The Effect of Thermal Treatment on SCC of Alloy 718: A Structure-Properties Study

Abstract

The stress corrosion cracking (SCC) behavior of alloy 718 is known to be controlled by the microstructure of the material. By evaluating the SCC susceptibility of alloy 718 as a function of thermal treatment and by characterizing the microstructure produced during various thermal treatments, it has been possible to identify those particular phases which are deleterious to SCC performance. The SCC response of two materials, conventional alloy 7 18 and "Ticolloy" has been evaluated using K1 see tests conducted in PWR water at 36O'C. Complimentary microstructural analysis of the various materials has been conducted using analytical electron microscopy techniques. The results show that the optimum SCC performance is related to a "clean" microstructure containing only fine 7" and 7' precipitates.

Analysis of stress corrosion cracking in alloy 718 following commercial reactor exposure

“…This is expected as the nominal irradiation temperature is < 0.3 Tm and therefore is below the range expected for solute redistribution through radiation-induced segregation [20]. This was also evident in the lack of precipitate-free zones near the grain boundaries, which have been reported [21] in alloy 718 following precipitate-hardening treatment at higher temperatures.…”

Revealing the stress corrosion cracking initiation mechanism of alloy 718 prepared by laser powder bed fusion assessed by microcapillary method

“…SCC initiation and propagation of conventional alloy 718 has been well established in the past decades and the presence of secondary precipitate hardening phases is reported to be the dominant factor in SCC risk assessment with a direct correlation with the density and size of the noncoherent δ phases as a consequence of prior heat treatments [5][6][7][8]19,57]. For instance, in a recent comprehensive study by Wang et al [19] on the effect of various heat treatment regimes on SCC behaviour of alloy 718, it is well clarified the effect of the presence of secondary phases on SCC susceptibility.…”

“…High creep resistance and mechanical integrity at high temperatures could be obtained resulting from the precipitation of γ'-Ni 3 (Al,Ti) and γ''-Ni 3 Nb throughout well-controlled heat treatment processes [4]. However, the type and shape of the precipitates are reliant on subsequent heat treatment parameters and could form unwanted phases such as δ-Ni 3 Nb, MC carbides, and Laves-(Ni,Fe,Cr) 2 (Nb,Mo,Ti), leading to a dramatic decrease in mechanical properties at high temperatures [1,4,5]. Another important aspect of the presence of secondary phases in the base matrix is their impact on the stress corrosion cracking (SCC) behaviour of austenitic superalloys.…”

Controlling Precipitation of Alloy 718 with Different Nb Content and Relevant Effect on Mechanical Behaviour

“…• gamma prime forms in an amount comparable and also higher to that of gamma double prime and without any apparent relationship with Nb content; • gamma double prime increases with Nb content and calculated amounts are always lower than those obtained by MatCalc; • some (very few) delta and eta phases are predicted to form at the temperature treatments; the fraction of these phases doesn't change with Nb content and decreases with reduction of ageing temperature; • delta phase is increased up to about 5% with the ageing treatment performed at 790°C for 8 hours. The presence of delta phase at grain boundaries, even when in low amount, has previously been documented as deleterious for stress corrosion resistance [10][11].…”