THE DISCOVERY OF STRONG ALUMINUM

Charles R. Simcoe

On rare occasions an individual searching in nature’s storehouse finds a gem so unique that it opens the door to another world, a world no one had imagined. One such instance occurred in a German laboratory at the beginning of the 20th century when a metals researcher was looking for a strong aluminum alloy

How did aluminum a metal that is light in weight, only one-third as heavy as steel, and in its unalloyed form is among the weakest of all metals become the structural material of modern airplanes? The development of aluminum alloys strong enough to build aircraft that can withstand the rigors of jet flight and carry passenger loads of 400 people with all their baggage and fuel enough to fly from continent to continent is a fascinating accomplishment in the field of metal making.

This story started in the very early 1900s, the infancy of the aluminum industry. At this time the aluminum market was mostly in cooking utensils, pots and pans; and as an additive to steel just before tapping into ingots. The leap from such small and mundane beginnings to building Zeppelins in a few years, Douglas DC-3s in 25 years, and Boeing 747s in 60 years is a remarkable feat of engineering..

The first character in this drama was a German engineer by the name of Alfred Wilm. Alfred Wilm was born in 1869 in Niederschellendorf, Silesia, Germany. He was interested in chemistry at an early age and later attended the Royal Institute of Technology at Charlottenburg where he was introduced to metallurgy. From 1897 to 1901 he worked as Chief of the Laboratory in the Goldschmidt Chemical Plant where he gained experience in the thermite process, especially its use in iron and steel. In 1901 Wilm made the fateful decision to work as a metallurgist at the government Central Laboratories for Scientific Research at Neubabelsberg near Berlin.

Wilm began a study to replace the heavy brass alloy, which was the time-honored jacket material for cartridges, with an aluminum alloy for weight savings. None of the aluminum alloys up to this time was strong enough for the application. Wilm, being familiar with heat treating steel, attempted to combine both alloying and heat treating in his research. No heat treatment known at that time could harden metals, except for steel.

THE ALLOY

Wilm was working with an aluminum-copper-manganese alloy, water-quenching it from a high-temperature, molten salt bath.. The strength of his heat-treated samples was not sufficiently high to interest the army. He needed to increase the hardness and next tried adding one-half percent of magnesium to his alloy. After heating and water-quenching Wilm reported that he gave a sample to his assistant, Fritz Jablouski, for hardness testing. Because it was a Saturday afternoon, Jablouski wanted to leave the laboratory early. Wilm urged him to take at least one reading and then finish on Monday. This reading showed only a small increase in hardness due to the magnesium addition. On Monday, however, the hardness was considerably higher.

After Wilm and Jablonski checked the results and convinced themselves that the alloy was indeed stronger on Monday, they repeated the experiment taking hardness readings on the heat-treated alloy every hour. Wilm reported "For almost two hours I saw no change, but after that time the hardness began to increase, which I followed from morning to evening. The next morning I continued, and determined that after about four days the material was quiescent." Wilm and Jablonski had observed a room temperature hardening (or strengthening) with time in a metal alloy, something never before observed by metal makers. Wilm's heat treatment was called age hardening.

Wilm worked for two years in his search before he discovered the optimum combination of aluminum, copper, manganese and magnesium that responded to his heat treatment. It would take another two years before he worked out the problems associated with producing the alloy, especially rolling techniques for making sheet material.

Wilm did not publish his discovery until 1911. In the meantime experimental quantities of the alloy were made in 1908 by the German metal working firm Durener Metallwerke-A.G. In 1909 they began producing the alloy under the trade name Duralumin, combining Durener and aluminum. Since the German Army showed no interest in the alloy, Wilm gained title to the patents. He sold rights to Vickers in England where the first major application was in the lighter-than-air ship "Mayfly". Unfortunately Mayfly broke in two during handling for its first flight. The alloy was used successfully, however, by Zeppelin to replace a lower-strength aluminum alloy in his lighter-than-air ships. With this limited experience, the German Navy ordered the alloy for their military Zeppelins. Each airship used as much as nine tons and 97 ships were built during "The Great War".

Interest in Wilm's alloy spread throughout the metal making world. Quantities were used in England and France during the war for aircraft parts but major airframe structures were still made of wood and canvas. Because the United States lagged behind the European countries in aircraft development, this country was not at the forefront in using Duralumin. However, samples of the alloy were obtained and studies were undertaken on the hardening mechanism at the United States Bureau of Standards in Washington D. C.

