Thermomechanical behavior of metal alloys

The specifications of a mechanical part express the functions to be fulfilled, the role to be played: to be light, to "resist" deformation and temperature, to have surfaces suitable for good contact, corrosion resistance...

Knowledge of materials, made objective by models and behavioral characteristics, is a necessary prerequisite to any selection problem. It requires us to consider three fields: the composition and microstructure, the properties, and the aptitudes of materials; and to link these fields in a rational way: materials science is interested in the links between the first two, while materials engineering seeks to account for aptitudes on the basis of properties.

The aim of this article is to show how the metallic nature and microstructure of metallic materials are responsible for their thermomechanical behavior, i.e. their deformation and fracture response when subjected to the action of stress and temperature.

Although aimed at readers with a mechanistic culture, strongly influenced by the mechanics of continuous media, and most often homogeneous, the article includes a significant microscopic approach.

Indeed, it would be a bit of a red herring to set macroscopic against microscopic, insofar as the behavior laws — linking stress, strain, strain rate, temperature... — can only be expressed correctly by taking into account the microscopic heterogeneity of the metallic material.

Progress in this area has been considerable over the past generation, and the initial training of technicians and engineers is increasingly open to this dual approach.

One might object humorously, as one corrosion expert does, that "early man knew nothing of the physical or chemical phenomena he was implicitly using, and was perfectly capable of judging them". And he adds, after recalling the evolution of knowledge, and the desire to "understand everything" down to the finest levels, that "while this evolution satisfies the curiosity of the mind, it is not certain that it improves the processes involved in choosing materials". It would be easy to point out that the reverse is not certain either. And a sociologist provides a clue: "Man doesn't live on information, but on communication, and communication doesn't happen without an intermediary.

The aim of this article is to provide a simple, broad overview of how metallic materials "work". We will therefore avoid referring to precise bibliographical references, and avoid heavy-handed demonstrations. And we'll refer to recent documentation in French wherever possible, for those who wish to "know more". The aim is first and foremost to facilitate dialogue between the players involved, which is not at all detrimental...