Extended Data Fig. 2: Microstructure of the invar alloy fabricated with an additional pressure-free sintering step and bulk property assessment.

a, Comparison between the two temperature profiles. The one utilized in main text Fig. 2f is sketched in blue, and the additional pressure-free sintering step is added right after the isothermal holding period at 700 °C. To hinder grain growth, a fast heating rate of 300 °C/min is applied to achieve 900 °C. b, IPF map showing the grain morphology after pressure-free sintering and the average grain size (~1.15 μm) is obtained by excluding the Σ3 annealing twin boundaries. c, Phase map validating the single-phase FCC microstructure where no detectable BCC or residual oxide phase is present. a and b are both overlapped with image quality maps and datum points with confidence index small than 0.1 have been removed. d, EDS maps taken across multiple grains validating the uniform distribution of Fe and Ni, and their contents are consistent with the designed values. e, Vickers hardness measurements of the bulk green invar alloys synthesized directly in one step from oxides (left and middle micrographs) and the one fabricated using the convention melting-casting-recrystallization method (right micrograph). The highest Vickers hardness of 226.6 ± 1.6 HV_100gf is seen in the green invar alloy synthesized with an additional pressure-free sintering step, which is ~1.5 times the hardness of the coarse-grain invar alloy fabricated using the conventional melting-casting-recrystallization route (138.0 ± 3.2 HV_100gf). Despite the presence of excessive porosity in the as-reduced green invar alloy (~17.4 %), no cracking event is observed at the corners of the indent, and its hardness reaches 153.7 ± 4.2 HV_100gf, revealing its load-bearing capacity, which may even serve to inspire meso-porous bulk invar foam design as a future work avenue.