Experimental Determination of TTT Diagram for Alloy 718Plus®

Abstract

The phase evolution of Allvac® alloy 718Plus® has been determined experimentally via construction of time-temperaturetransformation (TTT) and continuous-cooling-transformation (CCT) curves based on identifying the various phases, their sizes and volume fractions along with distributions as they evolved during thermal treatment. The solution temperature range of interand intra-granular  phases and of have been characterized via detailed microstructural investigations. The phase solvus of alloy 718Plus determined by dissolution during isothermal hold for 0.1 hours ranges between 1010-1071C, with clear distinctions between solutioning of grain boundary and complete solutioning. The temperature where the intergranular or grain boundary particles dissolve after 0.1 hours in the investigated material is 1015C. At this temperature, grain growth occurs owing to a lack of pinning by grain boundary phase particles. However, it is only at 1065C, that a complete dissolution of all particles has been observed. The nose of the TTT curve for  occurs at about 843C for start (25 % transformation) and end of transformation (100% transformation). The temperature ranges at which precipitation hardening (704-788C), coarsening (843C) and over aging (898C) take place in the alloy have been located by correlating the TTT and the time-temperature-hardness (TTH) curves. This investigation provides a solid basis for practical processing of the alloy, to be used as a candidate material for 700C applications.Allvac and 718Plus are registered trademarks of ATI Properties, Inc.

On the precipitation of delta phase in ALLVAC®718Plus

“…The XRD spectra ( Figure 5) demonstrate unambiguously the presence of stable η after 16 h, and a smaller amount of δ phase developing after 72 h. It should also be noted that our results regarding the presence of η are not necessarily inconsistent with other reports of the presence of δ phase alone in 718 ® Plus [11,12], as in these papers both thermo-mechanical histories and the heat treatments are different from the one performed in this study. Great care must be taken to distinguish the two structures, which have very similar diffraction characteristics.…”

“…The grounding of aircraft in the USA resulting from the tragic events of the 11 September 2001 allowed researchers to evaluate the effect of air traffic on the regional and global climate. Travis et al [1] reported a significant anomaly of +1.1°C in the diurnal temperature range during the flight interruption (11)(12)(13)(14). Without legislative intervention and consortia between aerospace companies, the projected increase in air traffic will further augment the effect on climate on a global scale.…”

“…The presence of the γ′ in 718 ® Plus instead of γ′′ in 718 increases the alloy's temperature capability whilst simultaneously improving mechanical properties [6]. The chemical composition was significantly modified with respect to 718 by the addition of Al, Co and W to favour γ′ stability [7], which also had the effect of stabilizing η over δ [8,9]; however, other publications have reported the presence of δ alone in the secondary precipitates [10][11][12]. One should note that the material thermo-mechanical history differs significantly and may account for some of the variability reported.…”

Morphology and Kinetics of Grain Boundary Precipitation in Alloy ATI 718Plus

“…Although early literature on 718Plus [5,7] reported 8 solvus temperatures as high as 1065 °C, obtained from thermodynamic calculations, the present result is in agreement with more recent studies, which placed the 8 solvus around 980 °C -1000 °C [28,30]. The exception is a study by Srinivasan et al [23], which indicates a solvus temperature between 1010 °C and 1071 °C, although after annealing times o f only 6 minutes. For heat treatments at 925 °C and 950 °C the etching revealed the presence o f spherical Y particles, indicating that the solvus temperature o f the Y phase is between 950 °C and 975 °C.…”

“…The interest in alloy 718Plus has prompted the appearance of several studies tackling the same issue, although the information already available is naturally more limited. The precipitation kinetics of the strengthening phase y' has first been studied in terms of hardness [7,23] and later completed with TEM observations [23] and modeling simulations [5,24]. TTT diagrams of n/5 phase precipitation have been published both in solution-treated (strain free) [5,7] and forged [5,13,25] material.…”