Over a period of some 70 years, the impact of Sir Alan Cottrell’s work on the basic understanding of materials and its application to engineering structures, his academic leadership, his role of scientific adviser to the Government and his contributions to safe nuclear energy, has been immense.
This special issue, dedicated to his memory, concentrates on the contributions he has made to Science and Technology. It consists of a series of papers relevant to the wide range of topics he researched at different periods of his distinguished career. In this Prelude we give a short biography and indicate the links to the papers which follow.
Alan Howard Cottrell was born in Birmingham in 1919, attended Moseley Grammar School and then read Metallurgy at Birmingham University, graduating in 1939. He was put on war work, introduced to a serious problem of cracking of armour plating of tanks along the heat affected zones bordering the electric arc welds. He demonstrated that locally this metal had gone into a particularly hard and brittle state, which could easily crack when stressed. He showed that the problem could be solved by first lining the faces to be welded with a thin layer of non-hardening steel. This early experience no doubt influenced his lifelong interest in fracture and structural integrity. Richard Dolby’s paper refers to this work and outlines the advances made in welding technology over the 60 or more years of Alan’s career.
Alan was made lecturer in 1943 and in 1944 he married Jean Elizabeth Harber, a marriage which lasted happily for 55 years. They had one son, Geoffrey in 1951, and much later adopted a daughter Ioana. It is said that one of his classic books Dislocations and Plastic Flow in Metals, published in 1953, was written during sleepless nights with baby Geoffrey. Towards the end of the war, Alan prepared a new lecture course ‘Theoretical Structural Metallurgy’ (which formed the basis of another classic book he wrote at this time) in which he discussed the structure and properties of metals in terms of the behaviour of constituent atoms and electrons. This course was very influential and ahead of its time. It helped transform a hitherto rather qualitative subject into a quantitative discipline, and was an important step in achieving his ambition to transform Metallurgy into Material Science. He was a brilliant lecturer, conveying complex phenomena in simple terms. As Mike Ashby puts it in his paper ‘Alan was an inspirational teacher’.
After the war Alan started research on the plastic properties of metals, with a view to establishing the role of dislocations in determining the mechanical properties. The yield point of structural steel was of major interest, and he explained it in terms of the interaction of interstitial carbon and nitrogen atoms with the dislocations (Cottrell locking). Explanations inter alia of the yield drop, strain ageing, the role of grain boundaries, blue brittleness of iron, the temperature dependence of the yield stress in steels, pinning effects in FCC crystals, followed. The researches were both experimental and theoretical, the latter with Bilby and Jaswon. The papers by JW Cahn, Bhadeshia and Friedel refer to this part of Alan’s work.
But there were also seminal contributions in other areas. In a series of elegant temperature cycling experiments, with Stokes on aluminium and Adams on copper, he showed, by cycling from a higher to a lower temperature, that the relatively small temperature dependent part of the flow stress is proportional to the main temperature independent part (the Cottrell-Stokes Law) which was explained in terms of dislocations cutting through other dislocations. The main effect was attributed to elastic interactions and the temperature dependent effect to thermally activated production of jogs. This led to the ‘forest’ model of flow stress. This is without doubt one of the most important contributions to our understanding of work hardening, and stimulated much further research. Friedel’s paper refers to this. The same workers also demonstrated the phenomenon of work softening, by cycling from a lower to a higher temperature. This part of Cottrell’s temperature cycling experiments (but not the Cottrell-Stokes Law) is referred to in Seeger’s paper in this issue. Other seminal contributions during this period included the explanation of Robert Cahn’s experiments on the recovery of bent crystals of zinc by the process of ‘polygonisation’ which is also described in Friedel’s paper.
Cottrell also showed that the Lomer dislocation formed by the reaction of two dislocations at the line of intersection of two different slip systems in FCC crystals, which was potentially glissile on a cube plane, would become sessile by dissociation into partial dislocations and stacking faults, resulting in the Lomer-Cottrell lock. This barrier in the work hardened state plays an important role in Seeger’s long range stress theory of work hardening, the development of which in the 1950s is described in his article in this issue. Lomer-Cottrell locks of finite lengths have been observed frequently by transmission electron microscopy in the forest of dislocations in work hardened FCC metals.
