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The precipitation behavior of different phases in a high-silicon stainless steel (6 wt% Si) during aging at 600–1050 °C for 24 h was investigated. The morphology, crystal structure and composition of various precipitates were detailly characterized using optical microscopy, scanning electron microscopy and transmission electron microscopy. Four phases were mainly identified: χ-phase, M6C carbides, σ phase and a new type of fcc-phase. During aging at 600–900 °C, the main precipitate was (Cr, Mo and Si)-rich χ-phase which was directly precipitated from γ matrix. The χ-phase was calibrated as bcc structure with a lattice parameter of 8.90 A. The peak temperature for the precipitation of χ-phase was 800 °C, and it was dissolved when aging at temperatures above 1000 °C. The σ-phase was observed only at 700 °C and grew next to χ-phase. Above 700 °C, a new fcc-phase was found to be precipitated along with χ-phase, with a space group of Fd3c and a lattice parameter of 12.56 A. The M6C carbides started to be precipitated at 700 °C in the vicinity of χ-phase. And its amount basically increased with the increasing of temperature. An orientation relationship between M6C/γ was found: [100]c//[100]γ, (001)c//(001)γ, i.e., the cube-on-cube relationship.

Amorphous Al2O3-reinforced Al composite (am-Al2O3/Al) compacted from ultrafine Al powders for high-temperature usages confronts with drawbacks because crystallization of am-Al2O3 at high temperatures will result in serious strength loss. Aiming at this unsolved problem, in this study, high-temperature Al materials with enhanced thermal stability were developed through introducing more thermally stable nano-sized particles via high-temperature pre-treatment of ultrafine Al powders. It was found that the pre-treatment at ≤ 550 °C could introduce a few Al2O3 in the Al matrix and increase the strength of the composites, but the strength was still below that of am-Al2O3/Al because without being pinned firmly, grain boundaries (GBs) were softened at high temperature and intergranular fracture happened. When the pre-treatment was carried out at 600 °C, nitridation and oxidation processes happened simultaneously, producing large numbers of intergranular (AlN + γ-Al2O3) particles. GB sliding and intergranular fracture were suppressed; therefore, higher strength than that of am-Al2O3/Al was realized. Furthermore, the (AlN + γ-Al2O3)/Al exhibited more superior thermal stability compared to am-Al2O3/Al for annealing treatment at 580 °C for 8 h. Therefore, an effective way to fabricate high-temperature Al composite with enhanced thermal stability was developed in this study.