Standards & Properties: Mechanical Properties of Copper and Copper Alloys at Low Temperatures

Publist#: 144/8

Copper alloys become stronger and more ductile as temperature goes down. They also retain excellent impact resistance to 20 K. These general characteristics have been revealed in tests on 15 copper alloys, including brasses, bronzes and commercially pure coppers. They were tested by the Cryogenics Div., National Bureau of Standards, for the copper and brass industry to check tensile strength, notch tensile strength, Youngs modulus, and impact properties at temperatures down to 4 K (-454 F). The information presented here is based on a report by R. P. Reed and R. P. Mikesell "Low Temperature Mechanical Properties Of Copper and Selected Copper Alloys," NBS Monograph 101, Institute for Materials Research, National Bureau of Standards, Boulder, Colorado 80302.

Tests — The compositions and condition of these alloys — that is, for standard, commercially available mill stock — are listed in Table 1. Tensile specimens with 1.5 inch gauge lengths provided the data on elastic modulus. Notch tension (also 1.5 inch gauge lengths) and tension tests were conducted using the cryostat and related equipment. Tests were conducted at 295 (ambient), 195, 76, 20, and 4 K. An Instron testing machine was used with a crosshead speed of 0.02 in. per min. Test procedures are described by R.A. Warren and R.P. Reed in NBS Monograph 63.

Results — The figures on the following pages present stress-strain behavior. In Table 2 the average data for each alloy are tabulated for all tests and temperatures. Data spread in most instances was less than ± 1%.

Considerable spread in the impact strength occurred for Copper 102 in the 60% cold-drawn condition. This was related to grain size. Metallographic examination revealed that the specimens with high impact strengths (113 to 115 ft-lb) had small grains while those with low impact strength (57 to 84 ft-lb) had large grains.

Aged Copper Alloy No. 647, although containing only about 2.5 wt. % alloying elements, proved to be considerably stronger than any other alloy tested. Its impact strength remains high, and the notch tensile strength, although falling off at 4 K, is good when compared to the tensile strength. In addition, elongation increases at low temperatures.

Except for the sand cast nickel-aluminum-bronze alloy, all alloys had notched-to-unnotched tensile ratios above one, and good or very good impact properties. Only the sand cast alloy was brittle at low temperature. For most alloys the tensile strength, yield strength, elongation and notch tensile strength increased in the temperature range from 295 to 20 K. Ultimate and yield strengths of most alloys are less at 4 K than at 20 K. Discontinuous yielding is evident in all stress-strain curves at 4 K.