Solid State Phenomena Vols. 172-174

Abstract: Ti-based alloys present unique properties and hence, are employed in several industrial segments. Among Ti alloys, β type alloys form one of the most versatile classes of materials in relation to processing, microstructure and mechanical properties. It is well known that heat treatment of Ti alloys plays an important role in determining their microstructure and mechanical behavior. The aim of this work is to analyze microstructure and phases formed during cooling of β Ti-Nb-Sn alloy through different cooling rates. Initially, samples of Ti-Nb-Sn system were prepared through arc melting furnace. After, they were subjected to continuous cooling experiments to evaluate conditions for obtaining metastable phases. Microstructure analysis, differential scanning calorimetry and X-ray diffraction were performed in order to evaluate phase transformations. Depending on the cooling rate and composition, α” martensite, ω phase and β phase were obtained. Elastic modulus has been found to decrease as the amount of Sn was increased.

Abstract: We have performed in-situ magnetization and high-energy X-ray diffraction measurements on two aluminum-based TRIP steels from room temperature down to 100 K in order to evaluate amount and stability of the retained austenite for different heat treatment conditions. We have found that the bainitic holding temperature affects the initial fraction of retained austenite at room temperature but does not to influence significantly the rate of transformation upon cooling.

Abstract: The development of ferromagnetic shape memory for practical applications needs to overpass brittleness issues, in addition to the control of the magnetoelastic domains. The Co-Ni-Ga system can provide adequate structural particularities to increase the ductility. This paper reports on structural observations of the martensitic transformation in a Co₂NiGa alloy, in the as-cast and in plastically deformed state. Characterization has been performed before and after the heat treatment, using in-situ X-ray diffraction, optical and electron microscopy, as well as DSC measurements performed on heating and cooling of the samples. The observations show a β + γ two-phase structure that can be further influenced by quenching. The structural contribution on the deformation capacity of the alloys is analyzed, based on the changes in the pattern of transformation. Severe plastic deformation by cold rolling leads to the disappearance of the thermoelastic phase transformation.

Abstract: Electric welding with energy stored in condensers offers the advantage of a reduced welding power and a better balance of the energy supplied in the joining area of the components. It can also supply hard thermal regimes in a short time, while the heat is generated only by the contact resistance. This is a major advantage for welding materials with high thermal conductivity, among which are the copper “based shape memory alloys. Since the energy accumulation only needs a few seconds and is released in very short time, it has favorable influence on welding, so that the effect on the shape memory effect is reduced. The paper details the process of electric stored-energy point welding, the structural and phase transitions in the welded seam and in the heat affected zone, as well as the influence of the welding process on the shape memory effect.

Abstract: Nanostructured, low temperature bainitic steels with remarkable combination of ultimate tensile strength of about 2.5 GPa and high uniform elongation have been developed in the recent decade. To accelerate the bainitic transformation in theses steels, new chemical composition was designed by MUCG83 thermodynamic model which was developed in Cambridge University by Bhadeshia. The samples, prepared from the hot rolled ingots, were transformed isothermally at the temperature range of 200 to 300°C for different times. The microstructure and mechanical properties of the samples were evaluated by XRD, SEM, hardness and tensile tests. The results showed a unique microstructure and good mechanical properties like other bainitic steels, while the needed time for the completion of transformation is shorter compared to that of others.

Abstract: This work presents a method of calculating the martensite fraction of an Fe-alloy, usingcooling curve analysis (CCA). It is based on a differential heat balance equation which takes intoaccount only convective exchange with the surroundings. By measuring a T(t) curve of an Fe-alloyand solving numerically the differential heat balance equation the martensite fraction can be calcu-lated. It is found that calculated martensite fraction using this methodology is comparable with resultsobtained using electron backscattering diffraction (EBDS).

Abstract: Two different kinds of experimental approaches yielding three-dimensional structural information on metastable semi-coherent precipitates are demonstrated. By combining high-resolution images from two independent viewing directions a full description of the strain field surrounding a nano-sized Ni₄Ti₃ precipitate in Ni-Ti can be obtained. The principal axes and strains correlate well with the transformation strain of the observed R-phase transformation close to the precipitate. Using a slice-and-view procedure in a FIB/SEM dual-beam instrument, a three-dimensional voxel dataset is produced from which morphological and distributional information on the same precipitates can be obtained yielding new insight into the particular transformation paths of these alloys, relevant for their functional behaviour.

Abstract: Morphology of phase-separated microstructure consisting of cubic disordered A1 and ordered L1₂ phases and tetragonal ordered D0₂₂ phase in Ni-V-X (X=Al, Si) alloys has been investigated by transmission electron microscopy. Ternary Ni-V-Al alloy showed the lamellar structure of D0₂₂ phase with spherical L1₂ particles, while Ni-V-Si ternary alloy indicated the anisotropic microstructure consisting of plate- or diamond-shaped D0₂₂ and plate-shaped L1₂ phases. When a part of element Al is substituted by Si, the morphology of three-phase microstructure (A1/L1₂/D0₂₂) varied depending on the amount of element substitution. When Ni-16.1V-4.0Al-2.8Si (at.%) alloy was aged at 1173K for long time, D0₂₂ plate-like particle aligned along <110> direction and L1₂ particles formed between them in the shape of sphere. In the case of Ni-16.6V-2.8Al-4.0Si (at%) alloy more substituted for Al by Si, cuboidal L1₂ phases were first formed aligning along <100> direction and then plate-shaped D0₂₂ phases precipitated along the <100> direction in the channel of cuboidal L1₂ particles. Such morphological changes with the substitution of the third elements X by Al or Si were able to be explained based on lattice mismatch.

Abstract: The motion of steps during the growth of hydride precipitates has been observed by in situ transmission electron microscopy. Precipitates in different orientation relationships (OR) are shown to obey to the rules of three-dimensional edge-to-edge matching. They form clusters in order to realize a more isotropic distribution of the volume expansion, and to decrease their total elastic energy.

Abstract: The influence of a crystallographic symmetry break on the morphology of precipitates during the coherent precipitation of a trigonal phase in a close packed hexagonal matrix is analyzed. It is pointed out that in spite of the isotropy of the stress free strain of the precipitate in the basal plane, the existence of an extra elastic constant in the precipitate (associated to the loss of symmetry) induces a morphological evolution from a shape having a symmetry of revolution around the threefold axis to a needle-like one oriented along the compact directions in the basal plane. These general considerations are applied to the case of zêta zirconium hydrides the crystallography of which has been recently identified to be coherent with that of the alpha Zr matrix. The influence of symmetry break and elastic heterogeneity on precipitation morphology has been numerically addressed by using different approaches. An analytical approximation to the elastic energy based on Eshelby equivalent inclusion method allows obtaining a qualitative criterion to determine the occurrence of a shape bifurcation of zêta hydrides.