, progress of the splitting process is analyzed when bonding flaws are present.

Abstract: We describe recent progress in first principles materials modelling applied to iron alloys. First principles methods in general have proven to be an effective way of describing atomic level phenomena in solids. When applied to alloys with chemical disorder, however, the widely used supercell methods turn out to be impractical due to the vast variety of different possible configurations. This problem can be overcome using the coherent potential approximation (CPA), which enables the description of a multicomponent alloy in terms of an effective medium constructed in such a way that it represents, on the average, the scattering properties of the alloy. A bulk alloy, in the case of substitutional random alloys, can thus be described with a single atom while a slab is needed to describe surfaces. The exact muffin-tin orbitals (EMTO) method provides a first principles method that can be combined with the CPA in order to describe steels and other multicomponent alloys. We describe the EMTO-CPA method and provide examples of both bulk and surface properties that can be modelled with this method.

Abstract: In this paper, dielectric and conductive properties of thermally sprayed Al₂O₃- and Cu-based coatings on steel and alumina substrates were studied. Alumina powders with nanoand micro-sized additions of Ni, NiO, TiO₂, silica, and commercial glass were used in High Velocity Oxygen Fuel (HVOF) deposition. The conventional commercial copper powder and three Ag, WC and H₂ -modified powders were used in Direct Write Thermal Spray (DWTS) deposition. Mixed phases of α-Al₂O₃ and γ-Al₂O₃ were found to be present in the as-deposited coatings. Sprayed alumina-based composites exhibited dielectric permittivity of 5.3-13.9 and losses of 0.002-0.178 at 1 MHz and 1 GHz while the additions tend to increase the values. Sprayed compositions with glass-type additions were found to retain α-Al₂O₃ crystalline phase after the deposition. Cu depositions, especially modified ones, realised by Direct Write Thermal Spray (DWTS) showed conductivity values as high as 4256 % of IACS values. The results demonstrate that ceramic and conductive coatings fabricated by thermal spray techniques show feasible properties for electrical applications, such as low-frequency components and insulation layers to be utilised in embedded 3D circuitry, in a way that is not possible through traditional manufacturing methods.