THE FIRST SCIENCE STUDY

Duralumin and modifications of it were studied by a trio of young metals researchers working at the bureau. They were Paul Merica, Howard Scott, and R. G. Waltenberg; and they would become famous as the next major contributors to the amazing story of age hardening. They found that nature had held a secret on a method of hardening metals, and that Wilm's alloy was just a single example of a universal behavior that was undiscovered since ancient man had learned to make alloys during the age of bronze.

PAUL DYER MERICA LED THE TEAM THAT DISCOVERED

THE PRINCIPLES GOVERNING PRECIPITATION HARDENING.

Paul Dyer Merica was the senior member of the research team. Merica was born in Warsaw, Indiana in 1889. He attended De Pauw University for several years and graduated from the University of Wisconsin in 1908. He taught for two years in Hangchow, China. Merica then entered graduate school where he earned his doctorate at the University of Berlin, Germany in 1914. Shortly after returning to the United States, he joined the research staff of the Bureau of Standards in Washington D.C. to work in physics and metallurgy.

The conclusions of the research of Merica, Scott, and Waltenberg were as follows:

Age hardening required an alloy where the second metal was soluble in the base metal at an elevated temperature but was considerably less soluble at low temperatures.

The alloy samples had to be heated at this higher temperature to take the second metal into solution and then water-quenched to a low temperature to maintain a supersaturated solution. At this point the metal alloy was relatively soft.

Aging represented the precipitation of the second metal or a compound between the two metals. This occurred at room temperature in the case of Wilm's alloy, although aging was accelerated by heating to a higher temperature

This knowledge not only explained the behavior of Duralumin but immediately set off a worldwide search for other alloys which could obey the conclusions of Merica, Scott, and Waltenberg.

THE REVOLUTION IN STRONG ALLOYS

Studies on alloy phase diagrams had been active since the first one published by Roozeboom on iron and carbon in 1900. These diagrams which show the degree to which one metal is effected by the addition of a second metal are road maps explaining how metals interact with temperature and increasing amounts of the second metal addition. Numerous alloy systems were now examined for candidates that would fit the conclusions of Merica, Scott, and Waltenberg. Throughout the 1920s and 1930s hundreds of alloy compositions were found in base metals of aluminum, cobalt, copper, gold, iron, lead, magnesium, nickel, silver, platinum, zinc, and others. Just about any base metal can be a candidate for age hardening provided that the appropriate second metal is added.

One of the most astonishing alloys developed during this period was copper with the addition of two percent beryllium. With the proper heat treatment it can be strengthened to 200,000 pounds per square inch of cross section (psi), as strong as heat-treated alloy steel. This copper-beryllium alloy is a preferred material for tools in use where explosive gases or mixtures are present, such as, mines and flour milling processes where sparks from steel tools can cause explosions.

An early commercial modification of Wilm's alloy developed by The Aluminum Company of America (Alcoa) was called 17S. It was brought out in the 1920s and was used to build the first commercial all metal airplane in the United States, the Ford Trimotor. Sheet metal was used for all the parts in the Ford Trimotor. The frame and all the interior structure were formed from sheet and the exterior skin was sheet metal shaped into a corrugated pattern. All the heat treating and forming operations were done at the Ford assembly plant. Parts had to be formed into final shape within several hours after heat treatment before natural aging started to increase the strength. They would fully harden in four to ten days. About two hundred Ford Trimotor planes were built in the late 1920s and early 1930s. Production ceased as the depression deepened and the aircraft design based on ten to fifteen passengers became obsolete for profitable commercial service.

The precipitation hardened 17S alloy lacked sufficient corrosion resistance in a salt spray atmosphere. Alcoa solved this problem by developing a process for bonding a layer of pure aluminum on both sides of the 17S sheet metal. These two layers make up about 10% of the total thickness of the sheet. This product is called Alclad and is still used for all precipitation hardened aluminum alloy applications exposed to a salt atmosphere.

