Fourteenth Century England aims to publish high-quality refereed academic research on topics relating to England, its dominions and neighbours. This volume follows in the footsteps of the previous eight volumes in the series by showcasing a wide-ranging selection of studies reflecting the concerns and trends of current scholarship in the field. We make no attempt to ‘theme’ the volumes, preferring to present whatever is good and innovative in current research in the period, thereby acting as a channel for the dissemination of new ideas, trends and debates. In this volume there are two essays on politics, two on religion, two on chronicle writing and two on aspects of the law. We are also delighted to introduce a new ‘Notes and Documents’ format for shorter, source-focused discussion. Although we do not rely on the proceedings of a specific conference, the continuing vitality of fourteenth-century studies owes much to the sessions organised under the auspices of the Society for Fourteenth-Century Studies at the annual International Medieval Congress (IMC), Leeds, and the Society of the White Hart at the annual International Congress on Medieval Studies, Kalamazoo, Michigan. A number of essays in this volume received their first ‘public airing’ in one of these contexts, and the cosmopolitan nature of these conferences allows us constant access to a vibrant source of medieval research from around the globe.

This volume marks a change in both the team responsible for organising the sessions at IMC Leeds and in preparing the Fourteenth Century England (FCE) volumes for publication. We are delighted and honoured to have been asked to join the FCE team by its existing members – Chris Given-Wilson, Jeff Hamilton, Mark Ormrod and Nigel Saul. We would like to thank them for their assistance in the organisation of the sessions at Leeds as well as the assiduousness with which they, along with various external referees, read and commented on submissions to this volume. Additionally, Alison McHardy in particular should be thanked for her willingness to look over submissions in this regard. We would also like to extend our collective thanks to Caroline Palmer and the staff at Boydell & Brewer for their continuing support of the series, and for the expertise and patience they have shown in seeing this volume through to publication.

What this monograph is about

Certain crystalline materials can exist in more than one solid phase, where a phase is identified by a distinct crystal structure. Typically, one phase is preferred under certain conditions of stress and temperature, while another is favored under different conditions. As the stress or temperature varies, the material may therefore transform abruptly, from one phase to another, leading to a discontinuous change in the properties of the body. Examples of such materials include the shape-memory alloy NiTi, the ferroelectric alloy BaTiO3, the ferromagnetic alloy FeNi and the high-temperature superconducting ceramic alloy ErRh4B4. In each of these examples the transition occurs without diffusion and one speaks of the transformation as being martensitic (or displacive).

Alloys such as Au–47.5%Cd and Cu–15.3%Sn are known to have a cubic lattice at high temperatures and an orthorhombic lattice at low temperatures. Therefore, if such a material is subjected to thermal cycling, it will transform between these two phases. Similarly, alloys such as Ni–36%Al and Fe–7%Al–2%C transform between a high-temperature cubic phase and a low-temperature tetragonal phase, whereas near-equiatomic NiTi has a high-temperature cubic phase and low-temperature monoclinic phase.

If a stress-free single crystal of such a two-phase material is slowly cooled from a sufficiently high temperature, it starts out in the high-temperature phase and at first, merely undergoes a thermal contraction.

Introduction

We next turn to the dynamics of the two-phase nonlinearly elastic materials introduced in Chapter 2. As in the theory of mixed-phase equilibria and quasistatic processes for such materials set out in Chapter 3, the notion of driving force plays a central role in the analysis when inertial effects are taken into account. The indeterminacy exhibited in Chapter 3 by even the simplest static or quasistatic problems for two-phase materials manifests itself again in the present much richer dynamical context. Moreover, the continuum-mechanical interpretations of nucleation and kinetics again serve to restore the uniqueness of solutions to the dynamic problems to be considered here. As in the preceding chapters, thermal effects are omitted; they will be included in the more general settings of later chapters.

The main vehicle for our study of one-dimensional dynamics of two-phase materials is the impact problem. There is an enormous body of experimental literature pertaining to the response of solids to shock or impact loading, much of it motivated by questions concerning the behavior of materials at extremely high pressures, as occurs, for example, deep in the earth. The reader will find some guidance to the experimental literature in this field of the dynamic behavior of materials in the books by Graham [11] and Meyers [25].

Introduction

In the preceding chapter we determined the kinetics of a certain phase transformation using experiments that involved fast loading in which inertia was important. In the present chapter we determine the kinetics of a different transformation using data from quasistatic experiments. The transformation studied here is a twinning deformation, not a phase transformation, a twin boundary being an interface that separates two variants of martensite; see Example 1 in Section 12.2. The change in lattice orientation across a twin boundary makes it analogous, in certain ways, to a phase boundary, and in particular, the motion of a twin boundary is governed by a kinetic relation.

As we have seen, the simplest form of kinetic relation governing the isothermal motion of an interface relates the driving force on it to its normal velocity of propagation: Vn = Φ(f). Since the kinetic response function Φ here is a function of a single scalar independent variable, one set of experiments, say uniaxial tension tests, completely determines Φ; and the function Φ thus determined characterizes all motions of this interface such as, say, in biaxial conditions. If the deformation field is inhomogeneous, and the phase or twin boundary is curved, one would use this same kinetic relation locally, at each point along the interface, relating the driving force at that point to the normal velocity of propagation of that point.