Physical metallurgy of alloy 718

Abstract

Abstract The physical metallurgy of alloy 718 is addressed with reference to Nb on the phase reactions found in alloy 718. The influence of Nb is observed during the solidification of alloy 718 as the large atoms Nb, Mo, and Ti segregate to the interdendritic regions and lead to the formation of Nb-rich Laves phase. In order to obtain desired properties, the Laves phase must be solutionized at high temperatures, and the Nb must be distributed throughout the dendrite areas. The normal phases found in alloy 718 are the metal carbide, TiN, δ, Laves, γ″, and γ′. The γ′ and γ″ are the main strengthening phases and their precipitation behavior is determined primarily by the amount of Nb, the temperature, and time of exposure. Any incomplete homogenization of the cast or wrought material will produce nonuniform precipitation of the δ, γ′, and γ″ phases during working or aging heat treatments. Two modes of strengthening mechanisms are combined in alloy 718 such as solid solution hardening (atoms of iron, chromium, molybdenum, and niobium can substitute to nickel within the metallic matrix) and hardening by a precipitation of ordered intermetallic phases, γ′ and γ″. Titanium and aluminum form by precipitation of the intermetallic phase γ′, Ni3(Ti, Al), metastable, and hardenable again by solid solution of niobium and titanium (at room temperature) and of tungsten or molybdenum (at high temperature). At a temperature close to 650°C, niobium combines to nickel to form by precipitation of the γ″ phase (Ni3Nb), which has very high mechanical properties at very low and moderately high temperatures. Although γ′ and γ″ are present in the aged condition, the amount of γ′ is much lower and γ″ is recognized as the primary strengthening agent. γ″ precipitates are disk-shaped, with a thickness of 5–9\xa0nm and an average diameter of roughly 60\xa0nm. The long-time stability of alloy 718 is related to the stability of the γ″ phase which transforms to delta and γ′ with increasing time and temperatures. Eventually the γ′ phase will be the solution, and the delta phase becomes the terminal phase in the 718 system. Alloy 718 contains significant amounts of iron endowing it with precipitation hardening effect. Iron’s low mobility in the matrix confers the main strengthening phase (γ″) a sluggish precipitation kinetics that reduces susceptibility to postweld cracking. An α-Cr phase is found in the grain boundaries in long-time exposures in the 593°C–732°C temperature range. The M23C6 carbide phase is not found in alloy 718 which is commonly found in Ni-based alloys. Indeed, alloy 718 was designed to overcome the low weldability of this class of materials, generally susceptible to cracks (microstructural segregation of alloying elements in the heat-affected zone of welds). Specific alloying elements give alloy 718 a strong resistance to corrosion up to 1000°C. For instance, nickel is useful in combating chloride-ion stress-corrosion cracking and protects from corrosion in many inorganic and organic oxidizing compounds, in a wide range of acidity and alkalinity. Chromium imparts an ability to withstand attacks from oxidizing media and sulfur compounds, while molybdenum is known to improve resistance to pitting corrosion. When stress and creep resistance are expected, applications are restricted below 650°C because γ″, metastable, rapidly overages under a prolonged exposure at or above this temperature. A rapid coarsening of γ″, solutioning of both γ′ and γ″ and microstructural shift, from the coherent disk-like γ″ phase to the stable, plate-like δ phase of Ni3Nb, is followed by a loss of strength and especially a creepy life.