The Three Most Significant Advances in Alloy Metallurgy Since Federation ( 1901 ) – An Australian Perspective

Topics Covered

Background

Physical Metallurgy at the Turn of the Century

The Three Most Significant Discoveries Since 1901

Aluminium Alloys and the Discovery of Age Hardening

Aluminium Production Since 1901

Major Uses for Aluminium

Age Hardening of Aluminium

The Age Hardening Mechanism

Age Hardening of Other Alloys

Stainless Steels

Classes of Stainless Steels

Martensitic Stainless Steels

Ferritic and Austenitic Stainless Steels

Duplex Stainless Steels

Alternative Materials to Stainless Steels

Precipitation Hardening and Maraging Steels

Nuclear Power

Problems for Materials to be Used in Nuclear Reactors

Nuclear Technology Induced Problems

Concluding Remarks

Background

Materials selection was much easier in 1901. The Steel Age had become dominant because of the availability of cheap steel produced by the Bessemer and Open-Hearth processes in the latter part of the 19th century. Mild steel, in particular, enabled the construction of large bridges, ocean liners, trains and then the motorcar. Already a country’s annual steel production was being used as a quick indicator of its industrial strength, a criterion that persisted for much of the ensuing century. World production of steel had reached some 30 million tons by 1900 which compared with 890,000 tonnes of lead, the next most used metal, and around 500 000 tonnes each for copper and zinc. Apart from the precious metals, tin was the only other base metal having significant industrial importance. Polymer science and technology was in its infancy, with “celluloid” (cellulose nitrate softened with camphor oil) being the only synthetic plastic so far invented. Ceramics were confined essentially to pottery and building materials.

Physical Metallurgy at the Turn of the Century

Physical metallurgy was in its infancy, with comparatively little being known, or appreciated, about relationships between the microstructure of metals and alloys and how they performed in service. What is interesting to record is that two important advances that did occur around that time involved men with connections to Australia. One was Walter Rosenhain who had qualified in physics and civil engineering at the University of Melbourne in the 1890s and went to England where he and E. A. Ewing, using optical microscopy, established that slip was the fundamental process of deformation in metals and alloys. The others were William Bragg and his son Lawrence, also working in England, who used x-rays to reveal the structure of crystalline solids and devised the famous Bragg’s Law, for which they were awarded the Nobel Prize in Physics in 1915. Previously, William had been Professor of Mathematical and Physical Sciences at the University of Adelaide from 1885 to 1909.

The Three Most Significant Discoveries Since 1901

During the 20th century, uses were found for most of the metals that make up close to three-quarters of the Periodic Table of the Elements and the number of alloys available escalated with time. Now, in retrospect, I have been asked to give a brief account of what I regard as the three most significant metallurgical developments since the Federation of Australia in 1901

Aluminium Alloys and the Discovery of Age Hardening

Aluminium was first isolated by Oersted in Denmark in 1825 and produced chemically as small ingots by Sainte-Claire Deville in France in 1855. At that time, aluminium was more expensive than gold but attracted the attention of Napoleon III who foresaw its use for military purposes such as lightweight body armour. Following the availability of high voltage supplies of electricity, independent discoveries by Hall in the United States and Heroult in France in 1886 led to the development of an economic method for extracting aluminium that remains the basis for production of this metal today.

Aluminium Production Since 1901

World production of aluminium had reached some 6,000 tonnes by 1900, 750,000 tonnes by the start of the Second World War and 4.7 million tonnes by 1960 when it surpassed copper as the second most used metal. Now annual world production is approximately 30 million tonnes of which approximately one third is recycled. Thus aluminium is very much a metal of the 20th century, Australia, where aluminium was not produced until 1955, has become the world’s fifth largest supplier of this metal which is now this country’s second most valuable manufactured export.

Major Uses for Aluminium

The majors sectors in which aluminium is used worldwide are building and construction, transportation, containers and packaging, and electrical. Wrought products dominate, traditionally accounting for some 85% of all aluminium used, although the steady replacement of cast iron components in the motor car by lighter aluminium alloy castings is changing this wrought-to-cast alloy ratio. The largest single use of aluminium is in the ubiquitous beverage can. This application commenced with the entry of aluminium into the can market in 1962 with the introduction of the tear-top tab, or so-called easy-open end, which was added to the lid of the soldered steel can. This was followed by the all-aluminium can with a seamless body that was produced from rolled sheet by cupping and then drawing and ironing the side walls. Producing this 0.30mm thick can body sheet from large (e.g. 600mm), direct chill cast ingots of the alloy Al-1%Mn-1%Mg is a technically complex process involving careful control of homogenisation, rolling and annealing cycles so that a correct balance of crystallographic textures is obtained to minimise tearing, combined with adequate strength and ductility. World consumption now exceeds 200 billion beverage cans per annum and, in the United States, aluminium enjoys more than 95% of this can market. A key factor in the economics of the aluminium can has been efficient recycling which, in some countries, amounts to more than two thirds of all cans used.

