http://esag.harvard.edu/rice/093_NeedlemanRi_PlasticCreepCavitation_AM80.pdf

Plastic creep flow effects in the diffusive cavitation of grain boundaries

The relationship between stress and strain rate is often sigmoidal in superplastic materials, with a low strain rate sensitivity at low and high strain rates (regions I and III, respectively) and a high strain rate sensitivity at intermediate strain rates (region II) where the material exhibits optimal superplasticity This relationship is examined in detail, with reference both to the conflicting results reported for the Zn-22 pct Al eutectoid alloy and to the significance of the three regions of flow

The mechanical properties of superplastic materials

Creep cavitation in metals

This chapter provides a comprehensive and detailed review of the topic of He effects in irradiated structural alloys Following an introductory overview section, the chapter reviews: (a) experimental approaches to studying He effects; (b) He effects on void swelling and high-temperature embrittlement, focusing on austenitic stainless steels, as well as a rate theory (RT)-critical bubble model (CBM) based framework for understanding and mitigating deleterious He effects; (c) the literature on spallation proton irradiation data on bulk helium effects in ferritic–martensitic and austenitic steels; (d) recent electronic–atomistic models of He behavior in steels; (e) nanostructured ferritic alloys (NFA) and in situ He injection data showing that these remarkable high-temperature materials hold great promise of managing He in a way that could turn it from a liability to an asset as well as a very promising comparison of this data to a master multiscale model of He transport and fate in real ferritic–martensitic and NFA microstructures The chapter concludes with a summary and list of some outstanding issues

1.06 – The Effects of Helium in Irradiated Structural Alloys

There are four distinct types of fracture in superplastic materials: failure by quasistable plastic flow, failure by necking, cavitation failure, and quasibrittle failure. The characteristics of these four types are described with reference to experimental examples. Maximum elongation occurs in a superplastic material when it pulls out to a fine wire in quasi stable flow. It is demonstrated that there are two basic requirements for this type of flow: (a) a suppression of localized (but not diffuse) necking, and (b) a suppression of significant cavity interlinkage (but not necessarily of cavity nucleation and growth).

Fracture processes in superplastic flow

Cavity growth at high temperatures may be controlled by vacancy diffusion, giving cavities which are approximately spherical and randomly distributed, or by power-law creep, giving cavities which are elongated and aligned in the direction of the tensile stress. In general, diffusion growth is favored at low total strains, and there is a transition to power-law growth at a critical cavity radius,rc. The value ofrc increases with decreasing strain-rate, so that there is also a transition from predominanly power-law growth at high stress levels to predominantly diffusion growth at low stress levels. Both types of cavities have been observed in superplastic materials, but the diffusion growth rate may be enhanced if the cavity intersects a number of grain boundaries. The analysis is in good agreement with experimental results reported for three diffent superplastic materials.

An analysis of cavity growth during superplasticity

Creep fracture processes may be plotted in the form of a map by using the equations for the times to fracture for each process. A new and simplified form of creep fracture map is introduced in which the logarithmic normalized stress is plotted against the reciprocal of the homologous temperature. In this form, both the field boundaries between the different fracture processes and the contours of constant time to fracture appear as straight lines. Maps are presented for 315 stainless steel using four different creep processes: transgranular creep fracture, triple point cracking, and intergranular cavitation controlled by either diffusion growth or power-law growth. It is demonstrated that a map constructed for 316 stainless steel having a grain size of 40 μm is in good agreement with published experimental fracture data.

Creep fracture maps for 316 stainless steel

Cavitation in a Superplastic Al–Zn–Mg Alloy

A mechanism for creep crack growth is proposed by which the crack grows by formation of grain boundary cavities ahead of the crack tip. Two cases are considered; firstly, when cavity growth is diffusion controlled and secondly, where growth is deformation controlled. The resultant crack growth rates predicted by these theoretical models are compared with experimental data.

David A. Miller

Papers

An analysis of cavity growth during superplasticity

Creep fracture maps for 316 stainless steel

Cavitation in a Superplastic Al–Zn–Mg Alloy

Diffusion and Deformation Controlled Creep Crack Growth

Independent and sequential cavity growth mechanisms