PhD Thesis Colloquium: Mr. Dova Kalyan (25/09/26)
Thesis title:
Immiscible Solute Engineering and Precipitation Control for Developing High-Strength High-Conductivity Copper Alloys
Faculty advisor(s):
Prof. Surendra Kumar Makineni
When?
25th September, 2026 (Friday), 11:00 AM (India Standard Time)
Where
KI Vasu Auditorium, Department of Materials Engineering
Abstract
Copper and copper-based alloys are widely used in high-heat-flux applications such as rocket combustion-chamber liners, heat exchangers, and electrical contacts because of their excellent thermal and electrical conductivity. However, their relatively low strength and rapid softening at elevated temperatures limit their use under severe thermo-mechanical loading.
Alloying and microstructural engineering can improve strength, but solute atoms, dislocations, grain boundaries, and second-phase interfaces also increase electron scattering and reduce conductivity. Among the available strengthening mechanisms, precipitation strengthening is particularly attractive because it strengthens the Cu matrix while removing solute atoms from solid solution and thereby recovering conductivity. Its effectiveness, however, strongly depends on precipitate size, volume fraction, coherency, distribution, and thermal stability. Accordingly, significant efforts have been directed towards high-strength and high-conductivity (HSHC) Cu alloys such as Cu–Ag–Zr, Cu–Al₂O₃, and Cu–Cr–Nb. Nevertheless, achieving high strength, high conductivity, and microstructural stability at both room and elevated temperatures remains a major materials-design challenge. The present thesis addresses this strength–conductivity trade-off through compositional and microstructural engineering of Cu-based alloys, with the aim of developing HSHC alloys with mechanical and thermal properties comparable to or superior to existing commercial Cu alloys.
Accordingly, the thesis is structured around three alloy-development strategies based on Cu-Ag systems: (1) Cu-Ag-Al alloy, where Al addition is explored to modify the Ag-precipitation and hetero-phase interface stability; (2) Cu-Ag-Cr alloy, where Cr addition is employed to control the grain boundary mediated discontinuous precipitation and promote finer, more homogenous precipitation via Cr precipitation; (3) Cu-Ag-Cr-Ge alloys, where Ge addition is investigated to modify the chemistry and structure of the Cr-rich precipitates. Together, these alloy systems provide a systematic framework to understand how solute additions, interfacial segregation, and precipitation can be exploited to improve the strength-conductivity trade-off in HSHC Cu-based alloys.
In the first part, a Cu–2.2 at.% Ag alloy was investigated with the objective of achieving a favourable balance between mechanical strength and thermal conductivity. Ageing of the binary alloy resulted in two competing precipitation modes: homogeneous precipitation of fine Ag-rich precipitates within the Cu matrix and discontinuous precipitation (DP) originating from grain boundaries. The DP reaction produced coarse lamellar Ag-rich precipitates behind a migrating grain-boundary reaction front, leading to a heterogeneous microstructure and limiting the strengthening efficiency of the fine precipitate population. To alter this precipitation pathway, 1.5 at.% Al was introduced into the Cu–Ag alloy as a ternary addition.
Atomic-scale compositional analysis revealed preferential segregation of Al at the Ag-rich precipitate/Cu heterophane interfaces. This interfacial segregation reduced the effective lattice misfit from approximately 12.7% in the binary alloy to about 6.3% in the ternary alloy, thereby stabilising the Ag-rich precipitate/matrix interface and favouring a finer and more homogeneous precipitation response. Consequently, Al addition shifted the precipitation behaviour from a DP-dominated heterogeneous microstructure towards predominantly continuous precipitation within the Cu matrix. This microstructural transition increased the yield strength from approximately ~88 MPa in Cu–2.2Ag to ~151 MPa in Cu–2.2Ag–1.5Al.
