Dennis M. Dimiduk

Gamma titanium aluminide alloys—an assessment within the competition of aerospace structural materials

Abstract In the late 1970s and early 1980s, intermetallic phases emerged from being alloys of laboratory curiosity, or precipitate structures to be avoided or controlled in many ‘real metals’, to being widely investigated alloys, especially as prospective structural materials. After nearly two decades of sustained world-wide research, selected materials are emerging with a balance of properties which improves engineered systems, while others are introducing compelling research questions and prospects for significantly improved materials. This manuscript briefly examines selected intermetallic alloys and how they compete with other structural materials. The examination focuses on gamma titanium–aluminide alloys, from the perspective of their balance of engineering properties, state of maturity, and prospects for impacting industrial needs. A variety of engineering and business challenges remain to be solved, some of which are discussed.

Scale-Free Intermittent Flow in Crystal Plasticity

Under stress, crystals irreversibly deform through complex dislocation processes that intermittently change the microscopic material shape through isolated slip events. These underlying processes can be revealed in the statistics of the discrete changes. Through ultraprecise nanoscale measurements on nickel microcrystals, we directly determined the size of discrete slip events. The sizes ranged over nearly three orders of magnitude and exhibited a shock-and-aftershock, earthquake-like behavior over time. Analysis of the events reveals power-law scaling between the number of events and their magnitude, or scale-free flow. We show that dislocated crystals are a model system for studying scale-free behavior as observed in many macroscopic systems. In analogy to plate tectonics, smooth macroscopic-scale crystalline glide arises from the spatial and time averages of disruptive earthquake-like events at the nanometer scale.

Strength and ductile-phase toughening in the two-phase Nb/Nb5Si3 alloys

The effect of heat treatment on the mechanical properties of Nb-Nb5-Si3 two-phase alloys having compositions Nb-10 and 16 pct Si (compositions quoted in atomic percent) has been investigated. This includes an evaluation of the strength, ductility, and toughness of as-cast and hot-extruded product forms. The two phases are thermochemically stable up to ∼1670 °C, exhibit little coarsening up to 1500 °C, and are amenable to microstructural variations, which include changes in morphology and size. The measured mechanical properties and fractographic analysis indicate that in the extruded condition, the terminal Nb phase can provide significant toughening of the intermetallic Nb5Si3 matrix by plastic-stretching, interface-debonding, and crack-bridging mechanisms. It has been further shown that in these alloys, a high level of strength is retained up to 1400 °C.

Oxidation mechanisms in Mo-reinforced Mo5SiB2(T2)–Mo3Si alloys

Abstract Results of a systematic investigation of the oxidation in a Mo–Si–B alloy containing the three phases, Mo, Mo 3 Si, and Mo 5 SiB 2 (T2) are presented. The relative kinetics of B-containing silica-scale formation, permeation of MoO 3 through the viscous scale, the viscosity of the B-containing SiO 2 scale, volatilization of MoO 3 and B 2 O 3 from the silica scale, are identified as key parameters that determine the kinetics of the oxidation of the alloy in the temperature range of 500–1300 °C. The oxidation is worst in the intermediate temperature range, 650–750 °C, where MoO 3 begins to volatilize but B 2 O 3 does not, resulting in gaseous MoO 3 bubbling through a low viscosity borosilicate scale. In this temperature range, the scale provides insufficient protection suggesting that attempts to improve the oxidation resistance of this system must focus on this temperature range.

Estimating the strength of single-ended dislocation sources in micron-sized single crystals

Three-dimensional (3D) discrete dislocation dynamics simulations were used to calculate the effects of anisotropy of dislocation line tension (increasing Poissons ratio, ν) on the strength of single-ended dislocation sources in micron-sized volumes with free surfaces and to compare them with the strength of double-ended sources of equal length. Their plastic response was directly modelled within a 1 µm3 volume composed of a single crystal fcc metal. In general, double-ended sources are stronger than single-ended sources of an equal length and exhibit no significant effects from truncating the long-range elastic fields at this scale. The double-ended source strength increases with ν, exhibiting an increase of about 50% at ν = 0.38 (value for Ni) as compared to the value at ν = 0. Independent of dislocation line direction, for ν greater than 0.20, the strengths of single-ended sources depend upon the sense of the stress applied. The value for α in the expression for strength, τ = α(L)µb/L is shown to vary from 0.4 to 0.84 depending on the character of the dislocation and the direction of operation of the source at ν = 0.38 and L = 933b. By varying the lengths of the sources from 933 to 233b, it was shown that the scaling of the strength of single-ended and double-ended sources with their length both follow a ln(L/b)/(L/b) dependence. Surface image stresses are shown to have little effect on the critical stress of single-ended sources at a length of ∼250b or greater. This suggests that for 3D discrete dislocation dynamics simulations of the plastic deformation of micron-sized crystals in the size range 0.5–20 µm, image stresses making the surface traction-free can be neglected. The relationship between these findings and a recent statistical model for the hardening of small volumes is discussed.

