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The authors regret that the caption for Figure 5 is incorrect, and should read as follows: Fig. 5. The transition from transgranular cracking to a mixed transgranular-intergranular cracking mechanism for 0.05 Hz samples cycled with R = 0.5 as ΔK was increased from 7.6 to 15.9 MPa√m. Data for 5 and 0.33 Hz are grouped in black, and data for 0.05 Hz triangle and trapezoid (1.5s-17s–1.5s) waveforms are grouped in gray. Circles on the plot indicate where each micrograph is located on the crack growth rate curve, and each micrograph is scaled identically. The authors would like to apologise for any inconvenience caused.

In this work, temperature dependent mechanical properties of conventional and additive manufactured Inconel 718 (IN718) after solution and aging (SA) heat treatment were measured at room temperature, 350 °C, and 650 °C. Nanoindentation technique was used to obtain material properties including hardness, reduced modulus, creep rate, creep stress exponent, and thermal activation volume. The indentation size effect (ISE) was analyzed at each temperature. The reduced modulus was measured to decrease by 26% for the conventional samples and by 22% for AM samples as temperature increases from room temperature to 650 °C. The nanoindentation hardness decreases by 19% for the conventional samples and 16% for the AM samples. During the creep tests at 650 °C, the creep stress exponent was found in the range of 8-18, indicating that the deformation at the examined nano/micro scale is controlled by the dislocation climb mechanism.

In this work, temperature dependent mechanical properties of conventional and additive manufactured Inconel 718 (IN718) after solution and aging (SA) heat treatment were measured at room temperature, 350 °C, and 650 °C. Nanoindentation technique was used to obtain material properties including hardness, reduced modulus, creep rate, creep stress exponent, and thermal activation volume. The indentation size effect (ISE) was analyzed at each temperature. The reduced modulus was measured to decrease by 26% for the conventional samples and by 22% for AM samples as temperature increases from room temperature to 650 °C. The nanoindentation hardness decreases by 19% for the conventional samples and 16% for the AM samples. During the creep tests at 650 °C, the creep stress exponent was found in the range of 8-18, indicating that the deformation at the examined nano/micro scale is controlled by the dislocation climb mechanism.

Fatigue crack growth is analyzed for steady state and overload conditions. The model combines an elastic-plastic strain gradient hardening material and an irreversible cohesive zone model. Plastic strain gradients are found not to affect steady-state fatigue crack growth but play a key role in the overload response. Plastic strain gradient hardening lowers plastic strain magnitudes and alters the spatial distribution of plastic strain. This leads to reduced crack closure and higher crack growth rates during and after the overload. Using classical plasticity alone to describe fatigue crack growth following an overload is thus nonconservative.

Crack growth mechanisms for Alloy 617 at 800 °C were investigated in air with specific emphasis on the transition from cycle-dependent to time-dependent crack growth mechanisms in the creep-fatigue regime. Crack growth studies were conducted using compact tension samples, a load ratio of 0.5, and triangular 5 Hz, 0.33 Hz, and 0.05 Hz waveforms, a trapezoidal 0.05 Hz waveform with 17 s hold time, and sustained loading. Fatigue crack growth rates were relatively insensitive to changes in frequency and hold times in air up to ∆K ≈ 11.5 MPa√m for R = 0.5, i.e, Kmax = 23 MPa√m. Above this threshold, the onset of time dependent crack growth was observed via a creep void nucleation and coalescence mechanism for triangular and trapezoidal waveforms with a loading frequency of 0.05 Hz, and during sustained loading. An estimate of the threshold for stress assisted grain boundary oxidation (SAGBO) crack growth was calculated to be 23 MPa√m, and oxidized grain boundaries observed near the crack tip were mostly uncracked, suggesting the SAGBO threshold was not reached before the onset of the void nucleation mechanism. A comparison of results across all available studies suggests that a threshold-based transition from cycle- to time-dependent crack growth at 800 °C likely exists. However, the stress intensity factor does not maintain similitude to accurately define a threshold across studies. Thus, gaining an understanding of the crack tip stress states that define the various time dependent mechanisms should be considered in future work.

Size effects occur in non-uniform plastically deformed metals confined in a volume on the scale of micrometer or sub-micrometer. Such problems have been well studied using strain gradient rate-independent plasticity theories. Yet, plasticity theories describing the time-dependent behavior of metals in the presence of size effects are presently limited, and there is no consensus about how the size effects vary with strain rates or whether there is an interaction between them. This paper introduces a constitutive model which enables the analysis of complex load scenarios, including loading rate sensitivity, creep, relaxation and interactions thereof under the consideration of plastic strain gradient effects. A strain gradient viscoplasticity constitutive model based on the Kocks–Mecking theory of dislocation evolution, namely the strain gradient Kocks–Mecking (SG-KM) model, is established and allows one to capture both rate and size effects, and their interaction. A formulation of the model in the finite element analysis framework is derived. Numerical examples are presented. In a special virtual creep test with the presence of plastic strain gradients, creep rates are found to diminish with the specimen size, and are also found to depend on the loading rate in an initial ramp loading step. Stress relaxation in a solid medium containing cylindrical microvoids is predicted to increase with decreasing void radius and strain rate in a prior ramp loading step.

Inconel 617 (IN-617) mainly contains nickel (Ni), chromium (Cr), cobalt (Co) and molybdenum (Mo). IN-617 is widely used in applications that require high temperature operation due to its high temperature stability and strength as well as its strong resistance to oxidation and carburization. The current work focuses on the measurement of temperature dependent mechanical properties of IN-617 from room temperature (around 25 °C) up to 800 °C. The properties measured are reduced modulus, elastic modulus, hardness, indentation creep rate, indentation creep exponent, and thermal activation volume. The indentation size effect is analyzed as a function of temperature. Using a combination of optical microscopy and scanning electron microscopy (SEM) imaging, the effect of precipitate distribution and oxidation on the measured properties is found to be negligible beyond a critical indentation depth. The mean hardness value ranged from 3.1 GPa at room temperature to 1.6 GPa at 800 °C. A relation between indentation depth and hardness as a function of temperature change was used to extract strain gradient plasticity associated length scales with values changing from 1.0 μm at room temperature to 1.8 μm at 400 °C and to 1.6 μm at 800 °C.