Microstructure and Mechanical Properties of Directionally Solidified Nb‐22Ti‐16Si‐7Cr‐3Al‐3Ta‐2Hf‐0.1Ho Alloy

Abstract

Compared to commercial Ni-based superalloy, Niobium silicides possess higher melting point, relatively lower density and excellent high-temperature strength. The attractive combination of properties makes Niobium silicides one of the promising candidates of the next generation materials for high-temperature structural applications. [1][2][3] However, the low ductility and toughness of Niobium silicides at room temperature have limited its practical applications as a structural material. Therefore, it is necessary to simultaneously improve toughness and high-temperature strength of this material to meet the requirement of engineering application. Directional solidification and alloying are usual approaches to achieve these goals. Previous studies showed that one approach to achieving ductility in Ni 3 Al was to directionally solidify samples to eliminate transverse boundaries. [4,5] It was also reported that directional solidification simultaneously improved room temperature toughness and high temperature strength of the Nb-17.5Si and Nb-10Ti-17.5Si. [6] In addition, our previous study showed that the addition of Ho to Nb22Ti-16Si-7Cr-3Al-3Ta-2Hf alloy increased its strength and ductility at the same time. [7] But the ductility of the alloy has not met requirement of practical application. To improve further the strength and ductility of Nbss/Nb 5 Si 3 composites, we combined the above-mentioned two benefits into one system. The aim of the paper is to study microstructure and mechanical properties of directionally solidified Nb-22Ti-16Si-7Cr-3Al-3Ta-2Hf alloy with 0.1 at.% Ho. ExperimentalThe master alloy with the nominal composition Nb-22Ti-16Si-7Cr-3Al-3Ta-2Hf-0.1Ho (at.%) was prepared by arcmelted using a non-consumable tungsten electrode under an argon atmosphere in a water-cooled copper crucible from starting materials of high pure Nb, Ti, Si, Cr, Al, Ta, Hf and Ho. The alloy button was remelted at least five times to ensure a homogeneous specimen. The master rods used for directional solidification were cut from the button by electrodischarge machining (EDM). The dimensions of the rods are 8 mm in diameter and 70 mm in length. Directional solidification experiments were performed in an optical floating zone (OFZ) furnace at the growth rates of 9 mm/h under an Ar gas flowing atmosphere. Microstructural analysis was performed by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). Mechanical properties of the alloy were evaluated by fracture toughness measurement and high-temperature compression tests. Three-point bending tests were performed using single notched specimens with the dimensions 3, 6 and 30 mm in thickness, width and length, respectively. A single notch with about a 3 mm length was introduced at half of the length by EDM with 0.1 mm diameter wire, while not pre-cracked. The tests were executed with a span of 24 mm and at an initial crosshead speed of 0.5 mm/min in air. After the tests, the fracture surfaces and side planes were examined by SEM. The compression specim...

On Nb Silicide Based Alloys: Alloy Design and Selection

“… Macrosegregation of Si (MACSi, at.%) (ordinate) in Nb-silicide based alloys versus (abscissa) the ΔH m sd , see [ 12 ]. …”

“…The most recent research on Nb-silicide based alloys has mainly focused on “forth” generation alloys, which have evolved from Nb-Ti-Si-Al-Cr alloys [ 9 ], MASC (Metal And Silicide Composite) type [ 1 ] Nb-Ti-Si-Al-Cr-Hf-(Sn, Ge) alloys, Nb-Si-TM-RM alloys to Nb-Si-TM-RM-X alloys (TM = Cr, Hf, Ti, Zr, RM = Mo, Ta, V, W, and X = Al, B, Ge, Sn, and exotic addition(s) [ 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 ]). To date, almost all the alloys have been studied in their cast and heat-treated conditions.…”

Microstructural characteristics and high temperature compressive properties at 1623K of a directionally solidified Nb-silisides based in-situ composite

“…Back-scattered electron images of Nb-22Ti-16Si-7Cr-3Al-3Ta-2Hf-0.1Ho (a and b) the CC alloy; (c and d) transverse and longitudinal sections of the DS alloy; (b) is higher magnification image in (a). The growth direction is indicated with a white arrow in (d) [14]. seen in Fig.…”

“…1. As reported previously [14], the microstructures of the CC and DS alloys were composed of gray-white (Nb, Ti)ss matrix, gray (Nb, Ti) 5 Si 3 , black (Ti, Nb) 5 Si 3 and a small amount of white Ho 2 Hf 2 O 7 . From Fig.…”