Mechanical and Creep Behavior of Advanced Materials: A SMD Symposium Honoring Prof. K. Linga Murty: High Temperature Creep of Structural Materials
Sponsored by: TMS Structural Materials Division, TMS: Mechanical Behavior of Materials Committee, TMS: Nuclear Materials Committee
Program Organizers: Indrajit Charit, University of Idaho; Yuntian Zhu, North Carolina State University; Stuart Maloy, Los Alamos National Laboratory; Peter Liaw, University of Tennessee - Knoxville

Monday 2:00 PM
February 27, 2017
Room: 24A
Location: San Diego Convention Ctr

Session Chair: James Earthman, University of California - Irvine; Rajiv Mishra, University of North Texas

2:00 PM  Keynote
Fundamentals of Creep in Aluminum Over a Very Wide Temperature Range: Michael Kassner1; Kamia Smith1; 1University of Southern California
    Some of the earliest characterization of steady-state creep in aluminum, was developed at UC Berkeley by John Dorn, his students and postdoctoral scholars, one being K.L. Murty. Research of the group included classic five power-law creep, Harper-Dorn creep, power-law breakdown and viscous glide. Many of the models and theories persisted for a relatively long period of time due to the thoughtfulness of the work. This paper discusses the more recent developments in these phenomena that may lead to new interpretations in aluminum creep, as well as other crystalline materials.

2:30 PM  Invited
Development of Creep-Resistant Austenitic Stainless Steels for High Temperature Applications: Philip Maziasz1; 1Oak Ridge National Laboratory
    Austenitic stainless steels are cost effective materials for high temperature applications, if they have the oxidation and creep resistance to withstand prolonged exposure to such conditions. This talk will highlight several examples of different types of cast and wrought austenitic stainless steels that creep resistance at 600-900C that compares to solid solution Ni-based superalloys due to carefully designed nano-scale precipitate microstructures using a microstructural engineering approach. These steels include the HT-UPS steels, the cast CF8C-Plus steel and the current AFA steels and alloys. This alloy development approach can deliver alloys that are ready for commercial applications and component trials in 1.5 to 2 years. Examples of commercial applications, particularly for the CF8C-Plus steel will be overviewed.

2:50 PM  Invited
Effect of Dynamic Strain Aging on Creep in Titanium Alloys: Priyanka Agrawal1; S. Karthikeyan1; Dipankar Banerjee1; 1Indian Institute of Science
    We explore in this presentation the interaction between dynamic strain aging and creep in titanium. Tensile tests were used to determine the strain rate domain of dynamic strain aging and PLC in a creep resistant, near alpha titanium alloy IMI 834 over the temperature range 350-500°C. Creep behaviour of the alloy was explored over this temperature regime. Anomalous primary creep behaviour has been observed in the form of abrupt decreases in strain rate beyond certain critical strains, as well as in instantaneous strain rates on loading. Creep behaviour also exhibited a large tension-compression asymmetry. Dislocation structure in primary creep was characterised and this revealed the preponderance of jogged-screw dislocations at small creep strains. A model for creep based upon the interaction of solute atmospheres with dislocations within the framework of a jogged screw dislocation model for creep has been developed.

3:10 PM  Invited
Mechanisms Governing the Creep Behavior of High Temperature Alloys for Generation IV Nuclear Energy Applications: Vijay Vasudevan1; Xingshuo Wen2; Laura Carroll3; Richard Wright3; T. L. Sham4; 1University of Cincinnati; 2Electrodiesel Corp; 3Idaho National Laboratory; 4Oak Ridge National Laboratory
    The mechanisms governing the creep behavior of Alloy 617 (Ni-20Cr-10Co-8Mo-1Al-0.6Ti) and Alloy 800H (Fe-33Ni-22Cr-0.4Al-0.3Ti), which are candidate materials for intermediate heat exchangers in the NGNP, are reported, including the effects of grain size, grain boundaries and second phases. Thermomechanically processed plates of alloys 617 and 800H were produced with different grain sizes and their creep behavior over the temperature range of 850 to 1050°C and stress levels ranging from 5 to 100 MPa was studied. The creep data was analyzed to decipher the various stages, the stress exponents and activation energies were determined and diffusional versus dislocation creep mechanisms were discriminated. Microstructural changes and damage processes following creep were characterized by SEM/EBSD and TEM, with particular attention given to the evolution of grain size, grain boundaries, grain boundary structure, second phase precipitates, dislocation structures and void formation. The various results relating the creep behavior to microstructural changes in these alloys and associated mechanisms will be presented and discussed.

