Influence of Both γ' Distribution and Grain Size on the Tensile Properties of UDIMET 720Li At Room Temperature

Abstract

The tensile properties of a forged UDIMET 720Li alloy have been investigated at room temperature. The aim of this study was to increase both yield and ultimate tensile stresses using adequate thermal treatments. Classical three steps heat treatments after hot forging were applied: a solution treatment followed by a quench and a two steps aging treatment. Several combinations were investigated: four hours solution treatments, either sub-solvus (1080 C -1120 C) or super-solvus (1160 C); two different cooling rates (10 C/min or 3600 C/min); four different two-steps aging treatments: 650 C/24h/Air Quench (AQ) + 760 C/16h/AQ, 760 C/16h/AQ + 650 C/24h/AQ, 700 C/24h/AQ + 815 C/16h/AQ, 815 C/16h/AQ + 700 C/24h/AQ. The tensile properties appeared to be maximized with the following combination of heat treatments: sub-solvus solutioning (1120 C or 1080 C), fast cooling rate (oil quench, 3600 C/min), and the 760 C/16h/AQ + 650 C/24h/AQ aging. Both EBSD measurements and systematic stereological analyses were performed to characterize for each condition, grain size and γ' distribution (primary γ', secondary γ', tertiary γ'), respectively. The increase of the 0.2% yield stress, is managed by a competition between keeping a small grain size (i.e. by using a low temperature sub-solvus solution treatment) and increasing the intragranular γ' content (i.e. by increasing the solutioning temperature). In order to evaluate the relative contributions to the deformation mechanisms of grain size and of γ' particles, especially intergranular one (primary γ'), Scanning Electron Microscope (SEM) insitu tensile tests have been performed at room temperature on both sub-and super-solvus samples. It is finally demonstrated that the main controlling parameter to reach a very high yield stress is grain size.

Impact of the Solution Cooling Rate and of Thermal Aging on the Creep Properties of the New Cast & Wrought Rene 65 Ni-Based Superalloy

“…8 also compares R65 to data generated by J.R. Vaunois and al. for C&W Udimet 720Li (U720Li) alloy also solution treated at 1080°C for 4 hours [11]. It is observed that the y' particles in R65 are smaller for given ST cooling rate.…”

“…C o o l in g r a t e (°C /m in ) Figure 8. y' size evolution as a function of the ST cooling rate for R65 (present study) and for a subsolvus C&W Udimet 720Li alloy [11] Impact of overaging on creep properties…”

“…The y' particle size distributions as well as the intermetallic area fraction, were determined using the Visilog® software and algorithms [11], based on at least 10 pictures taken in SEI mode for the y' particles and in BSE mode for intermetallic phases. A minimum of 10000 y' particles were included in each distribution.…”

On the prediction of the yield stress of unimodal and multimodal γ ′ Nickel-base superalloys

“…The model results have been tested mostly in powder metallurgy superalloys with grain size up to 55 lm for subsolvus and supersolvus heat treatments. However, for alloys processed by the cast and wrought route (C&W), the supersolvus heat treatment can lead to grain sizes as large as 1 mm [33]. A Hall-Petch constant of 710 MPa lm 1/2 was considered in our model, although this value could differ for other conditions since there is a concomitant alteration in the c 0 microstructure when modifying the grain size.…”

Toward a better understanding of strain incompatibilities at grain boundaries in the analysis of fatigue crack initiation at low temperature in the UdimetTM720 Li superalloy

“…In the following of the article, those microstructures will be denoted as the "Duplex" and the "Homogeneous" microstructures, respectively. The main microstructural characteristics (average grain size, volume fractions and sizes of primary, secondary and tertiary γ particles and average interparticle spacings) determined by image analyses techniques [7], [8] are detailed in Table 2. The average γ channels width was taken between the smallest γ precipitates.…”