Interaction effects and transport properties of Pt capped Co nanoparticles

We studied the magnetic and transport properties of Co nanoparticles (NPs) being capped with varying amounts of Pt. Beside field and temperature dependent magnetization measurements we performed delta-M measurements to study the magnetic interactions between the Co NPs. We observe a transition from demagnetizing towards magnetizing interactions between the particles for an increasing amount of Pt capping. Resistivity measurements show a crossover from giant magnetoresistance towards anisotropic magnetoresistance

A molecular dynamics view of hysteresis and functional fatigue in martensitic transformations

Shape memory alloys (SMA) exhibit a number of features which are not easily explained by equilibrium thermodynamics, including hysteresis in the phase transformation and ?reverse? shape memory in the high symmetry phase. Processing can change these features: repeated cycling can ?train? the reverse shape memory effect, while changing the amount of hysteresis and other functional properties. These effects are likely to be due to creation of persistent localised defects, which are impossible to study using non-atomistic methods. Here we present a molecular dynamics simulation study of this behaviour. To ensure the largest possible system size, we use a two dimensional binary Lennard-Jones model, which represents a reliable qualitative model system for martensite/austenite transformations. The evolution of the defect structure and its excess energy is investigated in simulations of cyclic transformation/ reverse transformation processes with 160,000 atoms. The simulations show that the transformation proceeds by non-diffusive nucleation and growth processes and produces distinct microstructure. Upon transformation, lattice defects are generated which affect subsequent transformations and vary the potential energy landscape of the sample. Some of the defects persist through the transformation, providing nucleation centres for subsequent cycles. Such defects may provide a memory of previous structures, and thereby may be the basis of a reversible shape memory effect

ОНОМАСИОЛОГИЧЕСКИЙ ПОДХОД К ИЗУЧЕНИЮ СЕМАНТИКИ ПРОИЗВОДНОГО СЛОВА

Изучение спектра семантических связей и синтаксических валентностей производящего наряду с “сочетаемостными особенностями морфем”, “ограничениями, которые кладутся в комбинаторику морфем их семантическими свойствами выявляет логику и тенденции деривационных репрезентаций актов номинации в синхронном срезе языка. За всем этим закономерно следует перспектива регулирования и управления словообразовательными процессами (т. е. наполнение пропозициональной структуры типизированной лексикой), обсуждаемая сегодня в исследованиях лингвисто

Intrinsic nano-diffusion-couple for studying high temperature diffusion in multi-component superalloys

We present a new approach for the quantitative study of high-temperature diffusion in compositionally complex superalloys on the nano-scale. As key element, the approach utilizes the γ/γ\u27-microstructure itself as intrinsic nano-diffusion-couple (NDC). By establishing equilibrium at one temperature followed by annealing at a different temperature, well-defined transient states are generated which are studied using STEM-EDXS. We demonstrate this approach for a multi-component superalloy of CMSX-4 type. The temporal evolution of element concentrations is consistently revealed for γ- and γ\u27-forming elements and is compared to diffusion simulations based on DICTRA. Excellent agreement is obtained for Ni, Co, and Cr whereas diffusion of Al and, in particular, Re lacks behind in experiment. Finally, it is demonstrated that transient states can also be captured by in situ TEM using chip-based heating devices. The NDC approach offers great opportunities for diffusion studies in compositionally complex superalloys and might be extended to other two-phase multi-component systems

Improving the intermediate- and high-temperature strength of L12_{2}-Co3_{3}(Al,W) by Ni and Ta additions

The effects of Ni and Ta additions on the mechanical properties in the L1$_{2}$ compound Co$_{3}$(Al,W), the strengthening phase of Co-based superalloys, have been investigated by compression tests between room temperature and 1000 °C, in order to elucidate the effects of stability of the L1$_{2}$ phase on the mechanical properties. The additions of Ni and Ta, both of which are L1$_{2}$-stabilizers that increase the L1$_{2}$ solvus temperature, increase the yield strength at intermediate and high temperatures. The strength increase is shown to be more significant as the amount of additions of these elements and thereby the stability of the L1$_{2}$ phase increases. Two factors account for the strength increase at intermediate temperatures: The reduction of the onset temperature of yield stress anomaly (YSA-onset) due to the increased complex stacking fault (CSF) energy and the increase in both the base strength and the intensity of the yield stress anomaly associated with an increased anti-phase boundary (APB) energy on (111) planes. The strength increase at high temperatures, on the other hand, arises from the increase in the peak temperature due to the increased L1$_{2}$ solvus temperatures. The increased strength of the L1$_{2}$ phase due to a higher phase stability thus partly accounts for the improved creep strength of Co-based superalloys upon alloying with Ni and Ta

High Apparent Creep Activation Energies in Mushy Zone Microstructures

Modelling represents an important tool in modern material processing which no longer follows the traditional trial and error route but rather represents what may be termed a right first time technology [1]. To successfully model technological solidification processes, thermodynamic and kinetic data are required. But mechanical aspects are important as well [2]: during solidification, temperature gradients or mechanical constraints imposed by the mold result in solidification stresses. These stresses must be considered for at least the following two reasons: first, they can lead to local air gap formation between metal and mold thus changing heat extraction, cooling rate and finally the cast microstructure [3]; second, at a larger scale they may influence the final product shape [4]. Moreover, they can assist in cavity formation and can produce cracking. Such stresses become important as soon as a significant amount of solid phase has formed during solidification. In principle, these stresses can be calculated using viscoelastic finite element stress analysis [5]. But, finite element calculations require as an input the constitutive law which governs the mechanical behavior. Therefore, there is an interest in mechanical data of solidifying alloys with mushy zone microstructures: Ackermann and Kurz [6] investigated the mechanical properties of a solidifying AIMg alloy perpendicular to the growth axis of the columnar crystals. The tensile behavior of solidifying AI-Cu alloys was studied by Wisniewski [7] and recently, Branswyck [8] proposed a modified indentation test which, in combination with FEM analysis, yields quantitative flow rules. Nevertheless, there is still a need for more mechanical data of solidifying alloys, especially creep data - where strain accumulates at a constant stress - only rarely exist for processing conditions

Effect of off-stoichiometric compositions on microstructures and phase transformation behavior in Ni-Cu-Pd-Ti-Zr-Hf high entropy shape memory alloys

High entropy shape memory alloys (HE-SMAs) show reversible martensitic phase transformations at elevated temperatures. HE-SMAs were derived from binary NiTi, to which the elements Cu, Pd, Zr and Hf are added. They represent ordered complex solid solutions. Their high temperature phase is of B2 type, where the added elements occupy sites in the Ni-(Cu, Pd) and Ti-sub-lattices (Zr, Hf). In the present study, advanced microstructural and thermal characterization methods were used to study the effects of the additional alloy elements on microstructures and phase transformations. The ratios of Ni-equivalent (Ni, Cu, Pd) and Ti-equivalent (Ti, Zr, Hf) elements in HE-SMAs were varied to establish systems that correspond to stoichiometric, under- and over-stoichiometric binary alloys. It is shown that basic microstructural features of cast and heat-treated HE-SMAs are inherited from the nine binary X–Y subsystems (X: Ni, Cu, Pd; Y: Ti, Zr, Hf). The phase transition temperatures that characterize the martensitic forward and reverse transformations depend on the concentrations of all alloy elements. The data obtained demonstrate how martensite start temperatures are affected by deviations from the composition of an ideal stoichiometric B2 phase. The findings are discussed in the light of previous work on the concentration dependence of SMA transformation temperatures, and directions for the development of new shape memory alloy compositions are proposed. © 2020 The Author