Issue 033, April 25, 2022
Patrick K. Bowen, Ph.D., Director of R&D
Deringer-Ney’s trade-named alloy Ney 75 is a silver-based material commonly used in sliding electrical contact systems. The alloy comprises 75 wt.% silver, 24.5% copper, and 0.5% Ni, and is governed by ASTM International standard B780. Owing to its high bulk conductivity, it is commonly applied in systems designed to conduct power-level currents through bundled fiber brushes.
One drawback of many silver-based alloys is a susceptibility to gradual deformation at elevated temperature when under an applied stress less than the material’s yield strength. In electrical contacts, this is commonly described as a stress relaxation phenomenon.¹ The consequences of stress relaxation are different from classical creep. The first difference is that creep is defined by transient and steady state regions, while stress relaxation is not. Additionally, classical creep leads to rupture of a component, while stress relaxation only results in a loss of contact load (“gram force”) and associated change in free height; not fracture of the component.
A study of Ney 75 stress relaxation utilized 0.010 in (0.254 mm) diameter wires in the heat straightened (HS) condition, which had 89.5 ksi (617 MPa) ultimate tensile strength and 83.9 ksi (578 MPa) 0.2% offset yield strength (YS). The standard modulus of the alloy was taken to be 13.3 Msi (91.7 GPa). Wires were placed into milled, circular pockets that were sized to impose a maximum bending fiber stress equivalent to 70, 75, 80, 85, and 90% off the yield strength. The elastically deformed wires were then aged at either room temperature (25°C), or elevated temperatures of 80, 100, or 125°C, and removed from the milled pockets after thermal insult. The radius of curvature was measured and used to calculate the magnitude of stress relaxation that had occurred.
Tabular stress relaxation results are presented in Table 1. It is observed from the data in this table that variations in temperature and time have a disproportionately large effect on stress relaxation over the maximum fiber stress range of 70-90% YS. In contrast, fiber stress in this range accounts for no more than about 6% variation in the observed total.
Table 1: Ney 75 HS wire stress relaxation, expressed as the lost % of originally applied stress, shown for selected temperatures, exposure times, and imposed stresses
A calculation of an annealing index, Φ, after Wright², was used to combine these two variables using the
following relationship:
Where
t is time, in seconds
k is an empirically derived constant, ~9.95 × 10³ for this experiment
T is temperature, in Kelvin
Stress relaxation is presented as a function of the annealing constant in Figure 1. The line of best fit has the form of an exponential growth curve, with the stress relaxation of Ney 75 HS wire described by:
From this empirical relationship, an approximate Ney 75 HS stress relaxation can be estimated for any combination of time and temperature within the bounds of this experiment. The uncertainty that arises from neglecting magnitude of fiber stress is estimated to be about ± 3% about mean behavior, with fiber stresses at 90% of YS erring to the higher, + 3% stress relaxation bound.
Stress remaining in Ney 75 HS wire were calculated using the above method at all conditions within the bounds of this study (i.e. temperatures and times between 50 and 125°C and 1 and 300 h, respectively). The calculated results were used to create a stress relaxation “map” in the style of Pitney¹, shown in Figure 2. Knowledge of the time-temperature dependence of Ney 75 HS on stress relaxation is crucial for its safe and effective application in challenging environments. This Ag-Cu-Ni alloy is provided by Deringer-Ney in critical sliding contact applications across myriad markets and applications.
References:
- K. E. Pitney, Ney Contact Manual: Electrical Contacts for Low Energy Uses, Revised 1st Edition. Bloomfield, Connecticut: Deringer-Ney Inc., 2022.
- R. N. Wright, “Chapter Thirteen – Relevant Aspects of Copper and Copper Alloy Metallurgy,” in Wire Technology, R. N. Wright, Ed. Oxford: Butterworth-Heinemann, 2011, pp. 175–197. doi: 10.1016/B978-0-12-382092-1.00013-0