‘Pearls of wisdom’ – thirty years of Materials Science and Technology

January 2014 marks the start of the thirtieth year of Materials Science and Technology: our ‘pearl’ anniversary. We have ambitious plans, which I expand upon below, to mark this occasion by launching a series of commissioned issues on topics of current research interest. However, I offer first a few thoughts prompted by consideration of this anniversary.

The immediate connotation that comes to mind is that of the ‘pearls of wisdom’ that are offered monthly to our readership. The origin of the phrase is not easy to find. It is clearly not from the book of Job, which states that ‘No mention shall be made of coral, or of pearls: for the price of wisdom is above rubies’ (chap. 28). It appears to derive from ‘The Task’ by William Cowper (published 1781) – an extremely long work extolling the pastoral life and divided into six books. Somewhere in the middle of Book 3, ‘The Garden’, there is the couplet:

But wisdom is a pearl with most success

Sought in still water, and beneath clear skies.

This conjures up images of pearl divers plying their trade in the surf and sunny waters of the Far East: something that even James Bond tried, in ‘You Only Live Twice’. For one ‘M’ character in IMMM, MST’s parent Institute (of Materials, Minerals and Mining), the couplet could be regarded as describing (a rather exotic form of) mining. A more fanciful extrapolation might be made to the way in which our MST ‘pearls of wisdom’ are sought by ‘information mining’ through ‘surfing’ the internet.

To the metallurgist, the word ‘pearl’ conjures-up ‘pearlite’: the lamellar mixture of ferrite and cementite occurring in normalised or annealed plain carbon steels. The name derives from the similarity of the interference colours, observed when pearlite is viewed in the microscope under white light, to those associated with ‘mother-of-pearl’ (nacre): more of this below. Pearlite does not hit the headlines as much as do quenched-and-tempered structures or the new ‘super-bainites’,¹ but it should be noted that extremely high strength can be developed in hard-drawn pearlitic steel wire, starting from ‘patented’ steel, named in recognition of a patented discovery by James Horsfall in Birmingham in 1854. Patenting consists of austenitising a steel of approximately eutectoid composition (∼0·8 wt-%C), followed by continuous cooling or isothermal holding to produce a uniform fine pearlite. This can then be drawn (in sequential stages) to true strains of order 3–4 (300–400%). Zelin² indicates values of yield strength and UTS of magnitude 3 GPa for a true strain of 3. Reductions of area of ∼50% are obtained (the wire fracturing in a ductile, ‘cup–cone’ manner), but the overall elongation is a function of gauge length, decreasing rapidly as the gauge length is increased. For such wire, the thicknesses of the ferrite and cementite plates are approximately 10 and 2 nm respectively: see also Ref. 3, which demonstrates that the very fine plates are able to deform, whereas coarser cementite develops microcracks. Zelin describes the final structure as a ‘nano-composite’: this leads me to draw attention to the recent (October 2013) issue of MST which contains a set of papers on other microstructures in steels, also ‘nano-engineered’ to enhance mechanical properties.⁴

Patented and drawn wires are used in a number of applications, including the strings of musical instruments, wire to reinforce rubber tyres and, in multi-stranded form, as cables for cranes, lifts, suspension bridges, transmission lines and marine applications. Olver et al.⁵ describe and analyse the mechanisms of failure of strings in the guitar, bass guitar and mandolin. Unused strings failed by ductile, ‘cup–cone’ fracture at a UTS of ∼2·7 GPa, but much-played strings were found to fail by fatigue, predominantly at the bridge of the instrument, where the stress concentration increased the surface tensile stress in the string to a level that exceeded the yield strength. The alternating stress producing fatigue was only 49 MPa. The service duty for such strings is severe. The ‘E’ string on the mandolin is only 0·17 mm in diameter and operates under a nominal tensile stress of 1·772 GPa (to produce a fundamental frequency of 659 Hz). Even in simple tension, the plastic displacement associated with ∼50% reduction of area and microscopically ductile fracture is extremely small. Hard-drawn wires exhibit little work-hardening capacity, so that the majority of the plastic elongation is that associated with the neck. This is why there is a dependence of elongation on gauge length. Associated with the small plastic elongation is a very high elastic strain: with Young’s modulus equal to 210 GPa, a UTS of 2·7 GPa implies ∼1·3% elastic strain at failure. When the string breaks, the stored elastic energy is released as kinetic energy, causing the two pieces of broken string to fly apart in an unpredictable manner. This is not a matter of great concern to life and limb for a simple string in a musical instrument, but such ‘whiplash’ would produce serious consequences if the failure were to occur in a (thick) cable, tens of metres in length. This is why cables are composed of multi-strands of wires wound in close contact with each other. If a single strand fails (due to the presence of a defect) it is restrained from flying apart by the neighbouring, intact strands. The safety factors for cables subjected to load control are also deliberately high, so that applied stresses are restricted to 10–20% of the yield strength, and only defective strands are liable to fail.

I return to the word ‘pearlite’, named from the similarity of the interference colours in etched sections to those seen in ‘mother-of-pearl’ (nacre). I was about to use the word to be found in all the dictionaries: ‘iridescence’ and expound on the Greek goddess Iris (the rainbow) from which the word derives, when I stopped, and reflected (an apt word as it turns out). It struck me that the interference colours seen in mother-of-pearl and other mollusc shells are not the strong ROYGBIV colours of the rainbow: they tend to be pale pinks and greyish-greens. I had not knowingly given consideration to colours of this sort for over thirty years, when making use of the optical wedge to look at optical interference in thin sections of minerals, using transmitted light. These colours appear to be second- or higher-order interference colours, whereas those in the rainbow are first-order.⁶ A former colleague, Dr John Leake of the Cambridge Department, drew my attention to two most informative articles. The first of these⁷ gives the general background to interference colours, with many examples of both first- and higher-order colours, including the following information on mother-of-pearl:

Mother-of-pearl (nacre) consists of thin aragonite (calcium carbonate) tiles held together by layers of an organic polymer (conchiolin, chitin or other). At each boundary between the crystals and the polymer, part of the incident light is reflected [hence, the aptness of my use of the word earlier]. Depending on their wavelength, the reflected waves may interfere constructively or destructively. The layers between the tiles are thin: about one third of the thickness of the tiles or less.