The paper on dissociation of [a + c] dislocations in GaN in this issue gives an example of a dissociation reaction of current interest, 60 years after Cottrell’s paper, but in a different context. In another influential paper involving partial dislocations, with Bilby, Cottrell proposed a pole mechanism for generating twins in BCC crystals. He was also interested in the interaction of point defects with dislocations, and with Maddin carried out seminal experiments on quench hardening of aluminium, subsequently illuminated by transmission electron microscopy observations at Cambridge and Harwell. With Rachinger he initiated research on ordered intermetallic alloys, in which deformation occurs by dissociated super-dislocations.
These were all pioneering studies carried out over a period of just 10 years and represent an impressive achievement. They are remarkable for their physical insight, their lasting impact and for showing the way how critical experiments should be carried out. Alan’s contributions in this field are second to none.
Alan’s work contributed much to making the Birmingham Department famous as a leading centre for the science of metals. He was given a personal Professorship in 1949 at the age of 30, and in 1955 was elected to The Royal Society at the early age of 35.
In 1955, he accepted an invitation from Monty Finniston, the Head of the Metallurgy Division at the Atomic Energy Establishment at Harwell, to become his Deputy. Alan accepted because he expected to find problems there of national importance which fell into his field. Alan’s aim was to advance the understanding of radiation damage relevant to the development of nuclear power reactors. Radiation damage in uranium rods, which were to be stacked vertically in channels in the graphite core in Magnox Civil Nuclear Reactors, was of particular concern. Swelling and growth of uranium were studied and in a brilliantly designed experiment Cottrell, with AC Roberts, showed that irradiation creep would produce a large buckle in the fuel rod within a few weeks. This led to a redesign of the fuel rods in the reactors. Another area studied was the radiation embrittlement of structural steels, resulting in a rise of the brittle-ductile transition temperature. This work has a direct bearing on the integrity of pressure vessels in pressurised water reactors of current design as well as in the older Magnox Reactors, and is referred to in Knott’s paper in this issue. Cottrell also wrote a review article in 1956 on the effects of neutron irradiation on metals and alloys, which was very influential at the time.
These studies led him to consider the problem of brittle cleavage of steels. Experimental evidence showed that cleavage cracks were nucleated by plastic deformation. In a famous paper, in 1958, Cottrell described an ingenious mechanism of elastic energy reducing coalescence of dislocations on intersecting slip planes for the nucleation of cleavage cracks on cube planes. The difficult step in brittle fracture was, therefore, the propagation of the crack nuclei across the grains. This led to the identification of refinement of grain size as being the important factor, not only in increasing yield strength (as recognised by Petch) but also toughness. This fact plays an important role in the development of modern steels.
Cottrell also became interested in fatigue failure. Following experiments by Hull (working at the Clarendon Laboratory in Oxford) that in copper fatigued at 20 K extrusions and intrusions occurred in persistent slip bands, he suggested a clever mechanism based on intersecting slip bands operating sequentially, published with Hull in 1957. The papers by Hull, Brown and Mughrabi in this special issue relate to this contribution by Cottrell.
On 10 October 1957, a reactor at Windscale caught fire during a gentle heating to anneal damage due to displaced carbon atoms in the graphite core, causing a national emergency. Unfortunately, the Wigner energy released in this process heated up the graphite so much that it caught fire. Bill Penny, then Chairman of the UKAEA, asked Alan to lead a team to investigate the reason for this. Cottrell set up a new laboratory in a few weeks and he and his team unravelled the problem and were able to give an assurance that the Magnox reactors would be immune to this self-heating effect. This was one of the last of Cottrell’s tasks at Harwell. Bullough’s article reviews his seminal contributions during this period to our understanding of radiation damage.
In 1958, Alan accepted an invitation to become Head of Department of Metallurgy at Cambridge. He modernised the Department by bringing in new people (Robin Nicholson, Tony Kelly, Jim Charles and later Graeme Davies) and new equipment, and by teaching the subject from the atomic point of view. This led to a revision of his earlier book “Theoretical Structural Metallurgy” to become an “Introduction to Metallurgy”, which was equally successful. Alan recognised that a major strength of Metallurgy at Cambridge was the existence of the Natural Sciences Tripos (NST) which required that undergraduates, during their first two years (to Part I of the Tripos), read three experimental subjects, typically not only Physics and Chemistry but also non-school subjects, such as Metallurgy or Mineralogy and others: the effect was that many students were exposed to Metallurgy in their first two years, and this provided a means for attracting them to read Metallurgy in their third year for Part II of the Tripos.
