A phenomenological model is proposed to predict both monotonic and cyclic softening in Inconel 718 superalloy at 650 °C. Contrary to the current cyclic softening models consisting of a modified isotropic hardening variable, an internal state variable acting on the amplitude of the back stress is proposed and implemented in a crystal-plasticity model. Crystal-plasticity finite-element simulations are performed to validate quantitatively the model on uniaxial strain-controlled monotonic and cyclic loading for several plastic strain rates and strain amplitudes as well as to gain insights on the ratcheting phenomenon depending on the mean stress and stress amplitude values when cyclic softening occurs.
Crystal-plasticity modeling of phase transformation-viscoplasticity coupling in high-temperature shape memory alloys
AbstractA coupling between phase transformation and viscoplasticity is observed in HTSMAs during actuation. The dislocations generated and retained phases accumulated are responsible for the coupling. It results in functional fatigue, which is reflected as an alteration in the functional properties of the alloy, and an increase in the irrecoverable deformations. The objective of the present study was to develop a theoretical framework to account for the interactions between viscoplasticity and phase transformation to simulate the alteration in functional properties and generation of irrecoverable deformations. A crystal-plasticity based multi-scale approach was followed to develop the framework. This approach accounts for the mechanisms of: phase transformation, transformation induced plasticity, accumulation of retained martensite, plasticity (in a rate-independent manner), and viscoplasticity (in a rate-dependent manner). The coupling was accounted by a direct effect of the dislocation densities produced by the irrecoverable mechanisms onto the transformation resistance and retained martensite. This resulted in an indirect effect on the functional properties of phase transformation. On simulating the response of single crystals and polycrystals, anisotropic responses are captured at the grain scale (from single crystals), and a nearly isotropic response is captured at the macro scale (from polycrystals). The results show consistency between the functional property trends (such as TT and hysteresis) and those observed experimentally at the macroscale. The contributions of this study are presenting an effect of: (i) texture, (ii) thermal cycling rate and (iii) functional fatigue, through the response of several randomly oriented single crystals and a polycrystal of Ni–Ti–Hf HTSMA.
Phase transformation and viscoplasticity coupling in polycrystalline nickel-titanium-hafnium high-temperature shape memory alloy
P. Chaugule, O. Benafan, J.-B. le Graverend* Acta Materialia ↗, December 2021
AbstractThe objective of this study was to investigate the interactions between phase transformation and viscoplasticity during uniaxial constant force thermal cycling (UCFTC) of a Ti-rich Ni-Ti-20Hf (at.%) high-temperature shape memory alloy (HTSMA). These tests were conducted (up till failure) at 1, 10 and 50 ∘C/min, to vary the duration of exposure to high temperatures, viz. the amount of viscoplasticity, and to examine the rate-dependency of actuation. The macroscopic results from the tests were used to investigate the evolution of transformation temperatures, hysteresis, transformation and irrecoverable strains for the cycles in which the effect of potential damage mechanisms could be assumed to be negligible. The phenomena that affected the behavior were: viscoplasticity at 1 ∘C/min, transformation-induced plasticity (TRIP) at 10 and 50 ∘C/min, and accumulation of retained martensite at all the three rates. More interestingly, the response at 1∘C/min indicated a unique interplay between the effect of viscoplasticity over phase transformation and static recovery. The retained martensite was identified through a series of DSC and XRD analyses, and its contribution to TRIP strain was quantified through a UCFTC test. Furthermore, a test involving alternating isothermal creep and UCFTC at 10 ∘C/min was conducted to investigate an effect of viscoplasticity produced by creep on the behavior, while reducing the viscoplasticity during thermal cycling. The alternating test revealed an effect of phase transformation over the viscoplastic strain rate. The experimental investigations demonstrated a rate-dependent phase transformation behavior, and a two-way coupling between phase transformation and viscoplasticity, bringing out the importance of understanding viscoplastic deformations in phase-transforming materials.
Ex-situ X-ray tomography characterization of porosity during high-temperature creep in a Ni-based signle-crystal superalloy: Toward understanding what is damage
Creep damage by void nucleation and growth limits the lifetime of components subjected to mechanical loads at high temperatures. For the first time, the porosity of a Ni-based single crystal superalloy subjected to high temperature creep tests (T≥1000 °C) is followed by ex-situ X-ray computed tomography. A large experimental campaign consisting of nine temperature/stress conditions is carried out to determine the kinetics of the damage accumulation by voids. It is, indeed, essential to characterize their evolution to create internal variables describing properly the evolution of damage in a Continuum Damage Mechanics framework. Nonetheless, it is pointed out that the increase in the plastic strain rate during the tertiary creep stage is not necessarily related to the increase in the pore volume fraction for the alloy and temperature range explored (1000–1100 °C). Therefore, it seems that the changes in the microstructure, i.e. precipitation coarsening and γ/γ′ topological inversion, and the shearing of the γ′ particles have to be considered further to properly describe the damage evolution. Thus, the Continuum Damage Mechanics theory is undermined and should be replaced by a transformative paradigm taken into consideration microstructural evolutions in order to improve the predictability of further damage models.