3:30 PM Break

3:45 PM  Keynote
Creep of Dispersion Strengthened Materials – Emergence of Paradigms Challenging the Old Theories: Rajiv Mishra1; 1University of North Texas
    Creep theories have evolved from 1950s to accommodate for microstructural complexities in advanced materials. Nabarro-Herring diffusional creep model is a classic example of an elegant theory set in a simple microstructural discussion. Engineering materials are often microstructurally complex and derive their strength from a number of mechanisms. The creep deformation is analyzed in terms of parallel and sequential processes. In this overview, the emergence of mechanistic understanding will be presented by considering recent examples that challenge the conventional thinking. The approach of using single crystal blades in engines is built on eliminating grain boundaries to suppress creep deformation and failure mechanisms. Can nanocrystalline materials be creep resistant? Recent results show this possibility. The role of dispersion strengthening can be direct or indirect for diffusional and dislocation creep mechanisms. These examples will be highlighted to emphasize a few fundamental creep-resistant microstructural paradigms.

4:15 PM  Cancelled
Uniaxial and Multiaxial Miniature Specimen Creep Testing of Single Crystal Ni-base Superalloys (SX): Gunther Eggeler1; Philip Wollgramm1; David Bürger1; Lijie Cao1; Xiaoxiang Wu1; Alireza Parsa1; 1Ruhr University Bochum
    Single crystal Ni-base superalloys (SX) are used to make blades for gas turbines for the aerospace and energy sectors. First stage blades of gas turbines have to withstand mechanical loads at temperatures far beyond half of the melting point in Kelvin. Creep is a key component of the load spectrum of these critical high temperature components. From an engineering point of view it is important to base engine design on a reliable creep data base. From a scientific point of view creep testing can help to understand elementary deformation and damage mechanisms which are responsible for high temperature strength. Three creep test techniques, all capable of providing creep data for temperatures up to 1100°C, are briefly reviewed: miniature tensile, miniature circular notched tensile and double shear experiments. Examples on how these test techniques can be used to progress our understanding of the creep behavior of SX are given.

4:35 PM  
TerraPower HT9 Mechanical and Thermal Creep Properties: Cheng Xu1; Micah Hackett1; 1TerraPower
    TerraPower has revitalized the manufacturing of HT9, and optimized its heat treatment to be used as fuel claddings in the Traveling Wave Reactor. TerraPower initiated a comprehensive test program to compare the mechanical and thermal creep properties of the optimized TerraPower HT9 to the historical HT9. The uniaxial tensile tests show TerraPower HT9 has improved yield strength compared to historical HT9 across all temperatures. Charpy impact toughness tests on TerraPower HT9 show lower upper shelf energy compared to historical HT9, but the ductile to brittle transition temperature (DBTT) between the steels remain similar. Three point bend tests between room temperature and 400°C show comparable fracture toughness to those of historical HT9. Thermal creep test data suggest TerraPower HT9 have improved creep strength compared to historical HT9 at temperatures below 650°C.

4:55 PM  
Creep Behavior of a Microstructurally Stable Nanocrystalline Alloy: K. Darling1; M Rajagopalan2; M Komarasamy3; M Bhatia2; B Hornbuckle1; R Mishra3; Kiran Solanki2; 1ARL; 2Arizona State University; 3UNT
    Nanocrystalline (NC) metals with a mean grain size below 100 nm, have superior room temperature strength in part due to a large reduction in grain size(Gleiter 2000). However, this high strength generally comes with dramatic losses in other properties, such as creep resistance. For instance, the creep rates in NC-Cu are about four orders of magnitude higher than typical coarse-grained Cu(Mohamed and Li 2001; Chokshi 2009). In this study, we will evaluate the creep response of a microstructurally stable nanocrystalline alloy (Cu-10 at.%Ta) at various homologous temperatures between 0.5 and 0.64 Tm under an applied stress range of 0.85% to 1.2% of the shear modulus. Further, various advanced electron microscopy techniques will be used to elucidate and discussed unique deformation mechanisms pertaining to this class of immiscible alloys.

5:15 PM  
On the Creep Behavior of Dual-Scale Particle Strengthened Nickel Based Alloy: Aniket Dutt1; Somayeh Pasebani2; Indrajit Charit2; Rajiv Mishra1; 1University of North Texas; 2University of Idaho
    A dual-scale particle strengthened Ni-20Cr based alloy was developed via mechanical alloying followed by spark plasma sintering. Approximately 1.2 wt.% nano-yttria and 5 wt.% sub-micron alumina were added to the Ni-20Cr matrix to improve the high temperature creep strength through dispersion strengthening mechanism and load transfer mechanism, respectively. Creep tests were conducted on three alloy systems: Ni-20Cr, Ni-20Cr-1.2Y2O3, and Ni-20Cr-1.2Y2O3-5Al2O3 (wt.%) at 800 oC and 100 MPa. Ni-20Cr-1.2Y2O3-5Al2O3 exhibited the highest creep resistance with minimum creep rate of ~2.3×10-9 s-1 under conditions of 100 MPa and 800oC. While major contribution towards high creep strength was generated from dispersion strengthening, significant creep resistance in Ni-20Cr-1.2Y2O3-5Al2O3 alloy implied effectiveness of the dual-scale particle strengthening mechanism. Transmission electron microscopy, 3D atom probe tomography and nano-indentation analysis were used to understand the correlation between the microstructural distribution of nano-Y2O3 and sub-micron Al2O3 particles and high temperature mechanical properties of Ni-20Cr based alloys.