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

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

Improving the intermediate- and high-temperature strength of L1₂-Co₃(Al,W) by Ni and Ta additions

The effects of Ni and Ta additions on the mechanical properties in the L1₂ compound Co₃(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 L1₂ phase on the mechanical properties. The additions of Ni and Ta, both of which are L1₂-stabilizers that increase the L1₂ 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 L1₂ 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 L1₂ solvus temperatures. The increased strength of the L1₂ 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

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

Cyclic degradation of titanium-tantalum high-temperature shape memory alloys - The role of dislocation activity and chemical decomposition

Titanium-tantalum shape memory alloys (SMAs) are promising candidates for actuator applications at elevated temperatures. They may even succeed in substituting ternary nickel-titanium high temperature SMAs, which are either extremely expensive or difficult to form. However, titanium-tantalum alloys show rapid functional and structural degradation under cyclic thermo-mechanical loading. The current work reveals that degradation is not only governed by the evolution of the ω-phase. Dislocation processes and chemical decomposition of the matrix at grain boundaries also play a major role.DFG/NI1327/3-1DFG/MA1175/34-1DFG/EG101/22-1DFG/FR2675/3-

Effects of aging on the stress-induced martensitic transformation and cyclic superelastic properties in Co-Ni-Ga shape memory alloy single crystals under compression

Co-Ni-Ga shape memory alloys attracted scientific attention as promising candidate materials for damping applications at elevated temperatures, owing to excellent superelastic properties featuring a fully reversible stress-strain response up to temperatures as high as 500 °C. In the present work, the effect of aging treatments conducted in a wide range of aging temperatures and times, i.e. at 300–400 °C for 0.25–8.5 h, was investigated. It is shown that critical features of the martensitic transformation are strongly affected by the heat treatments. In particular, the formation of densely dispersed γ’-nanoparticles has a strong influence on the martensite variant selection and the morphology of martensite during stress-induced martensitic transformation. Relatively large, elongated particles promote irreversibility. In contrast, small spheroidal particles are associated with excellent functional stability during cyclic compression loading of 〈001〉-oriented single crystals. In addition to mechanical experiments, a detailed microstructural analysis was performed using in situ optical microscopy and neutron diffraction. Fundamental differences in microstructural evolution between various material states are documented and the relations between thermal treatment, microstructure and functional properties are explored and rationalized