Alcoa developed a new, higher strength alloy in the 1930s called 24S. The major change from 17S to 24S was to increase the magnesium level to 1.5% from the 0.5% of the previous alloys. This increased the design strength of 24S to 50,000 psi from 40,000 psi of the earlier alloys. In addition it was found that some moderate cold working, such as stretching or rolling sheet material, immediately after water quenching and then aging could further increase the design strength of 24S to 57,000 psi. All of these properties could be produced with Wilm's original room temperature aging treatment called natural aging. When heated at temperatures of 300 to 400F, called artificial aging, the strength of 24S could be increased to 71,000 psi if cold worked after water quenching. This alloy was the major construction material for the first commercially successful passenger plane, the Douglas DC-3 (1935).

THE DC-3 THE FIRST SUCCESSFUL COMMERCIAL

ALL-ALUMINUM AIRPLANE(AMERICAN AIRLINES)

Over 10,000 DC-3s were built in the United States from 1935 to 1946. An additional 5,000 were built in Russia and 500 in Japan. It was the world standard for air travel during that time, as well as a useful military plane. Its rugged design allowed it to be used for dropping paratroopers, towing gliders, hauling freight (it was the plane that moved many of the supplies over the hump of the Himalayas to China in WWII until other planes were developed to fly at the high altitudes) as well as the major military passenger plane of the war. DC-3s are still in use around the world even though the last plane came off the assembly line almost 65 years ago. Some aviation buffs regard the DC-3 as the worlds most beautiful plane

Alloy 24S, both naturally aged and artificially aged, was the strong aluminum alloy used for the nearly three hundred thousand planes built in the United States during World War II. The quantity of alloy needed for this vast undertaking greatly exceeded the capacity of Alcoa, the only aluminum producer in the country. To fill this need numerous aluminum plants were financed by the government but were built and staffed by Alcoa. At the end of the war these plants were sold to Reynolds Metals Company and Kaiser Aluminum and Chemical Company. This ended the Alcoa monopoly that had existed since 1888. With this new aluminum industry, the old system of identifying alloys was modified. The new system used the 2000 series for the aluminum, magnesium alloys. Thus 24S became 2024 and it is still a major high strength alloy even though stronger alloys have been developed for the most critical applications.

Aluminum containing major alloy additions of zinc along with magnesium and copper were originally studied in Germany. This work was reported in the United States by a German researcher and teacher, Dr. Guertler, who claimed to be half-American and the grandson of a Civil War general. Early in his career he had taught at MIT. These alloys with zinc as a major alloying element produced very high strengths; but were prone to crack under stress when exposed to corrosion.

Nevertheless, research on these alloys was performed in the United States at Alcoa, and the first composition used commercially was 76S for aircraft propellers (1940). Later the stress-corrosion-cracking problem was greatly reduced by adding small amounts of chromium to the alloy. This lead to the commercial alloy 75S (now 7075) which contained 5.5 percent zinc and was introduced during the second world war as the structural metal on the B-29 long-range bomber. (It is interesting to note that the Japanese used a similar high zinc alloy years earlier in the Zero fighter plane that was so successful in the first years of WWII.) This alloy could be artificially aged to design strengths of 73,000 psi. A modification of this alloy with 6.8 percent zinc called 7178 introduced in 1951 can develop strengths as high as 78,000 psi.

Another precipitation hardening alloy system of great industrial importance is based on adding small amounts of magnesium and silicon to aluminum. This alloy system is the structural material for a great tonnage of ordinary engineering applications. These alloys are outstanding for their ease of fabrication, corrosion resistance, and low cost compared with the high-strength aircraft alloys. They have design strengths of 40,000 to 50,000 psi, which is well below those of the aircraft alloys. However, they are stronger than low-carbon steel, which is normally used without heat treatment, or the stainless steels another alternative where corrosion resistance is of concern.

The alloys in this group have excellent characteristics for general industrial applications, such as; trucks, buses, rail cars, trailer tanks, storage tanks, building construction and light aircraft. One alloy in this group known as 61S (now 6061) was developed at Alcoa in the 1930s and has been referred to as the most important ever developed.

There are about a dozen alloys in this magnesium-silicon precipitation-hardening category. They range from simple alloys with only magnesium and silicon additions to alloys with other additions of manganese, chromium, copper, zinc, and/or even lead and bismuth. These last two alloys are used for parts that require extensive machining on high-speed screw machines. The very large number of mill products for these alloys includes; sheet metal, forgings, extrusions, bar, tubing, pipe, and wire.

