Microstructural Template Consisting of a Face-Centered Cubic Matrix with Ordered Precipitates: Microstructural Evolution and Properties: Complex Concentrated Alloys/High Entropy Alloys
Sponsored by: TMS Materials Processing and Manufacturing Division, TMS: Phase Transformations Committee
Program Organizers: Rajarshi Banerjee, University of North Texas; Eric Lass, University of Tennessee-Knoxville; Ashley Paz Y Puente, University of Cincinnati; Tushar Borkar, Cleveland State University; Keith Knipling, Naval Research Laboratory; Sophie Primig, University of New South Wales

Tuesday 8:30 AM
February 25, 2020
Room: 30D
Location: San Diego Convention Ctr

Session Chair: Eric Lass, University of Tennessee

8:30 AM  Keynote
Elevated Temperature Microstructure Evolution and Properties of an Equiatomic CrCoNi Superalloy Containing Al,Ti: Connor Slone¹; Easo George²; Michael Mills¹; ¹The Ohio State University; ²Oak Ridge National Laboratory; University of Tennessee
    A new medium-entropy superalloy was produced based on the compositions of equiatomic CrCoNi and Ni-base superalloy Inconel 740H. Initial alloy design was performed using Thermo-Calc. The aging response and microstructural stability were assessed following heat treatment at temperatures between 600 - 900°C and durations up to 100 h. Aging from a fully recrystallized state resulted in negligible grain growth and produced ã’ and ó phases. The same phases were present after aging from a cold-rolled state, but partially recrystallized microstructures resulted in multi-modal size distributions and heterogeneous spatial arrangements. Room temperature hardness measurements were used to correlate aging conditions with quantitative precipitate measurements, mechanical properties, and deformation mechanisms. Of particular interest is the contribution of twinning to deformation of this superalloy in comparison to that exhibited by the equiatomic NiCoCr solid solution alloy.

9:10 AM  Invited
FCC High-entropy Alloys Strengthened with L1₂ Precipitates: Alloying Strategies and Highlights: Jean-Philippe Couzinie¹; Thomas Rieger¹; Jean-Marc Joubert¹; Mathilde Laurent-Brocq¹; Guy Dirras²; ¹Université Paris Est, ICMPE (UMR 7182) CNRS-UPEC; ²Université Paris 13, Sorbonne Paris Cité, LSPM (UPR 3407) CNRS
     Since 15 years, the alloy design based on the so called "high-entropy" approach emerges as a powerful tool to develop new materials with enhanced and sometimes unexpected properties. Among the potential material solutions for structural applications, high-entropy alloys based on 3d transition elements with FCC microstructures are promising. However these alloys are currently suffering of poor mechanical properties for temperatures above 800°C. A possible way for improving the high temperature mechanical behavior is to consider the (Ni,Co)₃(Ti,Al) precipitation.In this framework, a short review of some key results of the topic will be addressed. The different alloying strategies leading to duplex FCC+L1₂ microstructures will be discussed. More particularly, a part of the presentation will be focused on the high throughput CALPHAD calculations on the Ni-Co-Cr-Fe-Ti-Al senary system, allowing to screen compositions with the desired microstructure. Experimental results on few compositions selected from calculations will be exposed.

9:40 AM
Novel Microstructural Template Based on Ordered Intermetallic Precipitation in FCC Based Complex Concentrated Alloys: Sriswaroop Dasari¹; Vishal Soni¹; Abhinav Jagetia¹; Rajarshi Banerjee¹; ¹University of North Texas
    The microstructural template consisting of a coherent mixture of FCC and L12 phases was widely explored and the mechanical behavior is well established in Ni-base and Co-base superalloys. A specific advantage with mimicking the same microstructure in Complex Concentrated Alloys (CCA’s) is that the FCC solid solution matrix remains concentrated even after precipitating out the ordered phases. In addition to a potentially enhanced solid-solution strengthening effect, novel microstructures can be engineered in these alloys by controlling the competition between L12 and B2/L21 phases. FCC based CCA’s have been designed using CALPHAD approach, fabricated by conventional arc melting and thermo-mechanically processed to produce microstructures with different volume fractions of L12 and B2/L21 phases. Detailed microstructural and mechanical characterization was performed using using transmission electron microscopy (TEM), atom probe tomography (APT) and room temperature tensile testing.

