Analysis of creep in thermally cycled Al/SiC composites

Currently no systematic approach exists to explicitly quantify the combined effects of anisotropic grain orientations and grain boundary behavior on the fracture of polycrystalline materials. A multiscale cohesive finite element (CFEM) based computational framework is developed to predict the fracture properties such as K_IC and J_IC of polycrystalline metals as functions of microstructural attributes. The focus is on characterizing the effects of crystallographic texture on fracture toughness. The framework uses computationally generated statistically equivalent microstructure sample sets with varying proportions of textured grains, allowing statistical variations and distributions of the fracture behavior due to microstructural variabilities and the influences of intergranular and transgranular fracture mechanisms to be quantified and analyzed. A crystal plasticity formulation is used to model the anisotropic deformation in the grains. A misorientation-dependent interfacial relation is used to model texture-sensitive crack growth through grains and grain boundaries. The framework allows exploration of the effects of microstructure on the macroscopic fracture measures via the manifestation of the different fracture mechanisms. Calculations carried out for Mo under 2D and a more general 2.5D plane strain conditions capture and delineate the competing effects between (a) intergranular and transgranular fracture and (b) plasticity and crack growth on the overall fracture toughness of the material. Both the 2D and 2.5D models use the same sets of microstructures; the 2.5D model uses columnar grains. The results indicate that, as the fraction of grains with preferred orientation increases, transgranular fracture dominates relative to intergranular fracture. Consequently, the relative contribution of plastic dissipation associated with transgranular fracture is enhanced, resulting in higher overall fracture toughness for the material setting analyzed. The effect is characterized in terms of the effective grain size distribution, fraction of grains with favourably oriented slip systems, and misorientation-dependent GB characteristics. Finally, an analytical correlation is established between the overall fracture resistance and the microstructure attributes.

In this interpretive review, fatigue in metallic systems is considered primarily from the perspective of interactions between the microstructure, the deformation mode and the mechanical state at both low and high temperatures. In Part 1 the development and early propagation of cracks is considered in terms of the basic damage mechanisms and the relative size of the crack with respect to applicable microstructural feature(s). In this section, a multistage grain scale approach to microstructure-sensitive fatigue crack formation and growth is presented which uses Fatigue Indicator Parameters (FIPs) to correlate these processes. Various FIPs parameters are discussed in terms of their indication of the state of fatigue. The development and early crack propagation is considered in the context of microstructure and notches, and probabilistic aspects of the notch fatigue problem are discussed. These features are integrated into a systematic approach for the selection of fatigue resistant microstructures for given applications. In Part 2, attention is focused on Ni-base superalloys and the interaction between oxidation, creep and microstructure (including coatings) in the formation and propagation of cracks. This part of the overview addresses both experimental and modelling aspects. Methodologies based upon fundamental physical processes are presented for understanding and predicting the development and propagation of fatigue cracks, including effects of sequential oxide type formation and of creep on either restraining or accelerating damage by oxidation. The variable fatigue resistance of discs in jet engines is seen to depend upon the variability of microstructure and its influence on the severity of creep/oxidation interactions. All of these factors are considered in the practical case where both temperature and loading parameters vary simultaneously (thermomechanical fatigue). A physics-based life prediction model considering the interactions of deformation and environmental damage is reviewed in terms of its applicability to life prediction of components.

Recent trends towards simulation of the cyclic slip behavior of polycrystalline and polyphase microstructures of advanced engineering alloys subjected to cyclic loading are facilitating understanding of the relative roles of intrinsic and extrinsic attributes of microstructure in fatigue crack formation, comprised of nucleation and growth of cracks at the scale of individual grains or phases. Modeling of processes of early stages of fatigue crack nucleation and growth at these microstructure scales is an important emerging frontier in several respects. First, it facilitates analysis of the influence of local microstructure attributes on the distribution of driving forces for fatigue crack formation as a function of the applied stress state. This can support microstructure-sensitive estimates of minimum life, as well as characterization of competing failure modes. Second, it can inform modification of process route and its manifestations (e.g., residual stress, texture) to alter microstructure in ways that promote enhanced resistance to formation of fatigue cracks. Third, microstructure-sensitive modeling, even conducted at the mesocopic scale of individual grains/phases, can facilitate parametric design exploration in searching for microstructure morphologies and/or compositions that modify fatigue resistance. Fourth, such technologies offer promise for integration with advanced nondestructive evaluation methods for prognosis and structural health monitoring. Finally, as a longer term prospect in view of uncertainties in modeling mechanisms of cyclic slip, crack nucleation and growth, such modeling can serve to support more quantitative predictions of fatigue lifetime as a function of microstructure. We first discuss computationally based microstructure-sensitive fatigue modeling in the context of recent initiatives in accelerated insertion of materials and integration of computational mechanics, materials science, and systems engineering in design of materials and structures. We then highlight recent application of such strategies to Ni-base superalloys, gear steels, and α–β Ti alloys, with focus on the individual grain scale as the minimum length scale of heterogeneity. Finally, we close by outlining opportunities to advance microstructure-sensitive fatigue modeling in the next decade.

In this study, a unified mean-field micromechanical model was applied to analysis of thermal-cycling creep of short fiber-reinforced metal matrix composites subject to a wide range of external loads from compression to tension. The model considered the rate process of the local mass transfer by diffusion along the fiber/matrix interface. The thermal-cycling creep of the composites can be divided into three groups observed in high compressive stress regime, low compressive and tensile stress regime, and high tensile stress regime based on the micromechanical examination of the deformation mechanism. Numerical and experimental studies were systematically conducted with directionally solidified Al–Al₃Ni eutectic composites. These studies demonstrated that even though compressive loads were applied, the elongate creep deformation occurred under given thermal-cycling conditions, and thermal-cycling creep was largely affected by the length of the fiber and temperature profile.

The combined experimental and numerical approaches are utilized to investigate the effects of the whisker misalignment and rotation on the stress–strain response of discontinuous SiC whisker reinforced aluminum matrix composites during the hot compressive deformation at 300 °C. It is found from the numerical results that the whisker orientation has significant effects on the stress and rotation of whiskers, but little influences on the flow stress and work hardening of the matrix. Both numerical and experimental results indicate that the whisker misalignment and rotation play an important role in hot deformation behavior of the composites. During the hot compressive deformation, the composites with whisker misalignment angles less than 30° exhibit strain-softening behavior due to the whisker rotation or breakage. However, the composites with whisker orientation angles more than 45° show work hardening phenomenon due to the matrix work hardening. The predicted stress–strain behavior of the composites is in qualitative agreement with the experiment results.

Internal stress plasticity occurs when a small external stress biases internal mismatch strains produced by, e.g., phase transformation or thermal expansion mismatch. At small applied stresses, this deformation mechanism is characterized by a deformation rate which is proportional to the applied stress and is higher than for conventional creep mechanisms. In this work, we demonstrate the operation of internal stress plasticity due to internal chemical stresses produced by chemical composition gradients. We subject specimens of β-phase Ti-6Al-4V to cyclic charging/discharging with hydrogen (by cyclic exposure of specimens to gaseous H₂), under a small external tensile stress. As expected for internal stress plasticity, the average strain rate during chemical cycles at 1030°C is larger than for creep at constant composition (hydrogen-free or -saturated), and a linear stress dependence is observed at small applied stresses. Additionally, we present an analytical model which couples elastic and creep deformation with a transient diffusion problem, wherein the diffusant species induces swelling of the host lattice. Without the use of any adjustable parameters, the model accurately predicts both the observed strain evolution during hydrogen cycling of Ti-6Al-4V and the measured stress dependence of the deformation.