Bronze is an important alloy. It is so important in the histories of our civilizations that one of the prehistoric ages is named after it:
The Bronze Age is, with respect to a given prehistoric society, the period in that society when the most advanced metalworking (at least in systematic and widespread use) included smelting copper and tin from naturally-occurring outcroppings of copper and tin ores, creating a bronze alloy by melting those metals together, and casting them into bronze artifacts.
Though the origins of bronze lies in the distant eras of our civilizations, it continued to play an important role in the cultures and civilizations in many parts of the world in the millenniums that followed — see the exquisite Chola bronze figurine of Parvati below for an example closer home (image courtesy: wiki):
Thus, it is no wonder that the first accurate non-ferrous phase diagram was determined in bronze . Heycock and Neville produced the first accurate phase diagram of copper-tin alloys (shown below) at the beginning of the twentieth century ; in this post, I would like to talk about phase (or, equilibrium diagrams), their importance, and the pioneering work of Heycock and Neville.
The origins of the phase diagram
The questions, What is a phase diagram? and Why is it important? are answered in a rather nice manner (albeit with specific reference to solid systems) by Paul Gordon in the very first chapter of his wonderful book Principles of phase diagrams in materials systems :
Materials in the solid state exist in many different forms, or phases. The number of such phases can be large even for pure substances; e.g., ice may exist in any one of the six different solid forms, and the important metal iron exhibits four solid phases. In systems containing more than one chemical species the number of phases may be correspondingly larger; not infrequently even in commercially important systems the available data cover only parts of the systems. In view of the fact that the properties of materials depend significantly on the nature, number, amounts, and forms of the various possible phases present and can be changed by alterations in these quantities, it is vital in the use of materials to know the conditions under which any given system will exist in its various possible forms. A wealth of such information on a large number of systems have been accumulated. To record this enormous amount of data, it has become customary to plot the number and compositions (and, indirectly, the amounts) of phases present as a function of temperature, pressure, and overall composition. These plots are called phase diagrams, constitution diagrams, or equilibrium diagrams. The latter name is derived from the fact that such diagrams purport to show the most stable phases that occur under equilibrium conditions.
Today, both the determination and the use of equilibrium diagrams are largely empirical. There is, however, a firm basis for such diagrams in the science of thermodynamics.
There are a few important things to note in the quote above:
- The first thing to note is Gordon’s last sentence in the first paragraph (emphasis added by me):
The latter name is derived from the fact that such diagrams purport to show the most stable phases that occur under equilibrium conditions.
The “purport to show” is there because, in some of the phase diagrams, a phase is shown even if it is not the equilibrium phase, if, in the ranges of temperatures, pressure and composition of interest, this phase occurs; a classic example, as Gordon notes, is the iron carbide shown in iron-carbon phase diagrams; given enough time, iron carbide will decompose into pure graphite and iron; however, since in the range of temperature, pressure and composition important to steel, almost invariably, iron carbide is seen, it is usually marked.
- The second point is the empirical determination of phase diagrams; as we will see, it was determined empirically by Heycock and Neville in the bronze system; and, more or less the same methodology was used even as late as 1968 when Gordon wrote the book (or, even now for that matter).
- The third point is the thermodynamics basis of these diagrams. And, not surprisingly, this thermodynamic origins of the phase diagram can be traced to Gibbs and his phase rule! As Cahn notes in his book , Gibbs phase rule, along with the Le Chatelier’s principle based on Gibbs’ principles are the guiding rules in the determination of phase diagrms; and, Gordon’s book is a nice exposition of the developments of these ideas and their interconnectedness.
As an historical aside, Cahn, in his book also describes the key role played by Roozeboom in advancing the work of Heycock and Neville; not surprisingly, the acknowledgement of Heycock and Neville reads thus in part:
During the course of the research Professor Bakhuis Roozeboom was kind enough to write to us several encouraging and instructive letters on the theory of the subject and we wish here to express our gratitude to him.
The innovations of Heycock and Neville
As can be seen in the figure above, Heycock and Neville reported on the temperature versus composition (with pressure held a constant) phase diagram in the copper-tin binary system. According to Cahn , the innovations that Heycock and Neville made are threefold:
Apart from the fact that in their work they respected the phase rule, they made two other major innovations. One was that they were able to measure high temperatures with great accuracy, for the first time, by carefully calibrating and employing the new platinum resistance thermometer, developed by Ernest Griffiths and Henry Callendar, … . Heycock and Neville’s other innovation was to use the microscope in the tradition of Sorby, but specifically to establish equilibria, notably those holding at high temperatures which involved quenching specimens from the relevant temperatures.
Heycock and Neville attribute the third innovation to G G Stokes in the very first sentence of their paper:
The immediate origin of the present paper lay in a suggestion of the late Sir G. G. Stokes, made early in 1900, that we should attempt the microscopic examination of a few bornzes as an aid to the interpretation of the singularities of the freezing point curve.
The representation of the phase diagram, in the form in which we understand it today, it described under the heading The Temeprature Concentration Diagram in the following words:
This information (…) can be presented in another and for some purposes more convenient form by taking the composition of each alloy as the horizontal ordinate, and temperature (…) for the vertical one. Our Plate 11 is constructed in this way.
Here is Plate 11:
The paper of Heycock and Neville is full of surprises; I do not know if they are the first ones to use the word solid solution. But, they did use the word with this apology:
We have, therefore, reluctantly decided to abandon the term [mixed crystals] and use instead the term “solid solution”.
And, here are some quaint sentences about dendritic morphologies observed in the solidified metals and alloys:
Our experience, and we believe it to be identical with that of most who have studied the subject, points to the comparative rarity of the formation of large crystals with place faces during the solidification of a metal or alloy. The first solid structure is generally a crystal skeleton, which in its simplest form consists of a stem with radial branches projecting from it. This may be compared to a fir tree, and in many cases the branches are at right sngles to the stem and to each other. Such a structure when cut by a plane, gives rise to the fern-leaf or dendritic forms so often seen on the surface of a cast metal, or in the etched and polished surface of a section of an ingot.
Let us think of the case in which such a crystal skeleton has been produced in an otherwise liquid mass of metal. As the branches give off what we may call twigs, and these may develop other systems of twigs, and so on indefinitely, …
Finally, of course, the micrographs from the paper are a great plasure to look at; as Cahn notes, micrographs of this sort became the bread and butter of metallurgists and ceramicists in the years that followed; and, it is wonderful to observe the beginnings of the trend in the paper.
An ability to read and use phase diagrams is an important skill that one develops as a materials scientist, metallurgist or, for that matter, anybody who deals with materials and their equilibria– be it physicists, chemists, ceramicists, petrologists, or, mineralogists. The first non-ferrous phase diagram that was accurately determined by Heycock and Neville (in bronze), thus, is a landmark study in our understanding and use of phase diagrams, and deserves its classic status; aside from that, the micrographs and the discussions of the sort noted above (on solid solutions, dendritic morphologies and so on) make the reading of this classic a pleasurable experience; I strongly recommend the paper (along with the commentary of Cahn) to the readers of this blog — you may download a copy of the paper here, if you go there early enough. Have fun!
 R W Cahn, The Coming of Materials Science, pp. 72-93, Pergamon, 2001.
 C T Heycock and F H Neville, Bakerian lecture: On the constitution of copper-tin series of alloys, Philosophical Transactions of the Royal Scoeity A (London), Vol. 202, pp. 1-69 (107 pages including the plates), 1904.
 P Gordon, Principles of Phase Diagrams in Materials Systems, pp. 1-2, McGraw-Hill Book Company, 1968.