Nicolas Cayetano-Castro

Precipitation Process in Fe-Ni-Al-based Alloys

This chapter covers first the precipitation and coarsening processes in Fe-Ni-Al alloys aged artificially at high temperatures, as well as their effect on the mechanical properties. These results show the precipitation evolution, morphology of precipi‐ tates, coarsening kinetics and mechanical properties such as hardness. Additionally, the effect of alloying elements such as copper and chromium is also studied on the precipitation and coarsening processes. The main results of this section are concerning on the coarsening kinetics and its effect on hardness. Besides, the diffusion couple method is employed to study the precipitation and coarsening process in different FeNi-Al alloy compositions, as well as its effect on the hardness. All the above aspects of precipitation and coarsening are also supported with Thermo-Calc calculations.

Analysis of Phase Decomposition in Fe‐Cr Alloys by Diffusion Couples

Diffusion-couples of pure Fe/Fe-40at.%Cr alloy were used to analyze the phase decomposition process in Fe-Cr alloys. Diffusion-couples were solution treated at 1050 °C for 1 h and subsequently aged at 475 and 500 °C for 1–1000 h. Interdiffusion zones were analyzed by HR-TEM and hardness measurements. The EDS-SEM analysis of solution treated, water-quenched and then aged diffusion-couple permitted to obtain the equilibrium composition of miscibility gap at 500 °C to be 14 at. % Cr. The HR-TEM analysis of the aged diffusion-couple showed the change in size and volume percentages of precipitates with composition. The hardness measurements of the aged diffusion-couple indicated the age hardening due to the presence of nanometric Cr-rich precipitates in a Fe-rich matrix.

Ostwald Ripening Process of Coherent β′ Precipitates during Aging in Fe0.75Ni0.10Al0.15 and Fe0.74Ni0.10Al0.15Cr0.01 Alloys

The Ostwald ripening process was studied in Fe0.75Ni0.10Al0.15 and Fe0.74Ni0.10Al0.15Cr0.01 alloys after aging at 750, 850, and 950°C for different times. The microstructural evolution shows a rounded cube morphology (Fe, Ni)Al β′ precipitates aligned in the ferrite matrix, which changes to elongated plates after prolonged aging. The variation of the equivalent radii of precipitates with time follows the modified Lifshitz-Slyozov-Wagner theory for diffusion-controlled coarsening. Thermo-Calc analysis shows that the chromium content is richer in the matrix than in the precipitates which causes higher hardness and coarsening resistance in the aged Fe0.74Ni0.10Al0.15Cr0.01 alloy.