Solid State Phenomena Vols. 172-174

Abstract: A micromechanical model was developed to account for the particular microtexture of upper bainite in low alloy steels, i.e. the non-random spatial distribution of variants within a given former austenite grain. A self-consistent scheme and an Eshelby approach, considering both transformation shape strain and viscoplastic strain as eigenstrains, was applied to estimate coupling between parent austenite and two or more bainite variants without any applied stress. Model predictions concerning self-accommodation between variants are sensitive to the plane of the first “lattice invariant shear” in the crystallographic model used to determine the shape strain. No obvious effect of the constitutive equations of phases and of the other model parameters was found.

Abstract: Spontaneous self-assembling of interacting atoms prototyping crystallization and reconstructive phase transitions in crystalline state is discussed in terms of Atomic Density Function (ADF) theory. It is demonstrated that a multi-mode potential can reproduce any crystal structure. 3D computer modeling of formation of the FCC crystal and crystal lattice rearrangement in the FCCàBCC transformation is presented.

Abstract: Recovery processes responsible for evolution of microstructures in 0.1mm thin cold-drawn Ni-Ti shape memory alloy wire heat treated by DC electric pulse were investigated by combination of in-situ tensile stress - strain, electrical resistance and X-ray diffraction measurements. The X-ray data were used to obtain direct experimental information on the evolution of the phase fractions, internal strain and defects in the microstructure evolving through activation of a sequence of recovery processes during the short time electropulse treatment. It is shown that superelastic functional properties of the treated Ni-Ti wire can be precisely set by controlling the progress of the recovery processes by prescribing the time evolution of temperature T(t) and tensile stress s(t) (displacement control) in the treated wire.

Abstract: A 0.79C-1.5Si-1.98Mn-0.98Cr-0.24Mo-1.06Al-1.58Co (wt%) steel was isothermally heat treated at 350°C bainitic transformation temperature for 1 day to form fully bainitic structure with nano-layers of bainitic ferrite and retained austenite, while a 0.26C-1.96Si-2Mn-0.31Mo (wt%) steel was subjected to a successive isothermal heat treatment at 700°C for 300 min followed by 350°C for 120 min to form a hybrid microstructure consisting of ductile ferrite and fine scale bainite. The dislocation density and morphology of bainitic ferrite, and retained austenite characteristics such as size, and volume fraction were studied using Transmission Electron Microscopy. It was found that bainitic ferrite has high dislocation density for both steels. The retained austenite characteristics and bainite morphology were affected by composition of steels. Atom Probe Tomography (APT) has the high spatial resolution required for accurate determination of the carbon content of the bainitic ferrite and retained austenite, the solute distribution between these phases and calculation of the local composition of fine clusters and particles that allows to provide detailed insight into the bainite transformation of the steels. The carbon content of bainitic ferrite in both steels was found to be higher compared to the para-equilibrium level of carbon in ferrite. APT also revealed the presence of fine C-rich clusters and Fe-C carbides in bainitic ferrite of both steels.

Abstract: A hybrid in-situ observation system has been developed to study the phase transformation behaviour simultaneously in both real and reciprocal lattice space. This paper presents the development of the observation system. Furthermore, as an example of the application of our developed system, martensitic transformation of Cr–Ni steel along a designed thermal cycle was in-situ tracked with the developed system. As a result of analysing the time-resolved X-ray diffraction data for the observed target, our system could directly detect the effect of transformation strain on austenite during martensitic transformation.

Abstract: The applications of laser scanning confocal microscopy (LSCM) system were introduced to the track the phase transformation of steel. For the low and high carbon steel, the microstructural change along various thermal cycles was directly observed with LSCM system. The nature of non-themoelastic martensite was directly observed. Furthermore, direct checking of the nucleation site of ferrite phase was carried out for aluminum killed weld metal and titanium killed-steel. It was carried out concerning to acicular ferrite formation in high strength and low alloy steel. The assessment of the pinning effect of alumiunu nitride on the austenite phase at high temperature was also carried out by using LSCM system.

Abstract: Interface reactions between liquid aluminium and ZnO single crystal substrates of <1-100> orientation (at 1273 K under vacuum) were examined using scanning and transmission electron microscopy techniques. The substrates were subjected to the “pushing drop” tests when liquid is deposited from the capillary on the substrate surface and then, after appropriate contact time, it is pushed away. After short time of interaction with <1-100>ZnO substrate, three phases were detected: α-Al₂O₃, the alumina of unknown type and ZnAl₂O₄ spinel formed due to the solid state reaction between Al₂O₃ and ZnO.

Abstract: The reaction product region, formed between molten aluminium and MgO and MgAl₂O₄ single crystals of three different crystallographic orientations, was investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) coupled with X-ray energy dispersive spectrometry (EDS). The Al/MgO and Al/MgAl₂O₄ couples were produced under ultra high vacuum at 800, 900 and 1000°C. The observations proved the redox reactions of Al with both MgO and MgAl₂O₄. Independently of crystallographic orientation of initial oxide single crystals, the reaction product region (RPR) was formed and it was built of oxide particles surrounded by continuous metallic phase. For Al/MgO couples, the RPR was composed of two layers, where in the first layer, the oxide phase was Al₂O₃ while in the second layer, the MgAl₂O₄ was identified. In the case of Al/MgAl₂O₄ couples, a single layer was distinguished and only the Al₂O₃ phase was recognized.

Abstract: The localized plastic flow auto-waves observed for the stages of easy glide and linear work hardening in a number of metals are considered. The propagation rates were determined experimentally for the auto-waves in question with the aid of focused-image holography. The dispersion relation of quadratic form derived for localized plastic flow auto-waves and the dependencies of phase and group rates on wave number are discussed. A detailed comparison of the quantitative characteristics of phase and group waves has revealed that the two types of wave observed for the stages of easy glide and linear work hardening are closely related. An invariant is introduced for localized plastic flow phenomena occurring on the micro- and macro-scale levels in the deforming solid.

Abstract: Ni₄Ti₃ precipitates with a heterogeneous distribution growing in a polycrystalline Ni_50.8Ti_49.2 alloy have been investigated in a Dual-Beam FIB/SEM system. The volume ratio, mean volume, central plane diameter, thickness, aspect ratio and sphericity of the precipitates in the grain interior as well as near to the grain boundary were measured or calculated. The morphology of the precipitates was classified according to the Zingg scheme. The multistage martensitic transformation occurring in these kinds of samples is interpreted in view of the data of this heterogeneous microstructure of matrix and precipitates.