Age Hardening of Aluminium

Aluminium alloy development began when the properties of the unalloyed metal were insufficient to meet the needs of potential customers. In 1906 a German metallurgist, Alfred Wilm, was investigating the effects of adding copper and other metals to aluminium in the hope of finding a stronger replacement for brass in cartridge cases. One such alloy, Al-3.5%Cu-0.5%Mg, was heated and quenched into water to see if it would harden like steel given a similar treatment. Initial results were disappointing but, to his surprise, tests made by chance some days later revealed that the alloy had become significantly harder and stronger. The phenomenon was called “age hardening” and it represented the only new method of hardening alloys by heat treatment since the effects of quenching of steel were discovered in the second millennium BC In 1909, Wilm gave to the Durener Metallwerke in Duren sole rights to his patents and this firm produced the first sheet in the famous composition known as “Duralumin”. This alloy was quickly adopted in 1911 for structural members of the Zeppelin airships and then for the first all metal aircraft, the Junkers F 13, that first flew in 1919. Age hardened aluminium alloys have continued to be the principal materials for aircraft construction which, in turn, has provided continuing stimulus for new alloy development.

The Age Hardening Mechanism

The fundamental reason for age hardening of Duralumin was unknown to Wilm and this situation persisted for some time because the structural changes taking place were beyond the resolution of the optical microscope. The concept of supersaturation induced through quenching because of the decreasing solid solubility of copper in aluminium with decreasing temperature was proposed in 1919. Then, in 1921, it was suggested that hardening may be due to the formation of unseen, “submicroscopic precipitates” during ageing which might cause strengthening by interfering with slip. Independent X-ray studies by Guinier in France and Preston in England in 1937 revealed some fine scale structural changes but it was not until the advent of transmission electron microscopy that a detailed understanding of precipitation processes in aluminium and other alloys was possible. Even so, important ageing phenomena, such as the role of pre-precipitate atom clustering, are still being revealed with the advent of the technique of atom probe field-ion microscopy.

Age Hardening of Other Alloys

Many alloy systems have been found to respond to age hardening but none as effectively as some aluminium alloys. Recently an Al-Cu-Li-Mg-Ag alloy was developed that has a yield strength exceeding 700 MPa which is seventy times that of unalloyed aluminium and approaches the theoretical strength of this metal. Because of its relatively low melting point and ease of handling, aluminium has often been chosen as the base metal to model and develop new processes that have later been applied more widely. The most notable examples are the semi-continuous casting of large ingots and billets for rolling and extrusion and the continuous casting of rod and sheet. Furthermore, the association of some aluminium alloys with aerospace industries tends to place them at the forefront of advanced and emerging technologies. Thus, much of the recent work on laminated and metal matrix composites, mechanical alloying and rapid solidification processing has been carried out on aluminium alloys.

Stainless Steels

Credit for discovering the first stainless steel is usually attributed to H. Brearley of Sheffield who, in 1913, was experimenting with alloy steels for gun barrels that might resist corrosion in service. Some months later he noticed that one of the compositions he had rejected, which contained 14%Cr, had not tarnished. This steel was relatively soft and, initially, he regarded it as a curiosity. He is said to have made cutlery for friends warning them that the knives would not cut! Since then, many stainless steels have been developed, some indeed for domestic use, and without which it is difficult to imagine how chemical and other industries could function.

Classes of Stainless Steels

Four important classes of stainless steels are now in general use. All have in common the presence of more than 12% Cr which promotes formation of a protective surface film rich in chromium oxide. They are classified according to their microstructures as martensitic, ferritic, austenitic and duplex stainless steels.

Martensitic Stainless Steels

The martensitic group commonly contain around 13%Cr with small amounts of carbon and, as the name implies, can be hardened by heat treatment by quenching from the austenite region of their respective phase diagrams. Some contain additional alloying elements so that they respond to secondary hardening and display good creep resistance up to 600 °C. Martensitic stainless steels do now make good cutlery and have many key uses in industry such as for the blades in steam turbines and for ball bearings.

Ferritic and Austenitic Stainless Steels

Ferritic and austenitic stainless steels contain higher levels of chromium and, as they are single phase, they can only be strengthened by cold working. The former are produced mainly as sheet and tubing and are commonly used in the motor car for body trim, pollution control units and exhaust systems. Austenitic stainless steels are the most widely used of all. They also contain substantial amounts of nickel and many are derived from the famous “IS-8” (18%Cr-8%Ni) composition that is universally used for deep drawn products such as the kitchen sink. They are widely used for critical components in the chemical industries (with some compositions being resistant to oxidation at high temperatures), the food processing and pharmaceutical industries, and for many architectural purposes. They also have the useful property of being non-magnetic.

Duplex Stainless Steels

Duplex stainless have ferrite / austenite microstructures and have desirable combinations of properties such as relatively high strength and resistance to stress-corrosion cracking.