The interference colours are computed for a stack of 32 layers.⁷ A general property observed was that vivid ROYGBIV colours were found for thin layers; but, for a periodicity interval above 2 μm, only pale greens and pinks were seen; these being the most common ‘nacreous’ colours. The second reference⁸ goes back to Benham, writing in Nature in 1895, who associated the colours with ‘films’ within the mother-of-pearl, rather than with striations on the surface. These observations provide something of a puzzle with respect to the (quasi-) ‘iridescence’ of etched pearlite in steel, because neither ferrite nor cementite is transparent. Dr Leake suggests that the interference effects are arguably due to the ‘not-quite’ regular undulations on the etched surface (unlike the very regular surface of a compact disc). Following his reply, I have felt compelled to examine a number of compact discs, at various viewing angles, and, indeed, all seem to exhibit only strong, first-order colours. There is surely opportunity here for an in-depth study of pearlite’s interference colours, by means of controlled experiments, with more- or less-well-aligned pearlite; different plate spacings, different lengths of etching time, etc.

The growth of pearls (in oysters or some other molluscs) and ‘mother-of-pearl’ is a fascinating process. A pearl forms when an oyster ingests an irritant, in the form of a particle of grit in its food. To reduce the irritation, it deposits layers of nacre sequentially around the particle, until it is completely covered. There is an argument that the original particle has to incorporate some protein content because aragonite does not bond directly to inorganic material. The multiple, internal reflections within the nacreous layers produce a lustrous sheen, which is certainly ‘pearlescent’, if not iridescent (and a bit of research reveals other terms relating to the optical behaviour of minerals and gemstones: opalescence, adularescence, labradorescence and aventurescence!). ‘Cultured’ pearls are produced by taking small bits of mollusc shell and inserting these into pearl oysters. The ‘mother-of-pearl’ or nacre structure is a nano-composite with thin platelets (‘bricks’ or ‘tiles’) of aragonite (interspersed with organic polymer) stacked in a regular array, as described in the preceding paragraph.

Further fascinating examples of how crystal growth of inorganic materials can be affected by the presence of organic substances are described in the runner’s-up entry for the 2013 MST Literature Review Prize (LRP). Submitted by Heather Greer, of the University of St Andrews, this review has the title ‘Non-classical crystal growth of organic and inorganic materials’.⁹ To the conventional materials scientist, used to growing crystals from melts of the same material, the review presents entirely new concepts of growth from aqueous solutions, when modified by the presence of polymers. To quote (not quite verbatim) from the abstract, the review treats ‘inorganic crystals with structures resembling naturally occurring biominerals that have been biomimetically synthesised in the presence of a polymer or surfactant. This leads to a new category of crystals termed ‘meso-crystals’, consisting of orientated nano-crystals interspaced by organic components’. I will not give away details at this stage, but I suspect that connections exist with surfactants used in froth-flotation processes for concentrating minerals. I thoroughly recommend both Heather’s review⁹ and the 2013 winning entry by Peter Walker from Cambridge.¹⁰ We continue to receive LRP reviews of high quality and the 2014 competition is now well under way. Rose Worrell is responsible for LRP administration and, on behalf of the judging panel, I would like to thank her for all her efforts.

Developments for 2014

Different forms of growth apply also to the journal as we try to accommodate extra amounts of new, high quality submissions. In January 2011, I introduced¹¹ a ‘hybrid’ concept, in which the ‘mainstream’ print-plus-online version of the journal was augmented by a further 300 pages of online-only material. This pattern was to be adopted more generally in the future and indeed forms the basis of an important development in 2014: that of the inauguration of regular issues published in online-only format. Our intention is that these issues will be commissioned and thematic, allowing specialist guest editors to compile series of high quality contributions on topics of current research interest. The launch of this initiative has to a large extent relied on the efforts of Katerina Busuttil, Maney’s Materials Science Commissioning Editor, in identifying and recruiting participants. The rationale of publishing these issues online-only is to provide the Editors with flexibility to structure the length and content of issues, to ensure themes are covered in appropriate depth.

Three online-only thematic issues are planned for 2014, in addition to the twelve ‘mainstream’ issues. An issue guest edited by Zoe Barber and Bill Clyne at Cambridge and Petr Šittner at the Institute of Physics, Prague will cover smart materials, addressing a number of functional properties. The second, on biodegradable materials – ‘functional obsolescence’ that mimics the intrinsically smart behaviour of natural materials – is being compiled by MST’s associate editor, Devesh Misra. Finally, ‘Adventures in the physical metallurgy of steel’ will present a careful selection of papers from the ‘by invitation only’ conference run by Harry Bhadeshia and his associates in Cambridge last July. Further issues are in development or commissioning, and we shall continue to publish regular thematic sections in the ‘mainstream’ issues, starting with this month’s group of papers on high temperature creep and fatigue.¹²

We hope that readers will appreciate the ability to learn about new and exciting fields of materials science and will continue to find pleasure in what I might claim to be our ‘cultured’ pearls of wisdom.