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Probing deeper into the mysteries of the atom, Dr. L. R. Van Wert of the Graduate School of Engineering has devised a new method of investigating the atomic structure of metal alloys. The effects of pressures as high as 20,000 atmospheres on the speed with which certain alloys undergo agehardening are studied in his process.
Dr. Van Wert's experiments in the High Pressure Laboratories of the Jefferson Physics Laboratory, Harvard University, disclosed that pressures of 12,000 atmospheres applied to the alloys during the process of age-hardening measureably decreased the rate of aging of five age-hardenable alloys which he tested.
The final hardness was not affected by pressure; only the time necessary to attain this hardness was changed, the experiments showed. Alloys initially but partially aged under high pressures showed no irregularities when the aging was concluded under normal pressures.
During the age-hardening process in an alloy there is a migration of atoms of the alloying metal to strategic points in the atomic lattice work of the basic metal. The rate of aging depends on how rapidly these solute atoms can congregate at these positions, or diffuse.
Dr. Van Wert states, "The accelerating effect of temperature on age-hardening is assumedly the result of an increase in the diffusion rate with increase in temperature. Is it not conceivable that pressure's decelerating effect on aging comes about through interference with the diffusion process? High hydrostatic pressures, as we know, compress the metal lattice, in this case, the solvent metal lattice; conceivably the 'viscosity' of, and the difficulty of atomic movement within, the solid solution is proportionately increased, Diffusion then becomes slower and the progress of age-hardening retarded."
Six alloys were subjected to the pressure tests. They were commercial duraluminum, three aluminum alloys, a lead-calcium alloy, and an iron nitrogen alloy. Each alloy was submitted to the proper temperature, for the time necessary to insure complete solution of the precipitant. It was then quenched in water. The hardness was immediately taken. The alloy was permitted to age at room temperature, one series under high pressures, and a second series at ordinary pressures. After aging for definite time periods, hardness tests were again made.
The effects of pressure on the age-hardening period were most marked in the lead alloy; the aluminum alloys came next; while the iron nitrogen alloy was not affected even when pressure was run up to 20,000 atmospheres.
Since lead is the most compressible of the three metals, and iron the least, Dr. Wert notes that this relative compressibility might be thought to explain the nature of his results although "one would hardly anticipate from their comparative values that the lead alloy would be many times as sensitive to pressure effects as are the aluminum alloys or that the iron nitrogen alloy would show,--if, indeed, it shows anything,--a sensitivity so small as to escape detection by the usual Rockwell hardness tester."
He states, however, that the iron alloy is a different kind of solution from the aluminum and lead alloys, and that the difference in solution structure might explain the conduct of the iron alloy under pressure.
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