7.1 INTRODUCTION

All materials grow or shrink with time, even in the absence of applied stresses or the influence of environmental conditions. The reasons are surprisingly many. They include thermodynamic driving forces towards internal equilibrium, such as phase transformations towards stable phases [1-3]. An example is the volume expansion associated with the transformation of retained austenite to martensite in steels-a major cause of dimensional instability in gage blocks [1, 4]. The tendency to reach a stable vacancy concentration was discussed in the previous chapter, and this too is accompanied by a volume change with time. Other phenomena that may lead to temporal instability include relief of residual stresses, settling of contacts, post-curing, thermal, physical and chemical aging, metallurgical processes such as annealing, recrystallization, precipitation and grain growth, settling due to gravity, field effects such as magnetostriction and slow crack growth. In this chapter we shall review some of these phenomena with emphasis on the long-term dimensional stability of composite materials.

Aging is a general term covering a variety of atomic and microstructural processes. In metals, it occurs with precipitation hardening transformation. Alloy aging and over-aging are accompanied by dimensional changes. Metallurgical aging results from processes causing supersaturated solutions, e.g., quenched Cu-Be, which not only shrinks with time but does so anisotropically [5]. Distortions of ceramic bodies due to temperature gradients, sintering, liquid phase formation, etc. are discussed in [6, 7].

In polymers, the term thermal aging can refer to physical and/or chemical changes [8]. Physical aging in polymers involves a...