After some difficult negotiations with Physics and Chemistry, who wanted to stream students into their subjects from the first year, the NST was revised from October 1965 to include a new subject “The Crystalline State” taught jointly by Metallurgy and Mineralogy in the first year to Part I A of the Tripos, and to provide Metallurgy as an option in Part I B in the second year. This greatly increased the amount of material taught by the Metallurgy Department in the first two years, and this proved very beneficial for recruitment to the final Part II subjects, which from 1965 included both ‘Metallurgy’ and ‘Materials Science’. The new structure and courses have successfully stood the test of time, but when they were implemented in 1965, Alan Cottrell, who had worked hard to get them established, was no longer in the University.
On the research side Alan started two new projects, on field-ion microscopy, to which the paper by Smith et al. in this issue is relevant, and on superconducting alloys, which Alan predicted correctly to become an important growth area in Material Science. His own researches focussed on brittle fracture of structural steel at freezing temperatures, responsible for many tragic accidents to ships and bridges, and secondly, with Tony Kelly, on the physics of fibrous composites, which although made from brittle materials, could be very strong and resistant to fracture. They identified the mechanism of fibre ‘pull-out’ as a means of providing a high work of fracture. This led to the development of new materials, such as fibre glass and carbon fibre. Much later Alan advised Edward Heath to use carbon fibre for the spars of his boat. All these developments in teaching and research activities transformed the department into a world class institution and it has remained so ever since.
Alan’s work on fracture included the development, with Bilby and Swinden, of the theory of elastic-plastic cracks, and the elucidation of the basic processes of failure at the tip of a sharp notch. A toughness parameter (critical crack opening displacement) was identified for a metal containing a crack, when extensive plastic yielding occurred at the high stresses at the crack tip, which was a characteristic of the material, and which, when measured in a test piece, could be used to predict behaviour in a large structure. This represented an important advance in understanding and in ensuring structural integrity, and had an enormous impact in this field. The analysis in the classic paper by Bilby, Cottrell and Swinden was used, suitably modified, as a basis for the failure line in the Failure Assessment Diagram used by industry to assess the likelihood of failure in cracked engineering structures. Knott’s paper in this issue refers to this. Cottrell’s theoretical work was accompanied by experimental studies carried out by Knott and Griffiths, and Knott’s research established a local tensile stress criterion for propagation of a microcrack.
Lewandowski’s paper describes how Alan Cottrell’s concepts, techniques and analysis have formed the basis of modern fracture mechanics applications to wide ranging materials systems.
In a classic and influential paper in 1967 Alan, together with Kelly and Tyson, considered the factors determining whether a material with a sharp crack would fail by brittle cleavage or in a ductile manner. They made estimates of the theoretical fracture strength and of the stress to create a dislocation at the crack tip, which enabled them to classify materials as inherently brittle or ductile. This paper stimulated much further research particularly on nucleation of dislocation loops at crack tips.
In 1964, Alan accepted an invitation from Sir Solly Zuckerman and Lord Mountbatten to become Solly’s Deputy in the Ministry of Defence. Although most reluctant to leave the Department and the University, he had become concerned with the need to invigorate British Manufacturing Industry with Scientific Technology, and felt that Whitehall was the place to do this.
Working on Denis Healey’s defence review, Alan led tri-services studies on the problems, in particular the excessive cost, of a military presence in the near and far East. This led to the cancellation of the Government’s East of Suez Policy. In 1966, he followed Solly to the Cabinet Office as Deputy Chief Scientific Adviser. There he tackled various problems with scientific aspect, including the brain drain, environment and pollution, the Advanced Passenger Train and the Torrey Canyon disaster, but efforts to transfer some government defence research funding to research in civil industry failed, because the Treasury was interested only in reducing research funding.
In 1971, Alan was knighted and became Chief Scientific Adviser. His position became complicated by the arrival of Victor Rothschild and his Central Policy Review staff. Victor suggested that the Research Councils should be attached to the relevant Government Departments to make their work more related to national needs, a suggestion which did not go down well with the Research Councils. Alan suggested that the Councils should remain independent but that each relevant department should have attached to it a Chief Scientific Adviser, with a view to making the departments well informed paying customers for Council Services. Rothschild seized on this idea and it became the foundation of the controversial ‘Customer-Contractor’ principle. Alan had to implement this unpopular policy, which involved transfer of some funds from the Research Councils to the Departments.
