Particle-coarsening, σ0 and tertiary creep

The effect of stress and temperature on the steady state creep rate of SiCw/2124 Al (T4) has been determined. The stress exponent for steady state creep of the composite is shown to increase from a value of 8.4 at 177 °C to a value of 21 at 288 °C. The activation energy for creep was determined to be 277 kJ/mol for testing in the temperature range from 149 to 204 °C and 431 kJ/mol for testing from 274 to 302 °C. These values are much greater than that for self-diffusion in aluminum. Such a severe temperature and stress dependence of the steady state creep rate is characteristic of precipitation and oxide dispersion strengthened nickel-base superalloys, where the creep behavior is explained by the particle strengthening contribution being a significant fraction of the applied creep stress. In contrast, the estimated particle strengthening for the composite is much less than the applied creep stresses. Alternate strengthening mechanisms are proposed to account for the observed creep behavior of the composite material, including the effect of temperature on the measured values of the stress exponent and activation energy for creep.

Analysis of the creep behavior of silicon carbide whisker reinforced 2124 Al(T4)

The dependence of creep rate on microstructure in a γ′ strengthened superalloy

It has previously been shown that, during creep deformation, secondary creep rates (es) can be related to stress (σ) by the equationes = A*(σ−σ0)p exp (−Qc*/RT).The exponent p ≃ 4, Qc*≃an activation energy for diffusion (usually self-diffusion), and σ0 is a friction stress. The parameter A* is material-dependent so that creep data for a variety of materials appear as a set of parallel lines on a log es/log (σ−σ0) graph. In the present paper, the general applicability of the expression with p ≃ 3.5 is confirmed on a variety of nickel- and iron-base alloys encompassing a range of microstructures commonly found in engineering materials. Further, a single universal equation to describe the creep behaviour of the various materials has been developed. It involves normalization of the effective stress (σ−σ0) by the proof (σ0.05) or yield (σy) stress of the alloy at the appropriate creep temperature, resulting in the expressiones = B(σ−σ0/σ0.05)3.5with B independent of material, crystal lattice, microstru...

The development of a universal equation for secondary creep rates in pure metals and engineering alloys

The physical mechanisms that control creep of single phase metals and alloys are reviewed Attention is given to both steady state and non-steady state properties The evolution of dislocation substructure in the course of primary creep is followed for both pure metals and solid solution alloys in an effort to demonstrate its dominant role in creep It is shown that the subgrain size and dislocation density scale in a natural way with the applied stress The long range internal back stresses that are responsible for creep anelasticity are shown to be determined not only by the scale of the substructure but also by the bias of the applied stress Creep transients following load changes are also described and interpreted in terms of the nature and properties of the creep substructure In addition, the characteristics of steady state creep are described by referring to the extensive phenomenological evidence which supports the concept that steady state creep at high temperatures is controlled by lattice self diffusion It is suggested that the diffusion controlled recovery and thermally activated glide approaches to creep should be complimentary rather than competitive In general, dislocation motion can occur either as a result of thermal activation past obstacles or as a consequence of obstacle recovery We argue that the inherently inhomogeneous dislocation substructures that form during creep produce inhomogeneous stresses that enhance the rate of obstacle recovery in the “hard” regions of the creep substructure and inhibit thermally activated glide in the “soft” regions This “composite” model of creep allows us to explain both steady state and non-steady state creep properties in a semiquantitative way

Mechanisms controlling creep of single phase metals and alloys.

The dependence of the steady creep rate, e s , on stress, σ, and temperature, T, for Nimonic 80A at 1023K can be described as ɛ ˙ s = A σ n exp ( Q c / R T ) The values of the stress exponent, n (∼ 8.3) and the activation energy for creep, Qc , (≃460 kJ/mol) are considerably higher than those for the nickel-20% chromium matrix material. These anomalously high values of n and Qc are accounted for in terms of the stress- and temperature-dependence of the friction stress, σ0, which is determined by a technique involving consecutive small stress reductio...

K. R. Williams

Papers

On the Stress- and Temperature-Dependence of Creep of Nimonic 80A