Alternative Materials to Stainless Steels

Chromite-containing ores are the major source of chromium metal and, because they occur mainly in what have been the politically uncertain locations of Southern Africa and Russia, much effort has been given to searching for corrosion-resistant alloys that could substitute, economically, for the present stainless steels. However, no significant success has been achieved to-date.

Precipitation Hardening and Maraging Steels

During the Cold War of the 1950s, a supersonic bomber aircraft was designed in the United States to fly at a speed of Mach 3 which would experience aerodynamic heating to 250-30O°C. This meant that conventional aluminium alloys could not be used for construction and the only viable alternative at that time was a stainless steel formed into honeycomb panels to reduce weight. It was also necessary to develop a stainless steel with a much higher strength:weight ratio. This was achieved by adding small amounts of aluminium and titanium to certain nickel-containing stainless steels which stimulated a strong response to age hardening due to precipitation of one or more of the gamma prime phases such as Ni3Al. Prototypes of this aircraft were built but the program was later cancelled in favour of missile systems and these so called PH stainless steels, some of which had yield strengths as high as 1500MPa, have since been used only for specialised applications. However, the exploitation of precipitation hardening was taken further with the development of the range of what are now known as maraging steels that were based initially on the composition Fe-20%Ni. Again elements such as aluminium, titanium and molybdenum were added that combine with nickel to form the gamma prime precipitates. Some maraging steels have yield strengths exceeding 2000MPa, and recent applications include fasteners and aircraft undercarriages.

Nuclear Power

Materials and energy are inextricably linked. Industrial materials cannot be produced without the input of energy and energy cannot be generated without using specialised materials. This situation has applied particularly to nuclear power, and the translation of nuclear fission from the realm of atomic physics into a commercial process for the controlled production of electrical energy, within a decade following conclusion of the Second World War, was a remarkable achievement. Now some 17% of the world’s electricity is generated this way.

Problems for Materials to be Used in Nuclear Reactors

The basic purpose of a nuclear power reactor is to sustain the fission of the uranium isotope U235 by thermal neutrons in a controlled manner so that the heat which is generated can be removed and used to raise steam and produce electricity. Many metallurgical problems had to be solved. Initially, the study of the physical metallurgy of uranium with its several allotropes and isotopes, and of its alloys, presented many challenges. In the core of a reactor, the uranium metal or uranium oxide fuel rods had to be contained in metal cans that had to meet a unique set of requirements. These cans had to be permeable to neutrons so that fission could be sustained, mechanically strong at the operating temperatures, chemically stable with respect both to the fuel contained inside and the external coolant, and allow efficient heat transfer to take place from fuel to coolant. The first requirement of neutron permeability immediately limited choice for the can to beryllium, magnesium, aluminium, zirconium and possibly stainless steel in increasing order of neutron capture cross-section. Despite major efforts in several countries, beryllium metal proved intractable to produce in quantity and fabricate, whereas aluminium was found to react with the uranium fuel at elevated temperatures. A Mg-Al alloy and a duplex stainless steel were finally selected for fuel cans for two different British gas-cooled reactors. Most other designs of reactors employed water cooling and, for these, zirconium alloys were found to possess the best combination of properties. As little was then known about this metal an urgent study of its general metallurgy had to be initiated. One early problem was to find an economical way to separate hafnium, which always co-exists with zirconium and has a neutron capture some six hundred times higher. Ivan Newnham with the CSIRO Division of Industrial Chemistry in Melbourne developed one such method.

Nuclear technology Induced Problems

Nuclear technology has introduced unique problems that arise because materials in the reactor core are continually exposed to energetic radiation, notably bombardment with neutrons. Internal structural or so called radiation damage, occurs that takes two forms - displacement damage arising because atoms are knocked out of their equilibrium positions and transmutation damage which produces xenon and krypton gases that can lead to swelling within fuel and can. Each of these phenomena may induce internal stresses and cause shape changes. Toughness is progressively reduced and irradiation-induced creep may become a problem. Understanding these effects required detailed studies using, in particular, electron microscopy and microstructures were developed in the alloys that could accommodate the radiation damage for the prescribed lifetimes of the components.

Concluding Remarks

As mentioned above, two of the nominated advances in alloying occurred by chance early in the 20th century. Now the understanding of physical metallurgy has advanced to a stage whereby it is becoming possible to design alloys having properties that closely match service requirements. In retrospect, the 20th century will be seen as the golden age of alloy development. Special achievements have been a more precise understanding of the actual roles of alloying elements and the ability to control microstructures. While further refinement of alloy compositions will no doubt continue into the future, opportunities for spectacular advances now seem more likely to come from the development of new methods for processing alloys.

A final comment is to note that all three examples were associated initially with war. For the 21st century, it is to be hoped that new developments in alloying, and with materials in general, will be stimulated more by economic, social and environmental needs.

 

Primary author: I. Polmear

Source: Materials Australia, Vol. 33 No. 1, pp. 16-18, January/February 2001.

 

For more information on this source please visit The Institute of Materials Engineering Australasia.

 

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