Effects of Nb Content on the Mechanical Alloying Behavior and Sintered Microstructure of Mo-Nb-Si-B Alloys

Abstract

Different from conventional Mo-Si-B-based alloys consisting of Moss, Mo 3 Si, and Mo 5 SiB 2 , Mo 3 Si-free Mo-Si-B-based alloys (Moss+Mo 5 Si 3 +Mo 5 SiB 2 or Moss+Mo 5 SiB 2 ) show great potentials for more excellent oxidation resistance and elevated temperature strength. In the present work, alloying element Nb was added to Mo-12Si-10B (at.%)-based alloy to suppress the formation of the Mo 3 Si phase. 20,22,24,26, 28, 30, and 40) bulk alloys were fabricated using mechanical alloying followed by cold pressing and then sintering at 1773 K for 2 h. Effects of Nb content on the mechanical alloying behavior and then sintered microstructure were studied. The addition of Nb with an amount less than 30 at.% accelerated the mechanical alloying process, but 40 at.% Nb addition decreased the process due to excessive cold welding and high powder volume. For the sintered bulk alloy prepared from the mechanically alloyed powders milled for 30 h, a critical Nb content between 24 and 26 at.% was found to suppress Mo 3 Si production and γNb 5 Si 3 phase formed in the alloys with the addition of Nb content more than 26 at.%. Prolongation of a prior milling process could facilitate the suppression of Mo 3 Si and delay the formation of niobium silicides.Metals 2019, 9, 653 2 of 15 are given in at.% unless otherwise stated) fabricated using an arc cast and directional solidification. A Moss+T1+T2 microstructure was achieved successfully. However, determined by their solidification paths, a large size T2 primary phase is inevitable in these alloys prepared via a smelting process [13,14]. The formation of this large size T2 primary phase is certainly detrimental to the fracture toughness of the alloys. Mechanical alloying (MA) followed by a sintering process facilitates the fabrication of Mo-Si-B based alloy with a continuous Moss matrix [15][16][17]. Byun et al. [18] fabricated Mo-16.9Nb-3Si-1B (wt.%) alloys using a powder metallurgical method. In their research, the Nb-containing alloy showed lower hardness (425 vs. 590 Hv) and higher fracture toughness (14.5 vs. 12.6 MPa·m 1/2 ) than those of a Nb-free alloy. It is worth noting that this Nb content is not enough to destabilize Mo 3 Si and the alloy still consists of Moss, Mo 3 Si, and Mo 5 SiB 2 . In order to suppress the Mo 3 Si phase, plenty of added Nb is necessary, which will certainly affect the MA behavior of Mo-Nb-Si-B powder mixtures. Different from the solidification process, the constituent elements in the sintering process show a lower diffusion ability. As a result, the distribution of elemental Nb in different phases in the sintered alloys may be different from that in the solidified alloys, which will affect the alloy phase constituents and the critical content of Nb addition for suppressing the Mo 3 Si phase. Besides, the prior MA degree of powder mixtures can also affect the microstructure of the sintered alloy. Therefore, the present article will focus on the effects of Nb content on the MA behavior and sintered microstructure of quaternary Mo-Nb-Si-B alloys to...

Review of Research Progress on Mo–Si–B Alloys

“…The impact of the inclusion of Nb on the characteristics of the Mo-Si-B alloys, therefore, continues to be of importance, and the main concern is to identify the minimum amount of Nb added that can effectively suppress the A15 phase. Based on research, Yang suggested that the optimal range for the critical value of Mo-12Si-10B alloy lies between 24 at.% and 26 at.% [62]. The Mo 3 Si phase was absent in the alloy containing 26 at.% of Nb.…”