Advanced High Temperature Structural and Transportation Materials

            There is a constantly increasing demand for improved lightweight, high-temperature engineering materials for applications in advanced aerospace, automotive and propulsion systems. Many long range ordered intermetallic compounds combine a high melting point with good oxidation resistance, a favorable strength-to-weight ratio, a resistance to softening at elevated temperatures, and good creep and fatigue resistance. Candidate systems that have been explored in recent years include the gamma titanium aluminides (for 600-850°C), niobium aluminides/silicides (for 850-1200°C) and molybdenum silicides (1000-1400°C). Our past and current work has focused on three key areas: 1) fundamental aspects of mechanical properties, deformation mechanisms and oxidation, 2) kinetics, thermodynamics and mechanisms of phase transformations and microstructural evolution, and 3) the effects of microstructure on the tensile and creep behavior.

A.   TiAl-Based Intermetallic Alloys (supported previously by NSF; Student: Krishna Cherukuri).

One area that is of focus involves the application of detailed quantitative high-resolution transmission electron microscopy (HRTEM) in a study of massive and other phase transformations in gamma titanium aluminides (g-TiAl-based alloys). Through our previous studies under NSF support, the kinetics, temperature dependence and thermodynamics of the a®g_M massive transformation were established using a novel temperature and electrical resistivity system (TERMS), and the microstructural changes accompanying the transformation were studied using HRTEM techniques to address two very important issues relating to the mechanisms of the massive transformation, namely: 1) the nature of the massive-parent interphase interfacial structure and 2) the role of defect structures. The HRTEM results are combined with those of the reaction kinetics to both gain new insight into how changes in atomic configuration during these transformations occur and the associated kinetic, thermodynamic and crystallographic features and to serve as input for testing theoretical models of nucleation and growth.
 In ongoing work we have been systematically investigating the effects of alloy chemistry and thermal treatment on the kinetics and mechanisms of evolution of microstructure in multicomponent alloys.

B.   Nb-Si-Ti-X Refractory Intermetallic-Based Alloys for High Temperature Aero Engine Applications (AFRL/Rolls-Royce supported; Postdoc: R. Tewari; Students: Hyojin Song, Mengyao Xu).

Alloys based on these systems are of interest for aero engine structural applications at temperatures exceeding the potential use limits offered by the gamma titanium aluminides and nickel-base superalloys. These materials systems, in which a ductile solid solution matrix is in equilibrium with a strong ternary silicide, present significant flexibility in tailoring microstructure and mechanical properties and are generally amenable to conventional alloy processing methods. The goal of this work, which is presently supported by AFRL/Rolls Royce, is to gain a scientific understanding of the effects of composition, processing, heat treatment and microstructure on the mechanical properties, oxidation behavior, embrittlement causes and performance of these newer materials. We have made significant progress in this work, including development of new alloys with enhanced oxidation resistance, based on which a patent disclosure is being prepared; two journal articles have been submitted and several others are in preparation for submission.

C.   Mo-Si-B–Based Refractory Intermetallic Alloys (DoE/NETL supported; Student: Brian Riestenberg).

Another area we have been involved with that is funded by DoE-NETL concerns a new class of oxidation resistant Mo-Si-B alloys that are candidate materials for very high temperature (1000-1400°C) structural applications. In the recent past, such efforts have been concerned with the MoSi₂ compound because of its outstanding oxidation resistance. However, lack of ductility has been a major problem and all the ductilization and toughening strategies pursued to date have yielded little or no success. More recently, it has been recognized that balanced properties maybe realized with a two/three-phase system based on Mo-Si-B alloys in which a ductile/tough Mo matrix is in equilibrium with a strong ternary silicide/borosilicide There are also promising indications from initial studies that these alloys offer good oxidation and creep resistance. However, extensive research efforts on composition-processing-microstructure-mechanical property relationships and deformation mechanisms are required to advance these alloys into use. The present effort is concerned with processing, microstructure and creep behavior of these materials, with strong collaborative work with scientists at AFRL and ORNL and use of testing facilities there. Significant progress in the determination of strength and creep properties has been made.

