Precipitate Hardened Nickel Based Superalloy – ME3
Dwell-fatigue crack growth studies are performed on ME3 alloy generated in two different microstructures, one with a planar grain boundary and one with a serrated grain boundary. Summaries of these studies are listed below:
Modeling of Creep-Environment Interaction Mechanisms
A multi-scale, mechanistic model is developed to describe and predict the dwell-fatigue crack growth rate as a function of creep-environment interactions. In this model, the time-dependent cracking mechanisms involve grain boundary sliding and dynamic embrittlement, which are identified by the grain boundary activation energy, as well as, the slip/grain boundary interactions in both air and vacuum.
Grain Boundary Deformation and Fracture Criterion
The deformation behavior of the grain boundary is described by the rate at which the time-dependent sliding reaches a critical displacement. A grain boundary damage criterion is introduced by considering the GB mobility limit in the tangential direction leading to strain incompatibility and failure. The mobility takes into consideration the influence of oxygen flux and resulting dynamic embrittlement of the boundary reflected in alteration of the critical sliding displacement.
Loading Frequency and Microstructure Interactions in Intergranular Fatigue Crack Growth
The role of the loading frequency on the dwell fatigue crack growth mechanism is examined by carrying out a set of crack growth experiments in air and vacuum at three temperatures; 650°C, 704°C and 760°C using a dwell loading cycle with hold times up to 7200 seconds imposed at the maximum load level. These tests are used to identify the transgranular/intergranular transitional loading frequency and to determine the apparent activation energy of the time-dependent crack growth process.
Precipitate Hardened Nickel Based Superalloy – IN100
Dislocation/Precipitate Interactions
The influence of γ’ size on critical resolved shear stress is examined by considering dislocation/precipitate interactions involving particle shearing and Orowan by-passing mechanisms. In this, the critical resolved shear stress which is calculated as the sum of yield components corresponding to solid solution and γ’ particles being sheared and looped. The critical particle size defining the shearing/looping transition is determined and used to calculate the relative volume fraction and size of particles contributing to the critical resolved shear stress.
Isotropic and Kinematic Hardening as Functions of Gamma Prime Precipitates
A microstructure-explicit constitutive model is being developed in which the isotropic and kinematic stress components are derived as a function of the size and volume fraction of γ’ precipitates, considering particle shearing and/or Orowan by-passing mechanisms. The critical shearing/looping particle size is determined from the knowledge of the yield stress of heat treated specimens having different γ’ sizes. This critical size is used to calculate the relative contribution of particles in a distribution based on their size being cut or looped by mobile dislocations.
Relative Contributions of Secondary and Tertiary γ‘ Precipitates to Intergranular Crack Growth Resistance
The role of secondary and tertiary γ’ on intergranular crack growth rate is examined by performing a series of different heat treatments to vary the γ’ statistics. Dwell fatigue crack growth experiments are performed at 650°C and 700°C in air with dwell time loading cycles. The dwell crack growth rate is shown to be sensitive to γ’ variations. Considering that the yield strength of the continuum is a function of γ’ statistics, the role of γ’ on intergranular cracking is illustrated by correlating the crack growth rate and the continuum yield strength.
A Damage-Based Cohesive Zone Model of Intergranular Crack Growth Process
A cohesive zone model is developed to simulate grain boundary interface decohesion under dwell loading cycle. The role of creep–fatigue and environment has been introduced in terms of a scalar damage parameter coupled with the traction–displacement laws governing the grain boundary cohesion as well as the surrounding time-dependent deformation field.
Solid Solution Strengthened Nickel Based Superalloy – Alloy 617
Dwell-Fatigue Crack Growth in Solid Solution Strengthened Alloy 617 with Non Uniform Grain Boundary Carbide Distribution
The objective of this work is to predict the long terms deformation and damage mechanisms in Alloy 617 subjected to creep-fatigue loadings at elevated temperatures. Microstructural changes in grain boundary carbide precipitates have been correlated with stress during creep at 950°C. Compact tension specimens have been extracted from specimens which have been subjected to creep testing at 950°C. Specimens, containing grain boundary carbide precipitates on all grain boundaries and specimens with discrete carbide distributions are subjected to dwell-fatigue crack growth testing in air and vacuum environment.
Low Carbon Steel
Time-dependent deformation of low carbon steel at elevated temperatures
The microstructure and deformation properties of structural steel at elevated temperatures are examined in terms of the amounts and morphology of carbides as a function of thermal exposure parameters. A viscoplastic constitutive model has been employed to simulate the flow behavior of the steel. The material parameters were shown to be sensitive to the microstructure and temperature. Variation in carbide amounts and morphology in the post thermal exposed specimens result in differences in the kinematic hardening. Furthermore, the temperature sensitivity of the isotropic hardening is indicated by the presence a cyclic hardening/softening transition in the temperatures 600–700 ◦C.
Simulation of Viscoplastic Deformation of Low Carbon Steel Structures at Elevated Temperatures
The deformation response of low carbon steel subjected to high temperature is simulated using a viscoplastic material constitutive model which acknowledges the evolution of the material hardening parameters during the loading history. Both the temperature dependency and strain-rate sensitivity of the material parameters have been examined by the analysis of a single steel beam and a steel-framed structure subjected to temperatures ranging from 300 to 700C. Sequentially coupled thermal-stress analysis is applied to a structure under simulated fire condition.
Deformation characteristics of low carbon steel subjected to dynamic impact loading
Low to moderate shock loading tests have been carried out on steel specimens using a single stage gas gun. Stress history at the back face of the target specimen and projectile velocity prior to impact were recorded via manganin stress gauges and velocity lasers, respectively. The amount of twinning within the α-ferrite phase was measured in post-impact specimens and an analytical approach is developed to determine the twin volume fraction as a function of blast loading.
Precipitate Hardened Nickel Based Superalloy – Inconel 718
Fully Lamellar α/β Titanium
Near α Titanium – Ti1100
Near α Titanium – IMI834
Metal/Matrix Composite – β21S/SCS-6