Age hardening (now generally called precipitation hardening) was of special benefit to aluminum alloys since it provided high strength in a lightweight metal. However, extremely important precipitation hardening alloys in other metals are now used in a wide variety of applications. These include nickel-based alloys for the hot areas of jet engines, stainless steels for many aerospace parts, alloys of magnesium, copper, lead, zinc, cobalt, silver, gold, and platinum. Our world of high technology would look very different without the high strength provided by precipitation hardening.

THE NEW SCIENCE

While the conclusions of Merica, Waltenberg, and Scott were sufficient to send metal workers off on the successful search for precipitation hardening alloys in both aluminum and other metals; they were not precise enough to explain to metal scientists how the hardening occurred. It was generally believed that at some early stage in the precipitation of the second material a set of particles of very small size (perhaps on the atomic scale) provided a keying action to plastic deformation. This theory was proposed in 1923 by two distinguished researchers at Alcoa, Zay Jeffries and Robert Archer.

Zay Jeffries was born on the frontier of the Dakota Territory in 1888. He graduated in geology and mining engineering at the South Dakota School of Mines in 1909. He joined the faculty of the Case School of Applied Sciences (now Case-Western Reserve) in Cleveland as an instructor of metallurgy. Jeffries soon developed a consulting practice in the new technology industries of Cleveland. He worked with General Electric Lamp Division on tungsten and with an aluminum casting company. He became a consultant to Alcoa in 1920 and hired Robert Archer from the University of Michigan to work with him. They developed a group of aluminum alloys during the 1920s for castings and forgings that depended on precipitation hardening

A better understanding of metal behavior during plastic deformation began in the late 1920s and the1930s with the research of such Europeans as Geoffrey Taylor, A.A.Griffith, M. Polanyi, U. Dehlinger, and E. Orowan. These metal scientists developed the concept of a crystal lattice defect that they called a dislocation. Dislocations were segments of a row of atoms that did not line up with the atoms on the next row as they would do in a perfect crystal. These dislocations could move if enough stress was applied to the metal. The movement or gliding of dislocations could account for plastic deformation in metals. Dislocations could interact with one another and with grain boundaries; they could multiply by various means to explain the shape of the plastic deformation curves.

The concept of dislocations as the instrument of plastic deformation was also useful in explaining the strengthening of alloys by precipitation hardening. If dislocations moving under an applied stress produced plastic behavior, then anything restricting this movement increased the 0.2% yield strength. The method by which precipitates accomplished this result became the challenge for many metal researchers over the period 1935 to the 1960s.

Starting about the mid-1930s there were several successful studies in identifying the initial development of precipitation. Researchers began to see anomalies in x-ray work that preceded the formation of the final precipitate. Two researchers in Europe, A. Guinier and G. D. Preston, independently identified the first stage of the movement of atoms to sites in the lattice in preparation for forming the final precipitate. These sites were rich in copper atoms in an aluminum-copper alloy, but still retained the crystal structure of aluminum. The sites of this first stage in precipitation are called G-P zones in honor of Guinier and Preston.

Other researchers found that with longer aging times a more advanced stage in progression towards the final precipitate occurred. This precipitate showed a new crystal lattice that was not yet the crystal structure of the final precipitate. As long as these particles were constrained to conform to the crystal lattice of aluminum a condition called coherency they provided additional strengthening. When they grew larger with additional aging and formed their own boundary between them and the aluminum, the strength decreased.

The present understanding of precipitation hardening is that the presence of G-P zones and intermediate precipitates, which are still coherent with the crystal structure of the host metal, provide barriers to dislocation movement. Higher stresses are necessary to force the dislocations to cut through these barriers. When the alloy is aged for longer times, the G-P zones and the intermediate precipitates are replaced by the final particles that have complex crystal structures and boundaries between them and the host metal lattice. These are larger particles and the distance between them allows dislocations to move more freely through the metal.

New tools are usually the bases for advances in science and engineering. In the late 19^th century metallurgy surged ahead with the invention of the high-temperature thermocouple and the metallurgical light microscope. In the 20^th century two new tools were X-rays and the electron microscope. X-rays are used in a special way to measure atomic distances and crystal structure. The electron microscope uses beams of electrons for magnifications many orders of magnitude above the light microscope.

X-ray techniques were commonly used after the work of the British father and son team, Sir William Henry Bragg and William Lawrence Bragg, on measuring distances between rows of atoms in crystals. For their research they won the 1915 Nobel Prize in Physics. The electron microscope was first used in the 1940s, but became much more effective when new experimental techniques were developed in the 1950s and 1960s.

