Issue 042, Sept 8, 2022
Patrick K. Bowen, Ph.D., Director of R&D
The Deringer-Ney-made alloy Paliney 7 is a metallurgically complex material with excellent environmental resistance and spring characteristics. These attributes have made it an enduring favorite in sliding electrical contact applications where absolute reliability is required.
It is well-known that the alloy age hardens from both the annealed and cold worked tempers at temperatures between about 600 and 900°F. However, there are relatively few data detailing the structural transformations that underpin the aging reaction. A better understanding of this behavior was desired, and so a X-ray diffraction¹ study was conducted on rolled alloy coupons in the annealed, cold worked, and age hardened tempers.
Samples were mounted in two-part epoxy, wet ground, and polished to a mirror finish. They were then etched and re-polished with a cyanide- persulfate solution and 1 µm diamond, respectively, in several cycles intended to remove surface deformation. Samples were then interrogated with a Scintag XDS-2000 θ/θ diffractometer equipped with a copper target X-ray tube. Data were collected from 20 to 140° 2θ, a step size of 0.03°, and with a count time of 20 seconds per step.
Figure 1: θ/θ diffraction results for Paliney 7 in the age hardened, cold worked, and annealed tempers
Data acquired for all three conditions are presented side-by-side in Figure 1. Strong texture is observed in all conditions, consistent with prior stress-relief annealing cycles. The width of diffraction peaks in the annealed temper is also significantly lower than the other tempers, consistent with the lower dislocation density. The texture is indicative of body diagonal ({110}) texture, consistent with other cold-rolled, face centered cubic materials.
The diffraction peak at about 68° exhibited an intriguing broadening and splitting after age hardening, as illustrated in Figure 2. In this figure, the diffraction peaks are labeled with their corresponding Miller indices. As opposed to the annealed and cold worked tempers, the {220} peak from the age hardened Paliney 7 resembles a composite of three or more peaks, suggesting that several phases exist in the matrix after age hardening heat treatment. The centermost peak (solid line, Figure 2 inset) is consistent in location with the comparison tempers. The appearance of a {110} peak may be suggestive of superlattice diffraction, although no additional evidence of long-range ordering was found.
Figure 1: θ/θ diffraction results for Paliney 7 in the age hardened condition with corresponding Miller indices; the inset shows a close-up of the {220} composite peak indicative of one parent (solid line) and two child phases (broken lines)
Nelson-Riley error function analysis was performed to determine the lattice parameter. The cold worked Paliney 7 was calculated to have a single FCC lattice parameter of 3.91 Å. After age hardening, the constituent phases were calculated to have FCC lattice parameters of 3.98, 3.91, and 3.86 Å. The
reaction characterized as α→α1+α2+α is in good agreement with solid-state decomposition by the spinodal decomposition mechanism. This is consistent with the work by Don Susan et al.² at Sandia National Laboratory. Susan and coworkers reported a similar spinodal decomposition in Paliney 7 as determined by X-ray diffraction and scanning transmission electron microscopy. They posited that the decomposition was driven by low-temperature immiscibility of Ag- and Cu-rich solid solutions in the alloy.
Deringer-Ney applies knowledge of Paliney 7 and related noble metal alloys—combined with advanced characterization and applications experience—to solve even the most critical engineering challenges.
Acknowledgment:
Special thanks to Dr. Edward F. Laitila of Michigan Technological University who performed the experimental work and much of the data interpretation.
References:
- B. D. Cullity and S. R. Stock, Elements of X-Ray Diffraction, 3rd edition. Upper Saddle River, NJ: Pearson, 2001
- D. F. Susan, Z. Ghanbari, P. G. Kotula, J. R. Michael, and M. A. Rodriguez, “Characterization of Continuous and Discontinuous Precipitation Phases in Pd-Rich Precious Metal Alloys,” Metall and Mat
Trans A, vol. 45, no. 9, pp. 3755–3766, Aug. 2014, doi: 10.1007/s11661-014-2334-x.