The influence of Cr, Al, Co, Fe and C on negative creep of Waspaloy

Negative creep using precision dilatation experiments is investigated on a broad variety of single, dual and multiphase nickel-based superalloys. Pure nickel and dilute binary nickel-based alloys show no signs of negative creep. However, with higher contents of Cr and Al and in highly alloyed ternary and multicomponent nickel-based alloys negative creep is observed. Short range ordering of Cr and/ or Al are identified to cause negative creep at 550 °C. Carbon additions leading to retarded carbide precipitation or transformation can enhance negative creep.

Room Temperature Ferroelastic Creep Behavior of Porous (La0.6Sr0.4)0.95Co0.2Fe0.8O3-δ

The time-dependent deformation of porous (La0.6Sr0.4)0.95Co0.2Fe0.8O3-δ (LSCF) under constant uniaxial compressive stress at room temperature has been studied. Both axial and lateral stress–strain deformation curves clearly show the non-linear ferroelastic behavior of LSCF perovskite during compression. The ferroelastic characteristics of deformation curves such as coercive stress and apparent loading moduli decrease when the porosity of the samples increases. Ferroelastic creep deformations at applied stresses of 25 and 50 MPa demonstrate that stress and porosity are influencing factors on creep deformation, which increases with increasing stress and porosity. A negative creep or axial expansion and lateral contraction were observed in the sample with 35% porosity under 50-MPa constant compression stress.

Negative creep of soils

After unloading, the deformation of soils cannot be stable immediately, but continues to expand over time even under constant pressure. In this paper, the expansive deformation over time when effective stress is kept constant is defined as the negative creep, while the compressive creep is described as the positive creep. The division between positive creep and negative creep is named the stable normal compression line (SNCL), on which the stress–strain behaviour of the soil is time-independent. Based on the concept of the SNCL and test results, a new formula for creep is proposed. This formula is simple in form and has less parameters, and both negative creep and positive creep can be well predicted. By incorporating this formula into the current yield function of the unified hardening model, a new time-dependent current yield function is built. Combining the yield function, a flow rule, and transformed stress method, a new three-dimensional time-dependent constitutive model considering both positive and negative creep for clays is derived and presented. The new model is then validated by test results, including multistage loading oedometer tests, triaxial undrained creep tests, and triaxial undrained compression tests at the constant strain rates.

Transient Effects in Creep of Sanicro 25 Austenitic Steel and Their Modelling

Transient effects upon stress changes during creep of the new Sanicro 25 steel were investigated experimentally using the helicoid spring specimen technique. The creep behaviour was found to be qualitatively the same as that observed earlier with the creep-resistant 9% Cr ferritic-martensitic P-91 steel, but the transient strains are considerably smaller. Negative creep rate, which is strain running against the applied stress, was observed with any stress decrease. Parameters for the complex creep model were estimated and model results were compared to the creep rates measured experimentally. The model can be used for the finite element method modelling of the creep and stress relaxation effects in the components made from the Sanicro 25 steel.

A Novel In-Situ Nanoindentation Characterization of Phase Transforming Materials

ABSTRACTMaterials with different allotropes can undergo one or more phase transformations based on the changes in the thermodynamic states. Each phase is stable in a certain temperature/pressure range and can possess different physical and mechanical properties compared to the other phases. The majority of material characterizations have been carried out for materials under equilibrium conditions where the material is stabilized in a certain phase and a lesser portion is devoted for onset of transformation. Alternatively, in situ measurements can be utilized to characterize materials while undergoing phase transformation. However, most of the in situ methods are aimed at measuring the physical properties such as dielectric constant, thermal/electrical conductivity and optical properties. Changes in material dimensions associated with phase transformation, makes direct measurement of the mechanical properties very challenging if not impossible. In this study a novel non-isothermal nanoindentation technique is introduced to directly measure the mechanical properties such as stiffness and creep compliance of a material at the phase transformation point. Single crystal ferroelectric triglycine sulfate (TGS) was synthetized and tested with this method using a temperature controlled nanoindentation instrument. The results reveal that the material, at the transformation point, exhibits structural instabilities such as negative stiffness and negative creep compliance which is in agreement with the findings of published works on the composites with ferroelectric inclusions.

The Relaxation Behavior of High Chromium- Ni Base Superalloys

The relaxation behavior of wrought high chromium Ni-base alloys Nimonic 80A, Nimonic 101 and Nimonic 105 was studied at different temperatures. All of these alloys exhibited increase in the residual stress during the relaxation tests e.g. at 450°C for Nimonic 80A and 650°C for Nimonic 105. The observed increase in the residual stress is a manifestation of the known phenomenon of “negative creep”. The stress free aging of specimens of these alloys exhibited dimensional contraction at different temperatures ranging from 450°C to 650°C. The abnormal relaxation behavior of these alloys and the observed contraction were attributed to the precipitation and ordering of the intermetallic phase Ni2(Cr,Mo) at the test temperature. The differential thermal analysis i.e. DTA results, demonstrated endothermic peaks to correspond with the order–disorder reaction of Ni2(Cr,Mo).