Development of Grain Boundary Precipitate-Free Zones in a Ni-Mo-Cr-W Alloy
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CIPITATION hardening is one of the most important strengthening mechanisms in structural alloys, such as steel, aluminum alloys, and Ni-based superalloys.[1] Precipitate-free zones (PFZs) adjacent to grain boundaries (GBs) are common in precipitation-hardened alloys and are detrimental as they act as ‘‘soft areas’’ compared to the grain interiors thereby leading to localized deformation and fracture.[1–4] Since PFZs also reduce yield strength and fatigue strength,[2,5–10] understanding and controlling their development are important to materials scientists and engineers. PFZs adjacent to GBs are normally attributed to two different mechanisms: solute depletion and vacancy depletion.[11,12] The effects of these two mechanisms on PFZ width with increasing aging time have been summarized by Chang et al.[11] as shown schematically JIE SONG, ROBERT FIELD, and MICHAEL KAUFMAN are with the Center for Advanced Non-Ferrous Structural Alloys, Department of Metallurgical and Materials Engineering, Colorado School of Mines, 1500 Illinois Street, Golden, CO 80401. Contact e-mail: [email protected] DOUG KONITZER is with GE Aviation, One Neumann Way M/D M89, Cincinnati, OH 45215. Manuscript submitted August 9, 2016. Article published online February 16, 2017 METALLURGICAL AND MATERIALS TRANSACTIONS A
in Figure 1. If PFZ development is driven by solute depletion, the PFZ width follows the blue-dotted line in Figure 1 and the PFZ has several characteristics: (1) the width of the PFZ increases with increasing aging time, (2) the width of the PFZ is normally less than a micron, and (3) the solute depletion is due to GB precipitation during aging.[11,13] In contrast, if PFZ development results from vacancy depletion (red solid line in Figure 1), the PFZs usually develop at shorter aging times, and their widths decrease with increasing aging time. This phenomenon is exacerbated in systems in which nucleation of intragranular precipitates is not only affected by diffusion rates within the matrix, but also by the formation of low-energy solute/vacancy clusters which significantly reduce the activation energy for nucleation, as will be discussed below. In addition, the PFZ widths can vary from a few microns to over 10 lm.[11,14,15] In some systems, PFZ development is influenced by vacancy depletion at short aging times and solute depletion at longer times, as GB precipitates form and grow. This is represented by the green dash-dotted line in Figure 1. For the solute depletion mechanism, solute atoms are depleted near GBs due to the formation of solute-rich GB precipitates, which consume these elements. These elements are crucial for intragranular precipitate formation, as they either concentrate in intragranular
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Fig. 1—PFZ width as a function of aging time for the solute depletion and vacancy depletion mechanisms (the figure is adapted from Ref. [11]).
precipitates or influence the solubility limits of the precipitate-forming elements.[2,8,9,12,16–19] For the vacancy depletion mechanism shown in Figure 2, GBs with
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