10:00 AM Break

10:30 AM
Coarsening Kinetics and Mechanical Properties of fcc Compositionally Complex Alloys Strengthened by L1₂ Precipitates: Thomas Rieger¹; Mathilde Laurent-Brocq¹; Ivan Guillot¹; Jean-Marc Joubert¹; Loïc Perrière¹; Didier Locq²; Zhao Huvelin²; Azziz Hocini³; Guy Dirras³; Jean-Philippe Couzinié¹; ¹ICMPE, UMR 7182, CNRS - UPEC, F-94320, Thiais, France; ²ONERA, Université Paris Saclay, F-92322 Châtillon – France; ³Université Paris 13, LSPM (UPR 3407), F-93430 Villetaneuse, France
     The primary purpose of the study is to develop new classes of alloys for low-pressure turbine blades for the aerospace industry supporting extreme conditions in the 800-1000°C range.The most obvious way to strengthen face centered cubic (fcc) compositionally complex alloys (CCAs) is to mimic nickel superalloys microstructure in which L1₂ ordered precipitates are used to reinforce the disordered matrix with fcc structure. Following this strategy, high throughput CALPHAD calculations were performed on the Ni-Co-Cr-Fe-Ti-Al senary system. From these calculations, several compositions of interest were selected. They have then been subsequently processed and characterized. Coarsening kinetics of the L1₂ phase in four model alloys has been measured to evaluate the evolution of precipitate size and morphology between 800°C and 1000°C. The impact of the strengthening phase on mechanical properties has also been demonstrated using micro-hardness and quasi-static uniaxial loadings both at room and elevated temperatures.

10:50 AM
A Nickel-based Superalloy Dual-reinforced by L1₂ Ni₃Al and D0₂₂ Ni₃V Ordered-fcc Precipitates: Alexander Knowles¹; Lucy Reynolds²; Vassili Vorontsov³; David Dye²; ¹University of Birmingham; ²Imperial College London; ³University of Strathclyde
     Nickel based superalloys are widely used as high temperature materials owing to their combination of strength, creep, toughness, environmental resistance and microstructural stability in the 650–1200°C range. These exploit an fcc Ni matrix with L1₂ Ni₃Al precipitates. However, there are other ordered-fcc structures where greater understanding is needed, for example, D0₂₂, as employed in Inconel 718, however, metastable Ni₃Nb may decompose to D0_a.Here a nickel based superalloy has been produced reinforced by two different ordered-fcc intermetallic compounds, γ' L1₂ Ni₃Al and γ’’ D0₂₂ Ni₃V. Primary ageing at 900–1000°C precipitated spherical L1₂ Ni₃Al, whose volume fraction and size were controlled by ageing temperature and time. Secondary ageing at 700°C precipitated laths of D0₂₂ Ni₃V. The dual precipitation increased hardness by ~85 HV, with ∼500 MPa compressive strength at 800°C. Electron microscopy studied the Ni₃Al precipitation and confirmed the form of the secondary Ni₃V precipitates and their long-term stability.

11:10 AM
Coarsening and Creep Behavior of Co-Ni-Al-W-Ti-Ta-B Superalloy with High γ′ Solvus Temperature: Fei Xue¹; Dingwen Chung¹; Eric Lass²; David Seidman¹; David Dunand¹; ¹Northwestern University; ²The University of Tennessee
    Increasing the γ′ solvus temperature of Co-base superalloys is an important challenge for their development and eventual commercial use. This work investigates two multinary Co-base alloys, with 0.5 at.% difference in Ti-content, which exhibit a high γ′ solvus temperature of 1196 and 1211°C. The γ-γ′ two-phase microstructure with ~80% γ′(L12) precipitates, observed in both alloys, coarsens slowly upon annealing at 900 and 1000°C for up to 1,000 h (and at 1100°C for up to 168 h), with the low Ti-content alloy showing a slightly less coarsened γ′ precipitates. No secondary phase precipitates are detected, even after the long-term annealing. Atom probe tomography shows enrichment of Ni, Al, W, Ti, Ta and B in the γ′ precipitates after annealing at 900°C for 168 h in both alloys. Comparable creep properties are found in both alloys at 850 and 900°C. A further composition design with Cr additions to these alloys is presented.