D.  Advanced Boiler Materials for Ultra-Supercritical (USC) Coal Power Plants (DoE/UT Battelle-ORNL supported; Student: Quanyan Wu).

This project, which began in August 2002, is part of a larger USC Steam Boiler consortium funded by DoE/NETL and Ohio Coal Development Office (OCDO) involving scientists at ORNL, industry (Alstom, Babcock & Wilcox, Foster Wheeler) and Universities (UC) on new advanced materials for applications in superheated tubing and boilers in USC coal power plants. The goal of improving the efficiency of pulverized coal power plants has been pursued for decades. The need for greater efficiency and reduced environmental impact is pushing utilities to ultra supercritical (USC) conditions, i.e. steam conditions of 760°C and 35 MPa. The long-term creep strength and environmental resistance requirements imposed by these conditions are clearly beyond the capacity of the currently used ferritic steels and other related alloys. The goal of USC Consortium is to investigate the use of three classes of newer materials for applications in coal-fired power plants: advanced ferritic steels, (Save12, P92, T23) advanced/modified austenitic stainless steels (Super304H) and nickel base superalloys (Haynes 230, CCA 617, IN 740). Our effort is aimed at gaining a scientific understanding of the effect of composition, processing, heat treatment and microstructure on the mechanical properties, deformation mechanisms, embrittlement causes and performance of these newer materials. The results of this research are already having a significant impact on the application of these newer materials as components in high temperature, corrosive environments, and several publications have resulted, with additional ones in preparation.

E.  High Temperature Aluminum Alloys for Diesel Engine Applications (Cummins Inc. supported; Student: Prashanth Prasad).

One part of this project area that is currently funded by Cummins combines atomic-scale characterization and detailed quantitative analysis in a study of the microstructure of new aluminum alloys for high–temperature diesel engine applications. Candidate alloy compositions—based on the Al-Si system with additions of a host of other elements to produce nanoscale precipitates are investigated using an arsenal of diffraction and analytical and high-resolution electron microscopy techniques. These results are correlated with mechanical properties and failure modes. Significant progress has been made in the project and two journal articles are planned based on the results. 

F.      Delamination and Fracture Behavior of Aluminum-Lithium Alloys (UES/AFRL supported; Student: Amrinder Singh Gill).

This research, which is currently funded by AFRL/UES and being conducted in collaboration with Boeing (K. K. Sankaran) and AFRL (Dan Evans), combines detailed atomic-scale characterization and quantitative analysis of microstructure with property determinations in a study of the delamination and fracture behavior of Al-Li alloys. In recent years, there has been enormous interest in the cryogenic properties of Al-Li alloys, following reports of a marked increase in ductility, fatigue resistance, and especially fracture toughness with decrease in temperature from ambient to 4K. Consequently, these materials have become attractive candidates for liquid-hydrogen/oxygen/natural gas fuel tanks, in particular for existing and future transatmospheric and hypersonic rocket motor cases/fuel tanks and aircraft applications. A major concern for these applications is fracture toughness and premature fracture. Typically, these materials have a partially recrystallized, elongated grain and textured microstructure, which leads to significant differences in the low-temperature fracture toughness for the longitudinal (L-T, T-L) and short-transverse (S-L, S-T) orientations. The mechanistic origin of the anisotropy in fracture toughness and causes of delamination are uncertain. It is expected that the tendency for delamination, and hence the fracture toughness values and fracture behavior, will be significantly influenced by the aging treatment, microstructure, precipitate type, size and distribution, grain boundaries, texture and stress state, but no studies have been performed that shed light on the influence of these factors in a quantitative way, so that the fundamental mechanisms and causes of delamination are not known with certainty.

This project is aimed at determining the fundamental causes and mechanisms of delamination in Al-Li alloys, particularly the effects of microstructure (precipitation, texture, etc.), grain boundaries, stress and temperature (as expected in actual service). The C458 (Al-1.8Li-2.7Cu-0.3Mg-0.5Zr-0.3Mn-0.8Zn) alloy is being studied, since it is already being evaluated by Boeing and others as the material for rocket motor cases, but is prone to cracking by delamination possibly because the through-thickness (S-L, S-T) stresses encountered under plane strain conditions may potentially be greater.