Alan was not very comfortable with the machinations of Whitehall politics. He played it straight, and used his powerful intellect to make his case, however unpopular. Alan was clear about one thing: Knowledge is power in Whitehall.
In 1973, in a minute to the Nuclear Power Advisory Board, and in 1974, in evidence to the Parliamentary Select Committee on Science and Technology, Alan expressed his concern about the integrity of the steel reactor pressure vessel, which is critical to the safety of the Pressurised Water Reactor, promoted by Walter Marshall at that time, for our Civil Nuclear Programme. This caused quite a stir. In response, Walter Marshall set up a High Level Pressure Vessel Committee in 1973 which examined the issue in great detail. In the early 1980s, following publication of the second Marshall Report, Alan said that he was now satisfied that a sufficiently robust safety case could be established, provided the recommendations of the report were implemented. The report and Alan’s endorsement had a major impact on the Sizewell B enquiry and on getting Nuclear Installation Inspectorate approval, and led more generally to major advances in the requirements for ensuring the integrity of pressure vessels and other large safety critical structures. Knott’s paper in this special issue describes the manufacture of PWR pressure vessels, the possible cracking mechanisms which can occur, the use of fracture mechanics to assess defect tolerance, the importance of non-destructive examination by ultrasonics, the effects of neutron irradiation on brittle fracture, and the onerous conditions on structural integrity imposed by a Loss of Coolant Accident, which was of particular concern to Alan Cottrell.
Cottrell believed that nuclear energy is an important source of power and also felt that the public should be able to form a rational view. To this end he set out the facts in simple terms ‘How Safe is Nuclear Energy’, published in 1981.
In 1974, Alan accepted an invitation to become Master of Jesus College, Cambridge. He was glad to return full time to his family and to academic life. Alan had to supervise a major revision of the College Statutes and prepare for the admission of women. This proved a great success.
In 1977, he became the Vice Chancellor for two years. During this period, he introduced the new Chancellor, Prince Philip, to the intricacies of the operation of the University. On returning full time to College his main activity was preparing for the arrival of Prince Edward who became an undergraduate in the College.
Alan retired in 1986. He returned to the Department, and researched a new topic: The application of modern electron theory of metals to metallurgical problems, such as embrittlement of metals by certain impurities. Alan mastered the quite difficult theory and published in 1988 an excellent book Introduction to the Modern Theory of Metals. This was followed by an impressive set of papers on applications to important metallurgical problems, and the publication of a book on Chemical Bonding in Transition Metal Carbides. The paper by Pettifor et al. discusses this topic.
From 1996, he cared full time for his wife Jean, who suffered from Parkinson’s disease. Sadly, she died in 1999. Her loss affected him greatly. During the last few years he published again on the plasticity of metals. Creep was of particular interest. Argon’s paper is relevant to this period. It is clear from his contribution to the British Library Oral History of British Science, that during his final years he was working on the problem of work hardening of single crystals of FCC metals, but was frustrated by the difficulties in obtaining the relevant experimental facts. Alas, unfortunately, he ran out of time.
Alan was the most outstanding and influential Physical Metallurgist of the twentieth century. Through his pioneering researches, and as an educator, he has influenced countless students, scientists and engineers over the years and will continue to do so. His papers and books are remarkable for their clarity. In his researches, he always knew what important questions to ask, and how to answer them. He had a brilliant intellect which he retained to the end. His life time achievement and impact have been immense.
Alan was a kind, gentle and sensitive person, with a sense of humour, and very supportive of people. He loved his family and was proud of Geoffrey working on nuclear fusion, which Alan considered to be an important future energy source. He was very eminent, but did not realise it, and was very modest. He received many awards and honorary degrees. At a Metal Society Conference in 1977 Alan received the Acta Metallurgica Gold Medal and commented that he was suffering from ‘medal fatigue’. In 1996 he received The Royal Society Copley Medal, the highest award of the Royal Society. He was the first metallurgist to receive the medal, since it was first instituted in 1731.
Sir Peter Hirsch FRSFebruary 2013