The improvement in strength, however, was accompanied by a reduction in thermal conductivity from approximately 382 W m⁻¹ K⁻¹ to 273 W m⁻¹ K⁻¹, which is attributed primarily to residual Al retained in solid solution and the associated increase in electron scattering. Thus, the Cu–Ag–Al system demonstrates that atomic-scale solute segregation at heterophase interfaces can be exploited to modify interfacial misfit, redirect precipitation pathways, suppress discontinuous precipitation, and substantially enhance mechanical strength, although careful control of residual solute content remains essential for maintaining the strength–conductivity balance in HSHC Cu alloys.
In the second part, Cr was introduced into the Cu–1.5 at.% Ag alloy with the objective of suppressing grain-boundary-mediated discontinuous precipitation and achieving an improved balance between mechanical strength and conductivity. In the binary Cu–1.5Ag alloy, ageing resulted predominantly in coarse lamellar Ag-rich discontinuous precipitates originating from grain boundaries. In contrast, the Cu–1.5Ag–1.3Cr (at.%) alloy exhibited a transition towards a finer and more homogeneous distribution of continuously precipitated Ag-rich particles within the Cu matrix. High-resolution STEM revealed that the Ag-rich precipitates in the binary alloy were predominantly semi-coherent, whereas the ternary alloy contained coherent Ag-rich precipitates together with Cr-rich precipitates distributed both along grain boundaries and within the matrix. The FCC Ag-rich precipitates exhibited a cube-on-cube orientation relationship with the Cu matrix, while the BCC Cr-rich precipitates followed a Nishiyama–Wassermann orientation relationship with the FCC Cu matrix.
Atom probe tomography further revealed enrichment of both Ag and Cr at the grain boundaries, with the Cr concentration reaching approximately 4 at.%. The Cr-rich precipitates contained approximately 20–40 at.% Cr, indicating pronounced nanoscale chemical partitioning during ageing. The formation of Cr-rich precipitates at grain boundaries generated a Zener-pinning pressure of approximately 1.1 MPa, which restricted grain-boundary migration and thereby inhibited propagation of the discontinuous precipitation reaction. Hence, Cr addition modifies the precipitation pathway through the combined effects of grain-boundary segregation, Cr-rich precipitation, and grain-boundary pinning. This transforms the heterogeneous discontinuous precipitation observed in the binary alloy into a finer and more homogeneous precipitation response in the ternary alloy.
The change in precipitation behaviour resulted in a substantial enhancement in mechanical properties. In the peak-aged condition, the Cu–1.5Ag alloy exhibited a yield strength of approximately 85 MPa and an ultimate tensile strength of approximately 250 MPa, whereas the Cu–1.5Ag–1.3Cr alloy reached a yield strength of approximately 257 MPa and an ultimate tensile strength of approximately 418 MPa. The ternary alloy also exhibited a higher strain-hardening response, which is attributed to enhanced interactions between dislocations and the dual population of Ag-rich and Cr-rich precipitates. More importantly, the alloy retained substantial mechanical strength at elevated temperature, exhibiting a yield strength of approximately 200 MPa and an ultimate tensile strength of approximately 270 MPa at 400 °C, corresponding to retention of nearly 78% of its room-temperature yield strength.
Importantly, the significant improvement in mechanical strength was achieved with only a modest reduction in transport properties. In the peak-aged condition, the electrical conductivity decreased from approximately 94.7% IACS in Cu–1.5Ag to approximately 85.7% IACS in Cu–1.5Ag–1.3Cr, while the corresponding room-temperature thermal conductivity decreased from approximately 382 to 356 W m⁻¹ K⁻¹. The thermal conductivity of the ternary alloy remained nearly stable with increasing temperature and was approximately 358 W m⁻¹ K⁻¹ at 400 °C. This combination of approximately 200 MPa yield strength and 358 W m⁻¹ K⁻¹ thermal conductivity at 400 °C demonstrates that Cr addition provides substantial strengthening while preserving the intrinsically high heat-transport capability of the Cu matrix.
