# Plastic Behavior of Laser-Deposited Inconel 718 Superalloy at High Strain Rate and Temperature
## 4 Experimental Results and Data Analysis
The serrated flow stress as observed in the diagrams of Figure 8 for temperatures higher than 800 \({}^{\circ}\)C could be related to the Portevin-Le Chatelier (PLC) effect. As widely known, PLC manifests as an unstable plastic flow that occurs during tensile tests of some nickel-based superalloys in a certain range of temperatures and strain rates. As reported in [26], if the diffusivity of the dissolved atoms is increased by heating, the diffusion speed becomes so large that they can move to dislocation as soon as it stops. There is an alternation between tearing off and re-pinning of the dislocation, causing a serrated flow curve. Another consequence of this effect is that when the strain rate is increased, the dissolved atoms may not be fast enough to catch up with the dislocation and pin it. In this case, the yield strength of the material could become smaller when the strain rate increases, in contrast to the usual behavior, and the serration of the flow curve vanishes. Consequently, the final material behavior at a certain strain rate and temperature is a complex result of the interaction between solute atoms and dislocations, and the effect of temperature is not limited to the thermal activation for overcoming the dislocation obstacle but also to the solute diffusion. Figure 9a shows the engineering stress versus engineering strain curves obtained for aD, DA, and HT specimens at room temperature in quasi-static and high strain rate loading conditions. Again, as expected, by comparing the three materials, it is possible to conclude that the aD specimens have lower mechanical strength because of the microstructure. The strain rate sensitivity obtained covering six orders of magnitude was very limited or at least negligible at room temperature. As previously mentioned, the final mechanical response is the result of the combined effects induced by the temperature and strain rate. This  
Figure 8: Engineering stress versus engineering strain curves obtained for high strain rate tests performed on (**a**) DA and (**b**) HT specimens in the temperature range between 25 \({}^{\circ}\)C and 1000 \({}^{\circ}\)C.  
Figure 7: Results of a high strain rate test at 1000 \({}^{\circ}\)C: (**a**) comparison of the input (V\({}_{1}\)) and output (V\({}_{2}\)) sides’ velocity time history profiles obtained by video analysis (markers) and strain-gauge signal processing (solid lines); (**b**) check of the equilibrium condition in terms of forces on the input (F\({}_{1}\)) and output (F\({}_{2}\)) sides of the specimen. implies that the effect of the strain rate obtained at room temperature could not be useful to describe the material behavior in a different temperature/strain rate condition. This was the reason for specifically performing tests at various temperatures at the strain rate of interest. In Figure 9b, the flow stress at 0.1 of the true strain is reported as a function of the testing (initial) temperature. The significant decrease of the mechanical strength in the range between 800 \({}^{\circ}\)C and 900 \({}^{\circ}\)C can easily be appreciated, as well as the stable and low reduction between 400 \({}^{\circ}\)C and 750 \({}^{\circ}\)C. In the same diagram, the results obtained for compression tests, obtained in [20], are also shown. Compression and tension tests were carried out on the same materials with identical heat treatments (aD, DA, and HT), at an equal nominal strain rate (1500 s\({}^{-1}\)) and in the same range of temperatures (from 25 \({}^{\circ}\)C to 1000 \({}^{\circ}\)C). An in-depth understanding of the results requires further investigation, but Figure 9b reveals the asymmetry of the tension-compression behavior, as expected for Ni-based superalloys [27]. A preliminary analysis of the fracture was performed by observing the fracture surfaces. A summary of the results is reported in Figures 10 and 11. In Figure 10, the longitudinal views are reported both for DA and HT specimens at different loading conditions.