APPLYING THE NEW SCIENCE—POST WWII

The new tools and techniques and theories of precipitation hardening and dislocations came at a propitious time. The need for higher strength alloys accelerated after World War II for larger aircraft and especially for jet engines to power them. Hundreds of engineers and scientists performed research and development in alloys of nickel, cobalt, titanium, and aluminum during this period. Most of the major universities with Departments of Metallurgical Engineering, many government laboratories, the metal producers, the manufactures of aircraft and of engines, and many subcontracting companies were part of this effort.

Two examples of the use of precipitation hardened aluminum alloys were selected to demonstrate the value of these materials in our modern technology. The first is the latest wide-body aircraft, the Boeing 777. The second is the research space module Destiny.

The Boeing 777 is a twin-engine jet with a capacity of 400 passengers for long-range travel. The major structure of the plane is constructed with two basic aluminum alloys. The upper wings, because they are the most highly stressed components, are built of 7055 alloy. The lower wings are constructed of modified versions of 2024. They are 2224 and 2324. These alloys have lower impurity content, which improves toughness. The fuselage is made of 2524 another version of 2024 that contains even lower levels of impurities for greater toughness. All versions of 2024 are naturally aged. The 7055 alloy is artificially age to beyond peak strength for improved corrosion resistance.

The new alloy 7055 contains 8.0% zinc, 2.3% copper and 2.0% magnesium. It was designed to provide exceptionally high strength in compression. Strength levels of 90,000 psi are achieved in plate and 97,000 psi in extrusions. The strength is 10% higher than the best previous alloy and 25% higher than the original aluminum-zinc alloy, 7075, developed during WWII. Excellent corrosion resistance and fracture toughness accompanies these strengths. The heat treatment for 7055 is T7751, a proprietary Alcoa process.

A group at the Alloy Technical Division of Alcoa Research Laboratory under the direction of Dr. William A. Cassada and the leadership of Dr. James Staley performed the research work for alloy 7055. Alcoa received the ASM International Engineering Achievement Award in 2002 for the development of 7055 and 2524 alloys. These two alloys are destined to become the materials of choice for future aircraft.

The Destiny Module is a research laboratory that was manufactured for NASA by the Boeing Company. It was launched in February 2001 and is attached to the Space Laboratory. The module is a cylindrical shell 14 1/2 feet in diameter and 27 ½ feet long. The body of the cylinder consists of rings forming the ribs of the shell with a case of machined and formed plates. The plates are welded to the rings and the individual plates are welded to each other to form a pressure tight cylinder. The ends of this cylinder are cone shaped and contain hatches for access to the interior. Finally, there is a debris shield that surrounds the module to protect it from micro-particles coming in from outer space. Destiny is constructed from precipitation hardened aluminum alloys.

THE DESTINY RESEARCH MODULE

LAUNCHED IN 2001(BOEING)

The shell with its rings and outer case are made of 2219 alloy. This alloy was selected, according to Dr. John Golden, Technical Fellow at Boeing on the Destiny Program, because of its ease of welding and the past experience using it in the Saturn V. Alloy 2219 contains 6.3% copper and minor amounts of titanium and manganese. It was developed in the early 1950s and is a high copper modification of an earlier alloy (2025), invented by Jeffries and Archer in 1921. After solution heat treatment, the rings are plastically deformed and artificially aged to about 50,000 psi. The panels are naturally aged to 45,000 psi. They are machined from one-inch plate, which leaves a series of reinforcing stiffeners running diagonally around the circumference of the vessel. The material between these one-inch stiffeners is machined to 0.125 inches thick and the seams to be welded are 0.190 inches thick. Welding to form a pressure tight vessel assembles the entire module.

The access hatches are made of the higher strength aluminum-zinc alloy 7075. They are made from plate and are four feet in diameter. After forming and machining, they are solution heat-treated, artificially aged to beyond peak strength for improved corrosion resistance, and plastically deformed in compression for stress relief.

The debris shield, which stands off from the pressure vessel, is held in place by metal clips. The shield is 0.080 inch sheet made from alloy 6061 artificially aged to peak strength of 40,000 psi. The 6061 alloy was selected because of the amount of data available on ballistic performance and for the ability to maintain a bright surface finish needed in reflecting radiation from the sun. It is interesting that 6061, the most common strong aluminum alloy used in numerous industrial applications, finds a place in the 21^st century space program.

