In 1964, Robinson presented an elegant modification of the test method. The basic design of a line-heatsource guarded hot plate was presented to a thermal conductivity conference sponsored by the National Physical Laboratory in England. The design was reported in Nature [6] as follows:

H. E. Robinson (U.S. National Bureau of Standards) discussed forms of line heat sources that could be used as heaters in apparatus for measurements at lower temperatures on insulating materials in disk and slab form. These new configurations lend themselves more readily to mathematical analysis, they are more simple to use and would appear to be able to yield more accurate results.

The design was novel. In contrast to a (conventional) guarded hot plate that used uniformly distributed heaters, line-heat-source guarded hot plates utilized circular line-heat sources at precisely specified locations. By proper location of the line-heat-source(s), the temperature at the edge of the meter plate can be made equal to the mean temperature of the meter plate, thereby facilitating temperature measurements and thermal guarding. The benefits offered by a line-heatsource guarded hot plate included simpler methods of construction; improved accuracy; simplified mathematical analyses for calculating the mean surface temperature of the plate as well as determining the errors resulting from heat gains or losses at the edges of the specimens; and superior operation under vacuum conditions.

After Robinson, another generation of NBS researchers continued development of the line-heatsource technology. In 1971, M. H. Hahn [7] conducted an in-depth analysis of the line-heat-source concept and investigated several design options. The design, mathematical analysis, and uncertainty analysis for a prototype line-heat-source guarded hot plate were published in 1974 by Hahn, Robinson (posthumously), and D. R. Flynn [8]. Construction of the prototype apparatus was completed in 1978 [9]. Because of the promising results from the prototype, NIST began plans for a second, larger line-heat-source guarded hot plate apparatus. In 1980, a ruling by the U.S. Federal Trade Commission concerning the labeling and advertising of home insulation dramatically accelerated the construction of this apparatus.

With the energy crisis in the 1970s, there was growing demand for thick insulation and a resulting concern from the Federal Trade Commission that the existing standards for insulation measurement, based on 25 mm

thick specimens, were not protecting consumers' interests. In response to concerns from industry and the Federal Trade Commission, NBS expedited development of the one-meter line-heat-source guarded hot plate. Near the end of 1980, the second line-heat-source guarded hot plate apparatus was completed with the efforts of Hahn, B. A. Peavy, F. J. Powell, and others [10]. Industry subsequently testified to Congress [11] that the improved accuracy provided savings of $90 million per year to consumers in insulation costs alone. It is not surprising that NIST now plans to continue developing the measurement technique into the 21st century with a new design that will extend the current temperature range.

After starting at NBS in 1903, Hobart C. Dickinson was the co-author of several fundamental papers on thermometry. In 1910, he returned to Clark University for his Ph.D. His doctor's thesis was on combustion calorimetry, and his calorimeter design, with some refinements, yielded the most accurate results attainable for its time. From 1912 to 1917 he was in charge of work at the Bureau of Standards on the constants of refrigeration in a program sponsored by the American Society of Refrigeration Engineers. His contributions to this program were papers on the calorimetry of ice and on the thermal conductivity of insulating materials. At the onset of World War I, the Bureau Director, Samuel Wesley Stratton, asked him to assist in the development of aircraft engines, and he participated in the design of the Liberty Engine, which was one of the engineering triumphs of that time. After the war the activities of the section were expanded to embrace automobiles and their behavior on the road. From 1921 to 1923 he organized and directed the research department of the Society of Automotive Engineers at the headquarters in New York City, and in 1933 he was president of that society.

Dickinson was an ardent hiker and preferred to spend his vacations in the mountains. In his later years he served the Potomac Appalachian Trail Club in its program of shelter building. On his first trip to the Canadian Rocky Mountains with the Alpine Club of Canada he took only a small spare blanket and waterrepellent sheet when he should have taken all his own bedding. Confronted with bleak prospects, he remembered his measurements on insulating materials, and with his usual resourcefulness gathered balsam boughs in considerable quantity to put between his blanket and his sheet. The result astonished him, and he soon learned that a relatively small thickness of balsam furnished sufficient insulation to keep him pleasantly warm even on freezing nights.

Prepared by Robert R. Zarr.

Bibliography

[1] H. C. Dickinson and M. S. Van Dusen, The Testing of Thermal Insulators, ASRE J. 3 (2), 5-25 (1916).

[2] M. S. Van Dusen, The Thermal Conductivity of Heat Insulators, Trans. Am. Soc. Heat. Ventilat. Eng. 26, 385-414 (1920). [3] D. R. Flynn and D. A. Didion, A Steam Calorimeter Apparatus for Refractories, in Conference on Thermal Conductivity Methods, Battelle Memorial Institute, Columbus, Ohio (1961) pp. 81-90.

