Qianying Guo

Coarsening behavior of MX carbonitrides in type 347H heat-resistant austenitic steel during thermal aging

In this work, the growth kinetics of MX (M = metal, X = C/N) nanoprecipitates in type 347H austenitic steel was systematically studied. To investigate the coarsening behavior and the growth mechanism of MX carbonitrides during long-term aging, experiments were performed at 700, 800, 850, and 900°C for different periods (1, 24, 70, and 100 h). The precipitation behavior of carbonitrides in specimens subjected to various aging conditions was explored using carbon replicas and transmission electron microscopy (TEM) observations. The corresponding sizes of MX carbonitrides were measured. The results demonstrates that MX carbonitrides precipitate in type 347H austenitic steel as Nb(C,N). The coarsening rate constant is time-independent; however, an increase in aging temperature results in an increase in coarsening rate of Nb(C,N). The coarsening process was analyzed according to the calculated diffusion activation energy of Nb(C,N). When the aging temperature was 800–900°C, the mean activation energy was 294 kJ·mol−1, and the coarsening behavior was controlled primarily by the diffusion of Nb atoms.

Effect of cold rolling and first precipitates on the coarsening behavior of γ″-phases in Inconel 718 alloy

The coarsening behaviors of γ″-phase particles in Inconel 718 alloy aged at 750, 800, and 850°C were investigated by scanning electron microscopy (SEM). Detailed observations and quantitative measurements were conducted to characterize the coarsening behavior of the γ?-phase under various aging conditions. The experimental results indicate that the existence of the δ-phase retards the formation and coarsening of the γ″-phase, without influencing its final particle size or amount. Moreover, when cold rolled with a reduction of 50%, the dimensions of the γ″ particles in Inconel 718 alloy decrease with increasing aging time. Furthermore, the coarsening behavior of the γ″-phase in the Inconel 718 alloy after a normal aging treatment (sample A) and that of the primary δ-phase (sample B) follow the Lifshitz–Slyozov–Wagner (LSW) diffusion-controlled growth theory; the thus-obtained activation energies for the γ″-phase are 292 kJ·mol-1 and 302 kJ·mol-1, respectively.

Microstructure Refinement in W-Y2O3 Alloy Fabricated by Wet Chemical Method with Surfactant Addition and Subsequent Spark Plasma Sintering

With the aim of preparing high performance oxide-dispersion-strengthened tungsten based alloys by powder metallurgy, the W-Y2O3 composite nanopowder precursor was fabricated by an improved wet chemical method with anion surfactant sodium dodecyl sulfate (SDS) addition. It is found that the employment of SDS can dramatically decrease W grain size (about 40 nm) and improve the size uniformity. What’s more, SDS addition can also remarkably improve the uniform dispersion of Y2O3 particles during the synthesis process. For the alloy whose powder precursor was fabricated by traditional wet chemical method without SDS addition, only a few Y2O3 dispersoids with size of approximate 10–50 nm distribute unevenly within tungsten grains. Nevertheless, for the sintered alloy whose powder precursor was produced by improved wet chemical method, the Y2O3 dispersoids (about 2–10 nm in size) with near spherical shape are dispersed well within tungsten grains. Additionally, compared with the former, the alloy possesses smaller grain size (approximate 700 nm) and higher relative density (99.00%). And a Vickers microhardness value up to 600 Hv was also obtained for this alloy. Based on these results, the employment of SDS in traditional wet chemical method is a feasible way to fabricate high performance yttria-dispersion-strengthened tungsten based alloys.

Effects of morphology of Mg powder precursor on phase formation and superconducting properties of Mg11B2 low activation superconductor

The effect of morphology with magnesium (Mg) powder on phase formation and critical current density (Jc,) of sintered Mg11B2 bulk was studied. As a precursor for the formation of Mg11B2, the morphology of spherical and plate-like of Mg powder were separately synthesized using atomization and chemical reduction methods. From differential thermal analyses of the sintering process, the solid–solid reaction of a Mg11B2 sample using spherical Mg powder was confirmed to be much weaker than the plate-like one, and the second exothermic peak place was shifted back and its intensity was increased due to a special microstructure of spherical Mg powder. XRD analysis of Mg11B2 samples showed that the volume fraction of Mg11B2 in the sintered sample with plate-like Mg was higher than that with the spherical one, and for all the samples MgB2 is the main phase. The Mg11B2 sample using spherical Mg powder had two different kinds of microstructures because of differences in their diffusivity and their contact interfaces. The Jc value of an Mg11B2 sample using spherical Mg powder enhanced the entire field; it was twice as high as that using plate-like Mg, and it was even better than all the Jc values reported in previous literature using the same kind of Mg powder as precursor.