LATER CAREERS

The young metals researchers who built the foundation for precipitation hardened metal alloys continued to have successful careers in metallurgy and engineering in later years, except for Alfred Wilm, the individual who made the initial discovery. Wilm entered into licensing agreements for his alloy with Durener Metallwerke-A. G.in Germany and with several companies in other European countries. The war starting in 1915 destroyed his income from these foreign producers, and with other poor business investments he never received the financial reward he deserved. In the aftermath of the war, Wilm abandoned his technical career and moved his large family to a mountain area in Saalberg, Germany to become a poultry farmer.

While Paul Dyer Merica’s greatest contribution to metallurgy was his work on precipitation hardening in aluminum, he had a brilliant career after 1919 as an employee of the International Nickel Company (Inco). He joined Inco as a research metallurgist and progressed through the company as Director of Research, Vice President, and President in 1952. Over the years he was recognized for his research contributions by many technical awards. These include the James Douglas Gold Medal by the American Institute of Mining and Metallurgical Engineers and the John Fritz Gold Medal by the combined societies of four engineering branches. He also received the Platinum Medal of the British Institute of Metals, the prestigious Franklin Medal of the Franklin Institute, and the Gold Medal of the American Society of Metals. He was elected to the National Academy of Sciences in 1942.

Howard Scott served a long career as a research metallurgist and Chief Metallurgist at Westinghouse Electric Corporation where he specialized in high temperature alloys for steam and gas turbines. Walter Waltenberg worked for Englehart Industries in special alloys for electrical, magnetic and thermostatic applications.

. Zay Jeffries was a very gifted metals engineer. At the same time that he was consulting with Alcoa on aluminum he was a consultant to General Electric on tungsten in lamp filaments. He maintained this dual employment from 1920 to 1936 when he resigned his duties at Alcoa to become a full-time employee of General Electric Company. He directed a new division at General Electric in tungsten carbide for cutting tools and later he was a made a vice president in charge of all their chemical operations. Jeffries was active in technical societies involving metals and engineering in general. He was elected a member of the National Academy of Sciences (1938) and was a consultant working with Arthur H. Compton at the Chicago Laboratory on the Manhattan Atomic Energy Project during World War II.

Robert Archer left Alcoa in 1930 to head research at the A. O. Smith Company. Later he joined Republic Steel Company and completed his career in metals research at the Climax Molybdenum Company of Michigan.

The public knows little about the work of metal makers and the great advances made in this century. Individuals such as Wilm, Merica, Scott, Waltenberg, Jeffries, and Archer, and the other metal scientists referred to are just a few of the many metals researchers who have contributed to our modern technological society. Also the companies, Alcoa in the case of our story on precipitation hardening in aluminum, are seldom given the credit for supporting the people and processes sometimes over many years that are needed for advances in materials.

Wilm’s discovery of age hardening is one of those seminal events in technical history. He didn’t invent an alloy, he discovered a new universe. This is a rare occurrence experienced by only a few researchers in all of science.

SOURCES

History of Precipitation Hardening, H, Y, Hunsicker and H. C. Stumpf, The Sorby Centennial On The History Of Metallurgy, Gordon and Breach Science Publishers, New York, 1963.

Meriica, P. D., Waltenberg, R. G., and Scott, H.; Science Paper, U. S. Bureau of Standards, 347, 1919.

Men of Metals, Samuel L. Hoyt, American Society For Metals, Metals Park, Ohio, 1979.

Zay Jeffries, W. D. Mogerman, American Society For Metals, Metals Park, Ohio, 1973.

Aluminum: Volume I; Properties, Physical Metallurgy, and Phase Diagrams; Prepared by engineers, scientists, metallurgists of Alcoa, Kent R. Van Horn, editor, American Society For Metals, Metals Park, Ohio, 1967.

Alfred Wilm, Aluminum (German), September 1935.

Paul Dyer Merica, National Academy of Sciences, 33, 1959

Alcoa Aerospace Technical Fact Sheet—7055 alloy-T7751 Plate and –T77511 Extrusions

Dr. William A. Cassada, Private communication on Alcoa 7055 and 2524

Dr. John Golden, Private communication on Destiny alloys

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