Читать книгу Engineering Physics of High-Temperature Materials - Nirmal K. Sinha - Страница 25

The Celsius scale of temperature is almost universally accepted as the standard scale for temperature. It is based on the thermal state of pure water, which is the source for life and most abundant material on Earth's surface. In this scale, and for conditions under normal atmospheric pressure at sea level, the solidification temperature of pure water is considered as 0 °C and the boiling point is assumed to be 100 °C. Consequently, temperatures of the solid state of water (that is ice) are assigned with “negative” signs. The problem with this scale is that any temperature less than 0 °C is described as negative (−sign); a better scale is kelvin, as there is no negative temperature. However, is it a rational temperature for materials scientists to use? We will go to this question later, but let us first discuss human issues with the Celsius scale.

Since ice is cold for the human body, “negative” temperatures are naturally considered as “cold.” Water temperature of +50 °C may be considered as “warm,” but any human body temperature of 40 °C is considered as rather “high” and feverish because the body temperature of about +37 °C is considered as “normal” for a human. These definitions for temperatures were convenient and extended to materials in general. Materials at temperatures greater than say 70 °C may be considered as “high temperature” for human safety. In this way, the title of this book may be misleading. For materials science and engineering, the definition of high temperature has to be based on the state of the solid material under scrutiny.

The solid state is also subject to question. We will, of course, generally avoid solids, like polymers, in order to simplify the scope of this book. Ordinary glass is a solid with amorphous structure, but it could, phenomenologically speaking, deform like crystalline materials as temperature rises. Some rationalization can be made with glass because its structure may not show a long‐range order, but may have crystal‐like nanoscale structure. The primary issue then is how to characterize the thermal states of crystalline solids in general and relate them to all natural, ice and rocks, and fabricated materials, metals, and alloys.

As for metallic alloys, in many sections of this book, we will concentrate on engineering properties of titanium‐base and nickel‐base superalloys due to the availability of relevant experimental results obtained directly by the authors of this book. These alloys are more complex than any metallic alloys being used today. Many metallurgists consider them as the most fascinating of all metallic alloys and they are mostly used for the hottest parts of gas turbine engines. Actually, their use encompasses the highest homologous temperature of any common alloy system (Ross and Sims 1987).