Ni-based superalloys play a crucial role in various high-temperature applications, where their exceptional mechanical properties and resistance to corrosion are highly desirable. However, their response to low temperatures, especially concerning strain hardening, microstructural evolution, and deformation mechanisms, requires further scrutiny. In this study, we investigate the influence of temperature on the stacking fault energy (SFE) and its implications on deformation twinning in Alloy 625. Uniaxial tensile tests are performed at 298 K, 173 K and 77 K. The study reveals a notable increase in strain hardening at intermediate strain levels, suggesting the activation of a secondary deformation mechanism. To gain deeper insights, crystal plasticity based simulations using the DAMASK framework are employed, complementing the experimental outcomes. Deformation twins are consistently observed at all temperatures, albeit with a small volume fraction and thickness. The critical strain for twinning decreased with decreasing temperature. Based on the numerous literature studies, experimental and computational observations, the SFE of the material is estimated to be constant over the studied temperature range. (Read more…)
Biaxial tensile tests are performed on duplex stainless steel using a newly developed multiaxial testing rig integrated with a digital image correlation mapping facility. A cruciform-shaped specimen geometry is subjected to equibiaxial tensile (EBT) loading, and the load–strain response is captured. Unlike uniaxial tensile testing, a well-defined gauge area is absent in biaxial specimens; hence, stress cannot be estimated directly. Finite element analysis is used to calculate the stress state. The phenomenological finite element model failed to capture the inherent anisotropy of the material; hence, a crystal plasticity finite element based method is used to estimate the stresses. It is observed that the 0.2% proof stress increased, but the strain to failure remained constant in comparison with uniaxial tensile properties. The work hardening behaviour is isotropic similar to that observed during uniaxial testing. The goss and rotated goss orientations in austenite accommodated most of the strain along RD and TD, respectively. During EBT, the change in austenite and ferrite textures is minimal and similar to their single phase counterparts reported in the literature. The EBT test results indicate that the influence of micromechanical interactions between the phases on the work hardening behaviour and texture evolution is weak. (Read more…)
DSS are characterised by vivid phase morphology and crystallographic textures. The typical microstructure consists of elongated phases (along the rolling direction, RD) stacked alternatively along the normal direction (ND). Uniaxial testing in multiple macroscopic directions is used to explore the anisotropic evolution of work hardening. Crystal plasticity simulations are used to gain insights into the same. The compressive stress and work hardening of samples loaded in the transverse direction (TD) are observed to be higher than those loaded in the RD and ND. Austenite and ferrite developed [110] and [111] parallel to the loading direction type fibre textures during uniaxial compression. The weak austenite texture behaves similarly to its single-phase counterparts regarding reorientation. The plastic anisotropy evolution is aided by the strong ferrite texture, which deviates from its single-phase behaviour. When loaded along TD compared to other directions, microstructural characterisation revealed the formation of deformation twins in austenite and lower effective slip length of both phases. (Read more…)
The crystallographic texture is known to have a significant effect on the plastic anisotropy and formability in materials. In single-phase materials, the texture is often a function of prior thermomechanical processing. However, in materials with multiple phases such as duplex stainless steels (DSS), the texture is also influenced by the orientation relationship (OR) maintained during the phase transformations. The role of OR and their Bain variant classifications on the plastic anisotropy and, in turn, on the formability of DSS is studied. Crystal plasticity modelling is used to calculate the in-plane anisotropy of the r-values for DSS with different initial austenite textures. Further, finite element simulations of the deep drawing process are done to obtain the earing profiles and show the Bain variant dependency on the formability. For ferrite having Bain-C variant type texture, the desirable deep drawing properties are obtained i.e. large normal anisotropy and small plastic anisotropy values. (Read more…)
In two-phase materials like duplex stainless steels that contain both the phases in nearly equal proportions, the crystallographic texture in the phases is related by a special orientation relationship (OR). Most of the studies related to ORs are focused on characterising the type of ORs due to phase transformations and recrystallisation. However, very little is known about the stability of ORs and their role in the texture evolution during deformation. A detailed study on the role of ORs in the texture evolution during rolling in fcc/bcc two-phase materials is done. Three different ORs; Bain, Kurdjumov–Sachs, Nishiyama–Wassermann and two different parent phases; fcc and bcc are considered. Using three-dimensional representative volume element (RVE) based crystal plasticity fast Fourier simulations, we show that the final textures strongly depend on the initial parent phase textures and the existence of an OR, and are independent of the type of the OR. A detailed investigation using a series of bicrystal studies further revealed that even within an OR, the type of Bain variant plays a vital role in the overall texture evolution. It is also observed that there is no marked impact of the initial CRSS ratios and strain hardening on final textures of both the parent and child phases. (Read more…)
Deformation twinning is known to be one of the reasons that cause texture transition (copper type to brass type) in single-phase fcc materials and is studied extensively. The role of deformation twinning in two-phase materials is an area yet to be explored. Using a combination of experiments and modelling, the role of deformation twinning on texture evolution is studied in duplex stainless steels. The material is cold rolled to 80% thickness reduction and texture evolution is studied at various strain levels. These are compared with a series of crystal plasticity simulations using the Taylor model and grain interaction-based LAMEL model which was extended to a two-phase material. Deformation twinning in austenite is incorporated by the predominant twin reorientation (PTR) scheme. It is observed that only by accounting for the strong local interactions between the phases, the correct textures are predicted. The texture transition from {001}⟨110⟩ to {112}⟨110⟩ orientation observed in ferrite at higher strain levels is attributed to deformation twinning in austenite. This was achieved computationally by saturating the CRSS of one of the slip modes in ferrite. (Read more…)