Microcoining ripples in metal foils

Experiments, upper bound models, and finite element simulations are used to determine forming loads needed to microcoin surface ripples in thin metal foils. Coining is traditionally performed in a closed die, however enclosing all non-patterned surfaces is difficult to directly scale down to sub-millimeter foils. We find different forming regimes can exist at this small scale in an open pressing configuration. We explore the effects of the metal foil thickness and its work hardening behavior, two primary factors controlling the microcoining ripple forming load. For very thin foils, the load needed to coin a ripple pattern is lower than the load needed to compress the foil so that the open pressing configuration behavior is effectively closed with pattern formation without thickness change. For moderate thickness foils, the load needed to coin significantly drops as the entire foil compresses. For thick foils approaching bulk materials, the pattern will not completely form as the die macroscopically indents into the metal. Work hardening is found to raise the forming load for the thin, effectively closed die scenario, however it is a secondary effect at moderate thickness. This insight is used to microcoin patterns in extremely hard, thin metal foils

Grain Boundary Deformation and Fracture Mechanisms in Dwell Fatigue Crack Growth in Turbine Disk Superalloy ME3

The objective of this paper is the development of a multiscale, mechanistic based intergranular crack growth model, which considers creep, fatigue and environment interactions in a nickel disk material, ME3. In this model, the basic cracking mechanism involves grain boundary (GB) sliding and dynamic embrittlement, which are identified by examining the apparent activation energy, as well as, the slip/GB interactions in both air and vacuum environments. Modeling of the damage events is achieved by adapting a cohesive zone approach (an interface with internal singular surfaces) in which the GB dislocation network is smeared into a Newtonian fluid element. The deformation behavior of this element is controlled by the continuum in both far field (internal state variable model) and near field (crystal plasticity model) regions and the intrinsic GB viscosity which is able to define the mobility of the element by scaling up the motion of dislocations into a mesoscopic scale. This process is characterized by the rate at which the time-dependent sliding reaches a critical displacement and as such, a damage criterion is introduced by considering the GB mobility limit in the tangential direction leading to strain incompatibility and failure. Results of simulated intergranular crack growth rate, at different temperatures in both air and vacuum, are compared with those obtained experimentally. The model sensitivity is examined by performing case studies of materials with varying GB viscosity and different partial pressures of oxygen. © 2012 The Minerals, Metals, & Materials Society. All rights reserved

Loading frequency and microstructure interactions in intergranular fatigue crack growth in a disk Ni-based superalloy

The paper examines the role of the loading frequency on the dwell fatigue crack growth mechanism in the super-solvus nickel-based superalloy, ME3. This is accomplished 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 time periods up to 7200 s imposed at the maximum load level. Results of these tests show that the transitional transgranular/intergranular loading frequency is 0.1 Hz, and are used to determine the apparent activation energy of the time-dependent crack growth process. Analysis of this energy in both air and vacuum showed that the intergranular cracking is governed by a mechanism involving grain boundary sliding. This mechanism is explained in terms of absorption of dissociated lattice dislocations into grain boundary dislocations. The gliding of these dislocations under shear loading is assumed to cause grain boundary sliding. A condition for this mechanism to occur, is that a critical minimum distance exists between slip bands impinging the affected grain boundary. This condition is examined by correlating the slip band spacing (SBS) and loading frequency using a model based on minimum strain energy accumulation within slip bands and that a unique configuration of number and spacing of bands exists for a given plastic strain. The model outcome expressed in terms of SBS as a function of loading frequency is supported by experimental measurements at both high and low loading frequencies. Results of the model show that a saturation of SBS, signifying a condition for intergranualr cracking, is reached at approximately 3 μm which is shown to coincide with the transitional loading frequency of 0.1 Hz. © 2012 Elsevier B.V. All rights reserved

Creep-environment interactions in dwell-fatigue crack growth of nickel based superalloys

A multi-scale, mechanistic model is developed to describe and predict the dwell-fatigue crack growth rate in the P/M disk superalloy, ME3, 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. Modeling of the damage events is achieved by adapting a cohesive zone (CZ) approach which considers the deformation behavior of the grain boundary element at the crack tip. The deformation response of this element is controlled by the surrounding continuum in both far field (internal state variable model) and near field (crystal plasticity model) regions and the intrinsic grain boundary viscosity which defines the mobility of the element by scaling up the motion of dislocations into a mesoscopic scale. This intergranular cracking process is characterized by the rate at which the grain boundary sliding reaches a critical displacement. A damage criterion is introduced by considering the grain boundary mobility limit in the tangential direction leading to strain incompatibility and failure. Results of simulated intergranular crack growth rate using the CZ model are generated for temperatures ranging from 923 K to 1073 K (650 °C to 800 °C), in both air and vacuum. These results are compared with those experimentally obtained and analysis of the model sensitivity to loading conditions, particularly temperature and oxygen partial pressure, are presented. © 2014 The Minerals, Metals & Materials Society and ASM International