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In polycrystalline metallic materials, quantitative and statistical assessment of the plasticity in relation to the microstructure is necessary to understand the deformation processes during mechanical loading. Plastic deformation often localizes into physical slip bands at the sub-grain scale. Detrimental microstructural configurations that result in the formation and evolution of slip bands during loading require advanced strain mapping techniques for the identification of these atomically sharp discontinuities. A new discontinuity-tolerant DIC method, Heaviside-DIC, has been developed to account for discontinuities in the displacement field. Displacement fields have been measured at the scale of the physical slip bands over large areas in nickel-based superalloys by high resolution scanning electron microscopy digital image correlation (SEM DIC). However, conventional DIC methods cannot quantitatively measure plastic localization in the presence of discontinuous kinematic fields such as those produced by slip bands. The Heaviside-DIC technique can autonomously detect discontinuities, providing information about their location, inclination, and identify slip systems (in combination with orientation mapping). Using Heaviside-DIC, discontinuities are physically evaluated as sharp shear-localization events, allowing for the quantitative measure of strain amplitude nearby the discontinuities. Measurements using the new Heaviside-DIC technique are compared to conventional DIC methods for identical materials and imaging conditions.

The reprecipitation mechanisms and kinetics of γ′ particles during cooling from supersolvus and subsolvus temperatures were studied in AD730TM Ni-based superalloy using Differential Thermal Analysis (DTA). The evolution in the morphology and distribution of reprecipitated γ′ particles was investigated using Field Emission Gun Scanning Electron Microscopy (FEG-SEM). Depending on the cooling rate, γ′ particles showed multi or monomodal distribution. The irregularity growth characteristics observed at lower cooling rates were analyzed in the context of Mullins and Sekerka theory, and allowed the determination of a critical size of γ′ particles above which morphological instability appears. Precipitation kinetics parameters were determined using a non-isothermal JMA model and DTA data. The Avrami exponent was determined to be in the 1.5–2.3 range, suggesting spherical or irregular growth. A methodology was developed to take into account the temperature dependence of the rate coefficient k(T) in the non-isothermal JMA equation. In that regard, a function for k(T) was developed. Based on the results obtained, reprecipitation kinetics models for low and high cooling rates are proposed to quantify and predict the volume fraction of reprecipitated γ′ particles during the cooling process.

The kinetics of γ′ size and volume fraction changes during solution treatment of the advanced Ni-based superalloy, AD730 TM are determined and the underlying mechanisms are investigated.High resolution Differential Thermal Analysis (DTA) and thermodynamic modeling were used to design and perform solution heat treatment experiments. Semi-analytical models are developed to describe the dissolution and coarsening processes. The results from the proposed models, supported by electron microscopy observations, indicate that coarsening occurs before complete dissolution takes place. Agglomeration is shown to be the governing coarsening mechanism for this alloy after calculation of the coefficients for both the Ostwald ripening and agglomeration mechanisms. Electron microscopy observations revealed that the early stages of agglomeration occur by neck formation between two neighboring particles. The splitting of γ′ particles was identified as one of the main dissolution mechanisms. Based on the obtained results, a dissolution kinetics model is proposed to quantify the volume fraction of dissolved γ′ particles and estimate the activation energy of this process for AD730 TM . A coarsening model based on the time-temperature dependence of the γ′ coarsening rate coefficient is also proposed taking the concentration of elements, γ′ volume fraction and the temperature into consideration. Based on this model, a method is developed to predict γ′ size evolution during aging heat treatment process and an optimum heat treatment to reach the desired γ′ distribution is proposed.The validity and accuracy of the proposed models were verified by carrying out different heat treatment experiments.

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The low-cycle fatigue behavior of two direct-aged versions of the nickel-based superalloy Inconel 718 (IN718DA) was examined in the low-strain amplitude regime at intermediate temperature. High variability in fatigue life was observed, and abnormally short lifetimes were systematically observed to be due to crack initiation at (sub)-surface non-metallic inclusions. However, crack initiation within (sub)-surface non-metallic inclusions did not necessarily lead to short fatigue life. The macro-to micro-mechanical mechanisms of deformation and damage have been examined by means of detailed microstructural characterization, tensile and fatigue mechanical tests, and in situ tensile testing. The initial stages of crack micro-propagation from cracked non-metallic particles into the surrounding metallic matrix occupies a large fraction of the fatigue life and requires extensive local plastic straining in the matrix adjacent to the cracked inclusions. Differences in microstructure that influence local plastic straining, i.e., the d-phase content and the grain size, coupled with the presence of non-metallic inclusions at the high end of the size distribution contribute strongly to the fatigue life variability. DAMIEN TEXIER, Postdoctoral Fellow, is with the Institut Pprime -UPR CNRS 3346 -ISAE-ENSMA,