[4] M. S. Van Dusen and J. L. Finck, Heat Transfer through Insulating Materials, American Institute of Refrigeration 17, 137-150 (1928).

[5] H. E. Robinson and T. W. Watson, Interlaboratory Comparison of Thermal Conductivity Determinations with Guarded Hot Plates, in Symposium on Thermal Insulating Materials: Cincinnati Spring Meeting, American Society for Testing Materials, March 7, 1951, (ASTM STP 119), American Society for Testing Materials, Philadelphia, PA (1951) pp. 36-44. [6] R. P. Tye, Thermal Conductivity, Nature 204, 636-637 (1964). [7] M. H. Hahn, The Line Heat Source Guarded Hot Plate for Measuring the Thermal Conductivity of Building and Insulating

Materials, Ph.D. Dissertation, Catholic University of America,
Washington, DC (1971).

[8] M. H. Hahn, H. E. Robinson, and D. R. Flynn, Robinson Line-Heat-Source Guarded Hot Plate Apparatus, in Heat Transmission Measurements in Thermal Insulations (ASTM STP 544), American Society for Testing and Materials, Philadelphia, PA (1974) pp. 167-192.

[9] M. C. I. Siu and C. Bulik, National Bureau of Standards LineHeat-Source Guarded Hot Plate Apparatus, Rev. Sci. Instrum. 52, 1709-1716 (1981).

[10] F. J. Powell and B. G. Rennex, The NBS Line-Heat-Source Guarded Hot Plate for Thick Materials, in Thermal Performance of the Exterior Envelopes of Buildings II: Proceedings of the ASHRAE/DOE Conference (ASHRAE SP 38), American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Atlanta, GA (1983) pp. 657-672.

[11] National Bureau of Standards Authorization, Hearing Before the Subcommittee on Science, Technology, and Space of the Committee on Commerce, Science, and Transportation; United States Senate; Ninety-Eighth Congress; First Session, on National Bureau of Standards Authorization; Serial No. 98-3, 67 (February 22, 1983).

Precipitation hardening, or age hardening, provides one of the most widely used mechanisms for the strengthening of metal alloys. The fundamental understanding and basis for this technique was established in early work at the U. S. Bureau of Standards on an alloy known as Duralumin [1,2]. Duralumin is an aluminum alloy containing copper and magnesium with small amounts of iron and silicon. In an attempt to understand the dramatic strengthening of this alloy, Paul D. Merica and his coworkers studied both the effect of various heat treatments on the hardness of the alloy and the influence of chemical composition on the hardness. Among the most significant of their findings was the observation that the solubility of CuAl2 in aluminum increased with increasing temperature. Although the specific phases responsible for the hardening turned out to be too small to be observed directly, optical examination of the microstructures provided an identification of several of the other phases that were present. The authors proceeded to develop an insightful explanation for the hardening behavior of Duralumin which rapidly became the model on which innumerable modern high-strength alloys have been developed.

In his Institute of Metals lecture [3], Merica summarized the Merica, Waltenberg, and Scott paper as follows: "The four principal features of the original Duralumin theory were these: (1) age-hardening is possible because of the solubility-temperature relation of the hardening constituent in aluminum, (2) the hardening constituent is CuAl2, (3) hardening is caused by precipitation of the constituent in some form other than that of atomic dispersion, and probably in fine molecular, colloidal or crystalline form, and (4) the hardening effect of CuAl1⁄2 in aluminum was deemed to be related to its particle size."

At a symposium devoted to precipitation from solid solution, held nearly four decades after the original papers, R. F. Mehl noted [4], "The early work of Merica, Waltenberg, and Scott was the first contribution to theory: it demonstrated the necessity of a solid solubility decreasing with temperature; this paper had not only science but even prescience, for it suggested that some sort of precipitate-matrix interaction might contribute to hardening, long before coherency was even conceived. There are few better examples of the immense practical importance of the theory in the history of science; before Merica no new age-hardening alloys were discovered the worker did not know where

to look; following Merica, new age-hardening alloys came in a flood."

The importance of the theoretical suggestion for the development of new alloys is clear from the historical record [5]. At the end of the 19th century, cast iron was the only important commercial alloy not already known to western technology at the time of the Romans. When age hardening of aluminum was discovered accidentally by Wilm [6], during the years 1903-1911, it quickly became an important commercial alloy under the trade name Duralumin.

The two NBS studies published in 1919 explored both the application of phase diagrams to the phenomenon and the consequences of various heat treatments on the subsequent time evolution of mechanical properties. The latter study tentatively concluded that age hardening of aluminum was a room-temperature precipitation phenomenon and suggested that it should be possible for other alloys to be hardened by a thermal treatment leading to precipitation. Merica et al. suggested that examination of the relevant phase diagrams would reveal which alloys were candidates for such precipitation hardening and would provide both the solutionizing temperature and the range of temperature needed for the precipitation process.