Hence, the Cu–Ag–Cr system demonstrates that grain-boundary chemistry and mobility can be deliberately controlled through Cr segregation and precipitation to suppress discontinuous precipitation and promote a finer, more stable precipitate distribution. The simultaneous contribution of coherent Ag-rich precipitates, Cr-rich precipitates, grain-boundary pinning, and reduced discontinuous precipitation enables a strong improvement in mechanical properties without severely compromising conductivity. The Cu–1.5Ag–1.3Cr alloy therefore represents a promising HSHC Cu alloy for high-heat-flux applications requiring simultaneous mechanical integrity and efficient heat dissipation at both room and elevated temperatures.
In the third part, 0.1 at% Ge was introduced into the Cu–1.5Ag–1.3Cr (at.%) alloy with the objective of improving the structural stability of Cr-rich precipitates while retaining a favourable combination of mechanical strength and thermal conductivity. In Cu–Cr-based alloys, Cr-rich precipitates can initially form with a metastable FCC structure and subsequently transform towards the equilibrium BCC structure during prolonged ageing. Atomic-scale compositional analysis of the Ge-containing alloy revealed preferential partitioning of Ge to the Cr-rich precipitates. After 10 h of ageing, the FCC Cr-rich precipitates contained approximately 10 at.% Cr and 1.5 at.% Ge, while prolonged ageing to 50 h resulted in substantial Cr enrichment together with approximately 2 at.% Ge. In contrast, the corresponding Ge-free alloy predominantly exhibited BCC Cr-rich precipitates after prolonged ageing. These observations indicate that Ge partitioning modifies the local chemistry of the Cr-rich phase and stabilises the metastable FCC configuration, thereby retarding its transformation towards the equilibrium BCC structure.
The stabilisation of FCC Cr-rich precipitates provides an additional route for retaining a fine strengthening microstructure during prolonged thermal exposure. Both FCC and BCC Cr-rich precipitates were observed in the Ge-containing alloy, with Ge partitioning to both phases, demonstrating that nanoscale solute redistribution strongly influences their structural evolution and stability. Thus, while Cr addition primarily controls discontinuous Ag precipitation through grain-boundary segregation, precipitation, and pinning, Ge introduces an additional level of microstructural control by modifying the chemistry and phase stability of the Cr-rich precipitates.
Importantly, this microstructural stability is accompanied by excellent mechanical and thermal properties. The peak-aged Cu–Ag–Cr–Ge alloy exhibited a yield strength of approximately 300 MPa and an ultimate tensile strength of approximately 450 MPa at room temperature. At 400 °C, the alloy retained a yield strength of approximately 250 MPa and an ultimate tensile strength of approximately 290 MPa, corresponding to retention of nearly 83% of the room-temperature yield strength. The alloy also maintained a high thermal conductivity of approximately 310 W m⁻¹ K⁻¹ at room temperature, increasing to approximately 335–340 W m⁻¹ K⁻¹ between 100 and 400 °C. This simultaneous retention of high mechanical strength and thermal conductivity demonstrates the effectiveness of Ge-assisted control of Cr-rich precipitate stability.
Hence, the Cu–1.5Ag–1.3Cr–0.1Ge (at.%) system demonstrates that nanoscale solute partitioning can be exploited to manipulate precipitate chemistry, crystal structure, and thermal stability. The combination of an approximately 250 MPa yield strength and ~338 W m⁻¹ K⁻¹ thermal conductivity at 400 °C highlights the potential of this alloy for HSHC applications that require simultaneous mechanical integrity and efficient heat dissipation at elevated temperatures.
Overall, this thesis demonstrates that the strength–conductivity trade-off in Cu-based HSHC alloys can be effectively addressed through controlled solute partitioning, interfacial segregation, and precipitation-pathway engineering. Together, these strategies enable Cu–Ag-based alloys to retain high strength and high thermal conductivity at both room and elevated temperatures, establishing interface and precipitate engineering as a promising route for high-heat-flux applications.