G.  Laser Shock Peening (LSP) of Materials (GE Aircraft Engines supported; Students: Yixiang Zhao, Amrinder Singh Gill).

This project, which has just begun, is to establish a Center for Laser Shock Processing of Materials and perform related research at UC, including LSP equipment donation. In recent years, there has been enormous interest in the use of LSP to enhance the service lifetimes of critical metal parts, from aircraft engine fan and compressor blades fabricated from Ti and Ni-base alloys to HIP joints. Studies to date have revealed that LSP leads to a dramatic improvement in the fatigue strength, life and resistance to crack propagation in materials and parts. There is evidence showing that the improvements in fatigue life are brought about by the shock wave-induced generation of a deep compressive residual stress in the material extending from the surface to the interior. The increased depth effectively improves the fatigue life by a factor of three to five times over that provided by conventional peening treatments. In fact, tests on deliberately nicked (and hence weakened) Ti and Ni fan and compressor blades show that LSP will actually render these parts stronger than new, unflawed—but not laser-peened—blades, which points to improved resistance to foreign object damage. In addition, other properties like corrosion resistance and hardness are also improved markedly, and there are indications that creep performance at elevated temperatures may also be improved. While the LSP method has been applied largely to fan and compressor blades, there is growing realization that the properties and performance of other turbine engine parts like rotors, disks, gears and shafts could also be improved appreciably. Furthermore, use of the LSP method to other high value industrial applications like oil tools (drill collars, bits, mud pumps, etc.); automotive, marine and rocket engine parts; golf equipment and even hip joints is a clear expectation, potentially leading to a huge market for this technology.

Although the LSP technology is already being applied with great success to turbine engine parts, fundamental knowledge of the changes that LSP brings about on materials is still lacking. This project will focus on: 1) development of a basic understanding of laser shock wave effects on changes in microstructure and residual stress distributions in Ti and Ni-base alloys, 2) establishing quantitative relationships between various laser shock fluence parameters (like pulse duration, number of pulses, pulse energy, etc), microstructural changes and residual stress distributions and their relation to properties, and 3) investigate nondestructive methods for evaluation of LSPed materials to serve as a confidence-indicator for process conditions and expected part performance.

H.  Study of Surface-Treated and Cold-Worked Nickel-Base Superalloys (UTC/AFRL supported; Student:Hyo-Jin Song).

The intentional introduction of near-surface compressive residual stresses, using methods like shot peening, laser shock peening (LSP) and low-plasticity burnishing (LPB), is a well known practice for enhancing the resistance to fatigue crack nucleation and growth of turbine engine parts. Nondestructive evaluation of residual stress gradients in such surface-enhanced materials has great significance for component life extension and their reliability in service, and, in this context, the NDE Branch at AFRL/MLLP has made significant progress recently towards the development of a quantitative eddy current conductivity method for residual stress measurement in nickel-base superalloys. However, the difficulty in separating the residual stress (elastic) and cold work (plastic) contributions militates against the achievement of a complete analysis of the AECC results, and, furthermore, there is a lack of basic understanding of how intrinsic and extrinsic factors like microstructural changes brought about by surface and/or cold-work treatments affect the conductivity. It is thus clear that complementary research is needed to achieve better fundamental understanding of the effects of elastic and plastic strains (i.e., residual stress and cold work) on the electrical conductivity and microstructure of nickel-base superalloys, in order to develop accurate calibration techniques for quantitative nondestructive residual stress assessment in surface-treated components. This project, which has just begun, will combine measurements of electrical resistivity/conductivity with detailed atomic-scale characterization of microstructure and quantitative analysis in a study of surface-treated and cold worked Ni-base superalloys. Using these techniques, we will gain insight into what changes in microstructure and atomic configuration occur in these materials during laser-shock/shot peening and cold working and their relationships with the measured frequency-dependent apparent eddy current conductivity (AECC) and nanoindentation results being established in other parts of this program.

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