Microstructural Template Consisting of a Face-Centered Cubic Matrix with Ordered Precipitates: Microstructural Evolution and Properties: Al Base 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
Monday 8:00 AM
February 24, 2020
Room: 30D
Location: San Diego Convention Ctr
Session Chair: Rajarshi Banerjee, University of North Texas
8:00 AM Introductory Comments
8:10 AM Cancelled
Evolution of Ordered Intermetallic Phases in Alloys Through Concomitant Clustering and Ordering Processes: Rumu Halder Banerjee1; Ashok Arya1; Srikumar Banerjee1; 1Bhabha Atomic Research Centre; Homi Bhabha National Institute
The ordering and clustering tendencies in alloys are often manifested either in the formation of superlattice structures with symmetry elements which are a subset of those of the parent structure or in the development of concentration modulations along specific crystallographic directions. A simultaneous or sequential operation of clustering and ordering processes may lead to formation of ordered particles aligned along elastically soft directions. This has been previously demonstrated based on first principles thermodynamic calculations, in the Al-Li and Ni-Mo systems, identifying the temperature-concentration-domains over which different sequences of transformations operate. A summary of these studies will be presented. The addition of Cr in Ni-Mo alloys changes the relative stabilities of the competing superlattice structures – a factor which determines the constituent phases in commercial Hastelloys. Phase stabilities in the Ni-rich side of the ternary Ni-Mo-Cr system will be discussed for illustrating the influence of ternary addition in the phase evolution process.
8:50 AM
Sc-free Nanoprecipitate-strengthened Aluminum Alloys with Exceptional Creep Resistance: Richard Michi1; David Seidman1; David Dunand1; 1Northwestern University
Dilute, nanoprecipitate-strengthened Al-Sc-Zr aluminum alloys that retain their strength for long periods of time at elevated temperatures have been studied for many years. These aluminum “superalloys” are strengthened by coherent, L12-structure Al3(Sc1-xZrx) nanoprecipitates. However, the high cost of Sc in these alloys limits their commercial viability. We have developed Zr-based, Sc-free alloys with similar aging behavior and coarsening resistance to the most advanced Sc-containing alloys developed to date. Nanoprecipitate cores enriched in Er provide a nearly 50% increase in dislocation creep threshold stress relative to similar Sc-containing alloys at 300 °C. This talk will discuss alloy precipitation behavior, temporal evolution of L12 nanoprecipitates as measured by atom-probe tomography, and creep properties at 300–400 °C.
9:10 AM
D022 + D022’ Dual-phase Microstructure in As-cast Al-Mo-Ti Alloys: Andreas Leineweber1; Mario Kriegel1; Stefan Martin1; ShunLi Shang2; Zi-Kui Liu2; 1TU Bergakademie Freiberg; 2Pennsylvania State University
Certain Al-rich Al-Mo-Ti alloys (e.g. Al73Mo22Ti5) obtained in the as-cast state contain a peculiar microstructure, which consists, according to powder X-ray diffraction analysis of tetragonal D022-Al3(Mo,Ti) as well as of an unprecedented orthorhombic variant thereof, called D022'-Al3(Mo,Ti). Microstructure analysis reveals intergrowth of both phases in a checkerboard microstructure with parallel c-axes and alternate orientations of D022'. Energy dispersive X-ray analysis demonstrates a higher Mo content in D022'-Al3(Mo,Ti) than in D022-Al3(Mo,Ti). The crystal structure of D022'-Al3(Mo,Ti) can be related to shear instability of hypothetical tetragonal D022-Al3Mo revealed by first-principles calculations. Analysis of the lattice metrics of D022' using so-called Aizu strain tensors, commonly used to assess spontaneous strain in ferroelastics, reveals that the strain of D022'-Al3(Mo,Ti) resembles that of the stable monoclinic fcc-based superstructures of binary Al3Mo and Al8Mo3. The checkerboard microstructure minimizes strain energy of the microstructure, which likely has formed by unmixing during or after solidification.
9:30 AM Break
10:00 AM Invited
High Temperature Microstructural Stability Mechanisms Revealed by Microscopy in Al-Cu-Mn-Zr Alloys: Jonathan Poplawsky1; Patrick Shower2; Lawrence Allard1; Matthew Chisholm1; Dongwon Shin2; Amit Shyam2; 1The Center for Nanophase Materials Sciences, Oak Ridge National Laboratory; 2Materials Science and Technology Division, Oak Ridge National Laboratory
Typical Al-alloys used for automotive applications can only withstand temperatures of 250C. Retention of a high number density of metastable 𝜃’ precipitates (Al2Cu) is crucial for these alloys’ strength. Recently, ORNL developed an affordable and castable Al-Cu alloy that maintains its strength after a lengthy exposure to 350C (> 200 Hrs). Mn and Zr additions have been shown to be critical for the high temperature stability through a series of atom probe tomography (APT) experiments, scanning transmission electron microscopy (STEM) experiments, and computational simulations. Mn and Zr segregate to both the coherent and semi-coherent 𝜃’ precipitate interfaces. APT was used to calculated interfacial Gibbsian excess that reveal segregation behaviors with time and temperature. The segregation stabilizes the precipitates due to a combined reduction in interfacial energy and solute drag mechanisms. APT was conducted at the CNMS, which is a U.S. DOE Office of Science user facility.
10:30 AM Invited
Nanoscale Precipitation-strengthened Al-Er-Sc-Zr-(V,Nb,Ta) Alloys: Keith Knipling1; 1Naval Research Laboratory
Al-Sc alloys are strengthened by nanoscale Al3Sc precipitates (L12 structure). By alloying with faster-diffusing Er and slower-diffusing Zr additions, core/shell precipitates are formed, consisting of an Er-enriched core surrounded by a Zr-enriched shell that enhances strength and thermal stability, respectively. The present study seeks ultimate strength and coarsening resistance by alloying Al-Er-Sc-Zr alloys with Group 5 additions (M = V, Nb, Ta), which are expected to be slower diffusers than Zr. By sequential nucleation of the constituent solutes we have engineered Al3(Er,Sc,Zr,M) precipitates with core/triple shell compositions. Relationships between the observed mechanical properties and the precipitate sizes, volume fraction, number density, and compositions are established using atom-probe tomography throughout the microstructural evolution of the alloys.
11:00 AM
Impact of L12-phase Dispersoids on the Hardening Behavior of Multi-phase Strengthened Aluminum Alloys: Viktor Wessely1; Robin Schäublin1; Stephan Gerstl1; Stefan Pogatscher2; Peter Uggowitzer1; Jörg Löffler1; 1Laboratory of Metal Physics and Technology, Department of Materials; 2Nonferrous Metallurgy, Montanuniversität Leoben
We focus on a new generation of hardenable aluminum alloys based on the concept of high-strength Al–Sc alloys that form coherent L12 structured precipitates. Replacing scandium by more cost-effective elements potentially opens new fields of applications. Suitable candidates are rare-earth or transition metals, such as Er, Zr, Hf or Yb. In this study we have deployed thermodynamic (CALPHAD) and kinetic modelling to design alloys and their heat treatments, with subsequent mechanical and microstructural characterization. 7xxx-series alloys with < 0.5 wt.% Er and Zr have been systematically studied. High-resolution transmission electron microscopy (TEM) and chemical mapping provided the composition, size distribution, and number density of the nanometric dispersoids and their structural relationship with the matrix. Correlative TEM-APT (atom-probe tomography) measurements suggest that the Al3(Er,Zr) phase detected significantly increases the precipitation kinetics of subsequent hardening phases which formed at lower temperatures. Additional TEM in-situ heating experiments validated the modelling study.