Issue 039, June 29, 2022

Wade A. Jensen, Ph.D., Sr. Research Metallurgist

Solid-state phase transformations occur when the crystal structure is changed through a reconfiguration of its constituent elements. A variety of phase transformations are utilized in different alloy systems: pearlitic steels eutectoidally decompose into lamellar two-phase systems, Al alloys precipitate hard particles, and Ti alloys change from a ductile BCC crystal structure (β) to a harder HCP structure (α). DNI’s highly engineered alloys and processing techniques are similarly designed to utilize phase transformations of solid solutions¹. This tech note will further explore the phase transformations of Paliney® 7² and Paliney® 25³,⁴ which exhibit spinodal decomposition and ordering transformation, respectively.

The spinodal decomposition and ordering transformations both arise from the atomic rearrangement of homogenous, disordered solid solutions. These transformations are the result of opposite modes of nearest-neighbor interactions and enthalpies of mixing⁵. If the enthalpy of mixing is positive, then similar atomic bonds (A-A and B-B) prevail, and diffusion causes the separation of elements. This creates daughter phases with a similar orientation and crystal structure as the parent phase but with different compositions; this process is known as clustering. Diffusion causes like elements to group together and is the mechanism for spinodal decomposition. If the entropy of mixing is negative, then dissimilar bonds (A-B) prevail, and diffusion results in atoms alternating themselves in a way that engenders long-range order. This is called ordering and results in atoms occupying specific positions in the crystal lattice.

Spinodal Decomposition – Spinodal decomposition is distinct from precipitation, and is shown in Figure 1.a). Precipitates must grow larger than a critical size to remain stable, and this creates an energy barrier that impedes precipitation. The precipitate is always at the equilibrium composition of the daughter phase and it grows by depleting solute in the surrounding matrix; it is often described as having large compositional fluctuations but being limited spatially. As seen in Figure 1.b), spinodal decompositions begin as small compositional fluctuations that increase over time until two distinct phases appear. The homogeneous phase spontaneously decomposes into two compositionally distinct phases without altering the crystal structure of the parent phase. Unlike precipitation, there is no energy barrier, and the transformation occurs rapidly and globally. It should be noted that it can be nontrivial to distinguish between equilibrium microstructures for homogenously precipitated particles and spinodal decomposition.

Figure 1: Diagram depicting the change in composition and representative microstructure for a) precipitation and b) spinodal decomposition phase transformations.

Ordering Transformation – Paliney 25 takes advantage of an ordering transformation. This type of phase transformation often occurs at, or near, stoichiometric compositions and the daughter phase does not change the original composition or crystal orientation. The A1 crystal structure is a disordered FCC solid solution with atoms randomly distributed along lattice sites. Upon ordering, certain atoms are more likely to occupy specific lattice sites, as seen in Figure 2. The L10, and the L12 may look like they are FCC crystals, but symmetry has been broken and these crystal structures belong to the primitive cubic group.

Metallurgists at Deringer-Ney exploit phase transformations to produce high performing metal contacts and utilize materials science expertise to continue to supply the market with specialty noble metal alloys.

Figure 2: Modified Cu-Pd binary phase diagram depicting the locations of the A1, L10, and the L12 phases. This phase diagram was taken from ASM international, Diagram No. 104101.

References:

1. W.A. Jensen, Overview of Hardening Mechanisms, Deringer-Ney Tech Brief. (2022). https://www.deringerney.com/resource-library/.
2. D.F. Susan, Z. Ghanbari, P.G. Kotula, J.R. Michael, M.A. Rodriguez, Characterization of Continuous and Discontinuous Precipitation Phases in Pd-Rich Precious Metal Alloys, Metallurgical and Materials Transactions A. 45 (2014) 3755–3766. https://doi.org/10.1007/s11661-014-2334-x.
3. A.S. Klein, E.F. Smith, S. Viswanathan, Palladium-Based Alloys, 10,385,424 B2, 2019.
4. A.Y. Volkov, O.S. Novikova, B.D. Antonov, Formation of an Ordered Structure in the Cu–49 at % Pd Alloy, Inorganic Materials. 49 (2013) 43–48. https://doi.org/10.1134/S0020168512110167.
5. W.A. Soffa, D.E. Laughlin, Diffusional Phase Transformations in the Solid State, in: Physical Metallurgy, Elsevier, 2014: pp. 851–1020. https://doi.org/10.1016/B978-0-444-53770-6.00008-3.