A review on thermal expansion control in Fe–Ni–Cr austenitic alloys: From Invar effect to advanced power systems

The reliable operation of ultra-supercritical (USC) and advanced ultra-supercritical (A-USC) power plants depends on the performance of thick-walled components under severe thermal and mechanical conditions. Austenitic steels, while offering an attractive balance between cost and high-temperature strength, are constrained by their high coefficient of thermal expansion (CTE), which accelerates thermal fatigue and complicates integration with ferritic components. This review critically evaluates recent advances in the design of low-CTE Fe–Ni–Cr austenitic alloys. Two principal strategies are examined: (1) conventional alloying and precipitation engineering, such as Ni, W, and Mo additions and γ ′ phase control, to suppress lattice expansion in paramagnetic alloys; and (2) compositional tuning to exploit the Invar effect, particularly through Ni optimisation, to achieve intrinsically low CTE. Key challenges include maintaining high-temperature mechanical integrity, ensuring oxidation resistance, and mitigating CTE mismatch at dissimilar metal joints. Emerging approaches, including Co–Cr balanced designs, Ni/Co ratio optimisation, and high-Al three-phase architectures, demonstrate promising pathways for achieving multi-property optimisation. The review concludes with perspectives on integrating computational thermodynamics and machine learning to accelerate alloy design, enabling simultaneous control of CTE, creep strength, oxidation resistance, and cost for next-generation power plant applications.