This prescription proved to be astonishingly successful for developing new alloys. It led to a "golden age" of phase diagram determination that lasted two decades. It contributed to the development of a variety of fields in materials science and launched a scholarly debate that overthrew old concepts and definitions concerning alloy phases.

In the 15-year interval between the discovery by Wilm and the suggestion by the Bureau of Standards group, only one other age-hardening system had been discovered, but not published. Aging of Duralumin was thought to be a unique and curious phenomenon. However, by 1932, Merica could tabulate experience with fourteen base metals that had been discovered to harden by precipitation in a total of more than one hundred different alloy combinations. Even that list was already incomplete and underestimated the true worldwide effort that the theoretical suggestion had stimulated. Most of today's high strength commercial aluminum and nickel-based alloys are precipitation hardened, as are many titanium and iron-based alloys.

Despite the practical success of the theory, there was skepticism since the precipitates did not grow to

optically observable size until long after the hardening had begun. Almost 20 years passed before the precipitates responsible for the hardening were detected experimentally by small-angle x-ray scattering. When finally detected, they became known as Gunier-Preston (GP) zones. Today, they are regarded as true precipitates of a metastable coherent phase, obeying the laws of thermodynamic equilibrium, and are depicted as a metastable feature in phase diagrams.

The precipitation hardening hypothesis is now credited with insights into other phenomena, most particularly slip motions in crystals as presented in the slip interference theory [7]. The latter theory is acknowledged as the precursor to modern dislocation theory [8].

Fig. 1. Paul Merica, ca. 1932 (Reproduced with permission of the AIME).

Paul Dyer Merica had a rather remarkable career [9]. After attending DePauw University for three years, he went to the University of Wisconsin, earning an A. B degree in 1908. He then taught chemistry in China before receiving his Ph. D. in Metallurgy and Physics from the University of Berlin in 1914. He joined the U.S. Bureau of Standards that same year, holding the positions of research physicist, associate physicist, physicist, and metallurgist. In 1919 he joined the International Nickel Company, rising to become president and director from 1951 until his retirement in 1955. Throughout the course of his career, Merica received numerous awards, including the Franklin Institute Medal, the James Douglas Gold Medal, the Robert Franklin Mehl Award from the Minerals, Metals & Materials Society, the Fritz Medal, and, in 1942, he became a member of the National Academy of Sciences.

Prepared by Sam Coriell with reference to the historical account by John Cahn [5].

Bibliography

[1] P. D. Merica, R. G. Waltenberg, and H. Scott, Heat Treatment of Duralumin, Bull. Am. Inst. Min. Metall. Eng. 150, 913-949 (1919); also P. D. Merica, R. G. Waltenberg, and H. Scott, HeatTreatment of Duralumin, Sci. Pap. Bur. Stand. 15, 271-316 (1919).

[2] P. D. Merica, R. G. Waltenberg, and J. R. Freeman, Jr., Constitution and Metallography of Aluminum and Its Light Alloys with Copper and with Magnesium, Trans. Am. Inst. Min. Metall. Eng., Vol. LXIV, pp. 3-25 (1921); also P. D. Merica, R. G. Waltenberg, and J. R. Freeman, Jr., Constitution and Metallography of Aluminum and Its Light Alloys with Copper and with Magnesium, Sci. Pap. Bur. Stand. 15, 105-119 (1919).

[3] P. D. Merica, The Age-Hardening of Metals, Trans. Am. Inst. Min. Metall. Eng. 99, 13-54 (1932).

[4] R. F. Mehl, Introduction and Summary, in Precipitation from Solid Solution, R. F. Mehl, et al., American Society for Metals, Cleveland, OH (1959) pp. 1-5.

[5] J. W. Cahn, A Historical Perspective on the Utilization of Phase Diagrams for Precipitation Hardening, Bull. Alloy Phase Diagrams 4, 349-351 (1983).

[6] A. Wilm, Physikalisch-metallurgische Untersuchungen über magnesiumhaltige Aluminiumlegierungen, Metallurgie 8, 225227 (1911).

[7] Z. Jeffries and R. S. Archer, The Slip Interference Theory of the Hardening of Metals, Chem. Metall. Eng. 24, 1057-1067 (1921). [8] C. S. Smith, Dictionary of Scientific Biography, Vol. VII, Charles Scribner's Sons, New York (1973) pp. 92-93.

[9] Zay Jeffries, Paul Dyer Merica, in Biographical Memoirs, Vol. XXXIII, National Academy of Sciences, Columbia University Press, New York (1959) pp. 226-240.