The geometry and nature of pinning points of ½ 〈110] unit dislocations in binary TiAl alloys

Abstract The b = ½ 〈110] unit dislocations in deformed TiAl alloys exhibit a unique morphology, consisting of numerous pinning points along the dislocation line aligned roughly along the screw dislocation direction, and bowed-out segments between the pinning points. The three-dimensional arrangement of these dislocations has been characterized in detail, based on post-mortem weak-beam transmission electron microscopy observations in deformed binary Ti-50 at.% Al and Ti-52 at.% Al alloys. The bowed segments glide on parallel {111} primary planes, and the pinning points are jogs with a range of heights, up to a maximum of about 40 nm. The substructure evolution is consistent with dislocation glide involving frequent double cross-slip and consequent jog formation. The dislocations experience a large glide resistance during the forward (non-conservative) motion of these jogs. Pinning of unit dislocations is an intrinsic process in these alloys and is not related to the presence of interstitial-containing prec...

The compositional dependence of antiphase-boundary energies and the mechanism of anomalous flow in Ni3 Al alloys

Abstract The theory of the anomalous flow behaviour of Ll2 compounds has developed over the last 30 years. This theory has a foundation in the early estimates of the crystallographic anisotropy of antiphase-boundary (APB) energy in these compounds. In spite of this critical aspect of the theory, it is only in the last 5 years that electron microscopy has been employed to quantify the APB energies and to determine the detailed nature of dislocation structures at each stage of deformation. The present study examined binary Ni3Al single crystals having compositions which span the single-phase region, and selected ternary compositions expected to have a dominant influence on either {001} or {111} plane APB energies. Crystals were deformed in compression at an orientation near the [001] direction over the temperature range from — 196 to 700°C. Weak-beam dark-field microscopy was employed to quantify the APB energies and the APB energy anisotropy over the composition range. Experimental uncertainties associated...

Computational methods for microstructure-property relationships

Introduction.- Methods for Generating 3D Image Data for Material Microstructures.- 3D Microstructure Simulation.- Microstructure-Based Domain Partitioning, RVE Definition and RVE Evolution for Intrinsic Property Computations.- Coupling Microstructure Evolution with Microstructure Characterization.- Materials Response Representations & Limitations in Present-day Design Codes.- Local Continuum Approaches to Constitutive Modeling.- Strain Gradient Plasticity Dislocation Coarse-Graining Challenges Representing Length-Scale Dependence.- Constitutive Laws for Time Dependent Thermally-Activated Processes.- Conventional Finite Element Methods.- Full-Field or Meshless and Homogenization Methods for Handling the RVE.- Limitations in Homogenization and Uncertainty.- Links to Fracture Mechanics & Probabilistic Methods.- Accelerated Time Scaling Methods for Fatigue Analysis.- Treating Challenges Below the Grain Scale.- Multiscale Methods for Material Modeling.- Representing Environmentally Induced Damage.- Emerging Methods for Matching Simulation and Experimental Scales.- Approaching Design Systems for the Future.

Effect of microstructure on fatigue and tensile properties of the gamma TiAl alloy Ti-46.5Al-3.0Nb-2.1Cr-0.2W

Abstract The relationships between the microstructure and tensile and axial load-controlled fatigue properties of the alloy Ti-46.5Al-3.0Nb-2.1Cr-0.2W (atomic per cent) have been studied. Two different microstructures, i.e. duplex (grain size, 20 μm) and fully lamellar (grain size, 300 μm), were produced, through two-step forging and subsequent heat treatments, giving similar yield strengths at room temperature. The fracture strains at room temperature were about 1.1% and 2.9% for the materials with the fully lamellar and the duplex microstructures respectively. At 600 °C, the duplex material shows a 15% higher fatigue strength than that of the fully lamellar material. At this temperature, the gamma alloy of both microstructures reaches high ratios of the fatigue strength at 10 7 cycles to the ultimate tensile strength (UTS), i.e. about 0.95. At 800 °C, the fully lamellar material exhibits higher fatigue strength values above 10 5 cycles, and both microstructures result in a two-stage behavior, in contrast to the test at 600 °C. The second stage features the characteristic conventional fatigue behavior, with a broad amplitude stress range, while the first stage is characterized by a narrow band of fatigue stress levels near the UTS. The fracture modes for the duplex material showed a general trend from transgranular to intergranular failure with increasing temperature. For the fully lamellar material, a change from predominantly translamellar failure to a mixture of inter lamellar and translamellar failure was observed, resulting in a microscopically and macroscopically rough fracture surface. The strain rate sensitivity of the fully lamellar material was negligible in the temperature range tested.

Overview of experiments on microcrystal plasticity in FCC-derivative materials: selected challenges for modelling and simulation of plasticity

An important frontier in both metallurgy and mechanics is the development of a modelling framework that accurately represents length-scale effects and dislocation structure. Emerging mechanics formulations incorporate a length scale tied to distortion gradients aimed at modelling the effects from geometrically necessary dislocations (GND). However, recent experimental studies show important intrinsic size effects exist separately from an evolving GND structure at a mesoscopic scale. The present studies probed for intrinsic size effects at this scale using experimental methods that more closely approach the size scales accessible via discrete dislocation simulation (DDS) in 3d. Uniaxial compression tests of single crystals of pure Ni, Ni3Al alloys, Ni superalloys and fine grained polycrystalline Ni showed material-unique size-dependent responses. Notable among these are a range of strengths and clear intermittency of flow that reveals self-organization. Mechanistically self-consistent simulation of such results, by either discrete dislocation or continuum methods, stands as an unsolved challenge for emerging materials modelling approaches.