γ′ precipitates with a twin orientation relationship to their hosting grain in a γ-γ′ nickel-based superalloy

Abstract

In a polycrystalline γ-γ' nickel-based superalloy, γ' precipitates with a close-to-twin orientation relationship to their surrounding matrix (named T-type precipitates) are found in recrystallized grains located near specific unrecrystallized grains (named recovered grains). This type of γ/γ' interface has not been reported in literature before. Inherited from the ingot conversion process, recovered grains are characterized by a high density of micrometric and close-to-coherent γ' precipitates. Resulting from the interaction of the recrystallization front with the latter precipitates, the T-type precipitates appear to form onto the recrystallization front. The present paper details the crystallographic characteristics of the T-type precipitates. KeywordsGamma -Gamma Prime nickel-based superalloy, Interphase boundary, Twin boundary, Recrystallization.Polycrystalline γ-γ' nickel-based superalloys are commonly used to manufacture aircraft engine rotative parts due to their good mechanical behavior at high temperature (tensile, fatigue, creep resistance) [1]. Parts are usually derived from cast ingots through forging sequences [2,3]. They are made of a γ matrix in which γ' precipitates of various sizes are distributed. Both γ and γ' phases have a cubic structure but, while the γ phase is a Face Centered Cubic (FCC) non-ordered solid solution, the γ'-Ni 3 (Al,Ti) phase has a L1 2 ordered structure. Yet, the mechanical properties of the alloy do not only depend on the size and spatial distribution of the precipitates [4], but also on the γ/γ' interface characteristics [5,6]. Indeed, γ' precipitates are reported in literature as coherent, semi-coherent or incoherent to their surrounding matrix [7]. Coherency means that the γ and γ' crystal lattices perfectly coincide at the γ/γ' interface. This is made possible by a low lattice parameter mismatch between the γ and γ' phases [8]. Coherent γ/γ' interfaces have very low energies [9,10] and form during the cooling stages of the forging process for example. However, if some misfit dislocations pile up at the γ/γ' interface, the lattice matching is only partial and the interface is semi-coherent [7]. Semi-coherency occurs when external stresses (high temperature plastic deformation) or internal stresses (e.g. induced by long aging treatments leading to particle coarsening) generate dislocation loops which are incorporated into the γ/γ' interface as misfit dislocations [11]. Finally, when there is no crystal lattice matching, the γ/γ' interface is incoherent and has a high energy [9]. Secondary γ' precipitates which form during the cooling stages of the forming process, are coherent with a cube-cube orientation relationship: they evolve from spheres to {100} bounded cubes as their size and/or the lattice misfit increase [12]. On the other hand, primary γ' precipitates, derived from the as-cast microstructure and high temperature billet forging sequences, are usually incoherent with no special orientation relationship, excepted in case of heteroepitaxial recrystallizat...

A Mechanism Leading to γ′ Precipitates with {111} Facets and Unusual Orientation Relationships to the Matrix in γ–γ′ Nickel-Based Superalloys

“…A. T and C type precipitates in the AD730 TM alloy : the results of a single mechanism T-type precipitates have been reported in the alloy AD730 TM , in some recrystallized grains which had extensively grown into a recovered grain and which shared a <111>axis with it [18] . Nevertheless, Fig.…”

“…Moreover, the longest sides of the rectangular shapes of the T and C type precipitates are parallel to the {111} plane trace common to the recrystallized and recovered grains. Yet, it has been evidenced in [18] that T-type precipitates are plate-like particles bounded by the {111} plane common to the T-type precipitates, the recrystallized grain and the recovered grain. Here, the similar morphologies and alignments of the C and T type precipitates strongly suggests that both types of precipitates are facetted by the same {111} plane, common to the recrystallized and recovered grains.…”

“…These unrecystallized grains are characterized by intragranular orientation gradients with more or less developed recovery cell structures; thus they will be called recovered grains in the following text. Yet, in the AD730 TM γ-γ' nickel-based superalloy a previous work [18] pointed out a new interaction between the close-to-coherent precipitates of the recovered grain and the recrystallization front passing through. This interaction sometimes led to γ' precipitates with a close-to-twin orientation relationship to their recrystallized host grain.…”

“…This interaction sometimes led to γ' precipitates with a close-to-twin orientation relationship to their recrystallized host grain. "Close to twin orientation relationship" means that the γ' and γ cubic lattice cells conform to a 60°<111> rotation within the 8.66° tolerance given by Brandon's criterion: both the misorientation angle and misorientation axis do not deviate more than 8.66° to the 60° angle and <111> axis of the perfect twin boundary [18,19] . Such precipitates close to the twin orientation relationship have been called T-type precipitates and revealed to be {111} bounded plate-like particles.…”

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Cast-and-wrought heavily-alloyed γ-γ' nickel-based superalloys may exhibit large recovered grains inherited from the ingot conversion and characterized by a high density of close-to-coherent micrometric γ' precipitates. In the AD730 TM nickel-based superalloy, a previous work [18] highlighted a new interaction between such precipitates and a recrystallization front passing through. This interaction resulted in γ' precipitates with a close-to-twin orientation relationship to their recrystallized host grain.

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