Refractory Metal Phases Research Articles

Strengthening nanocrystalline immiscible bimetallic composite by high-entropy effect

Immiscible bimetallic composites are a kind of transpiration cooling material with potential in high-temperature service. Aiming at boosting their load-bearing capacity, the refractory phase was replaced with the multi-principal refractory high-entropy phase in the present work. Bi-phase metallic nanocrystalline NbMoTaW–Cu composites were fabricated successfully by the powder metallurgy method. The average grain size of the NbMoTaW phase in the sintered composite was kept to be 15 nm. Two interfacial configurations of BCC/FCC and BCC/amorphous/FCC were found in the composite. Using atom probe tomography, notable compositional inter-diffusion between the immiscible metals was disclosed, and the thickness of the mutual diffusion layer in the NbMoTaW–Cu composite was 2.2 times that in the W–Cu composite. The mechanisms of entropy effect on the formation of amorphous configuration and interfacial mutual diffusion were explained based on thermodynamic calculations. The yield strength and Vickers hardness of the nanocrystalline NbMoTaW–Cu composite are 52% and 27% higher than those of the W–Cu counterpart, respectively. In addition, the NbMoTaW–Cu composite processes excellent resistance to high-temperature softening even at 900 °C. The improved mechanical properties were associated with solid solution strengthening of the refractory metal phase, as well as the constraint effect and strengthening of interface between the refractory metal and Cu phases. This work provides novel guidance for designing advanced immiscible metallic composites with excellent mechanical performance.

Surface Roughness Evolution to Identify Incubation Time for Hot Corrosion of Nickel-Base Superalloys: CMSX-4, CM247LC DS and IN6203DS at 550\xa0\xb0C

In the absence of protective scales, nickel-base superalloys have an extremely limited hot corrosion incubation period before increased rates of attack are experienced. This paper reports on the nickel-base superalloys: CMSX-4, CM247LC DS and IN6203DS subjected to 550 °C hot corrosion exposures of durations ranging from 0 to 800 h, during which none of the superalloys developed a fully protective scale. The aim of the research was to identify the incubation period of each superalloy and this was achieved by means of surface roughness evaluations. A metrology exercise was performed on the cross section of test specimens which produced Cartesian data points which were subsequently converted to Ra and Rz data. Statistical analysis of the results suggested the incubation period lasted approximately 400, 500 and 200 h, respectively, for each superalloy. It was concluded that refractory metal phases within the microstructure were associated with the relatively short IN6203DS incubation period. This paper demonstrates that monitoring the changes in surface roughness provides a plausible method to identify the transition from incubation to propagation when studying 550 °C hot corrosion attack.

Fracture and Fatigue of Niobium Silicide Alloys

AbstractThe fracture and fatigue behavior of refractory metal silicide alloys/composites is significantly affected by the mechanical behavior of the refractory metal phase. This paper reviews some of the balance of properties obtained in the alloys/composites based on the Nb-Si system. Since some of the alloy/composite properties are dominated by the behavior of the refractory metal phase, the paper begins with a review of data on monolithic Nb and its alloys. This is followed by presentation of results obtained on Nb-Si alloys/composites and a comparison to behavior of some other high temperature systems.

Internal stress superplasticity in the NiAl–Mo eutectic alloy

Internal stress superplasticity (ISS) in a NiAl–Mo based eutectic alloy was investigated by conducting thermal cycling creep tests. The alloy was annealed at high temperatures to spheroidize the refractory metal phase. Under thermal cycling creep conditions, the alloy exhibited characteristics of ISS, that is, at low stresses, the thermal cycling creep rates were much higher than the isothermal creep rates and the corresponding stress exponent was close to 1. These results were compared quantitatively with the predictions of a theoretical model of ISS. The experimental thermal cycling creep rates agreed with the predicted values within an error less than one order of magnitude. Superplastic elongation of >200% without fracture was attained during a thermal cycling tensile creep test.

Superplasticity of a multiphase Ni-25Al-25Cr intermetallic alloy

In the past decade significant improvement in high temperature strength, processing and design methodology for NiAl have been achieved. However, limited ductility and toughness as well as poor impact resistance continue to be critical issues which will impede near term production implementation. The most recent work on NiAl suggests that the room toughness of NiAl can be conveniently improved in-situ with refractory metal phases by directional solidification of pseudo-binary (e.g., NiAl-Cr) and -ternary(NiAl-Cr(Mo)) eutectic systems. Since it has been well established that the majority of eutectic alloys, such as Al-Cu, Al-Si and Al-Al{sub 3}Ni, display superplasticity under appropriate conditions, their enthusiasm for exploring whether the multiphase Ni-25Al-25Cr alloy, containing a ternary eutectic of {beta}Ni(Al,Cr) + {gamma}{prime}Ni{sub 3}(Al,Cr) is appropriate.

Nickel aluminide containing refractory-metal dispersoids: 1: Synthesis by reactive mechanical alloying

Nickel and aluminum powders were mechanically alloyed with 10 or 30 vol.% refractory metal powders (molybdenum or tungsten) for times up to 50 h. Intermetallic synthesis takes place after 10–20 h alloying, with concomitant dispersion of the inert refractory metal phase, resulting in an NiAl matrix containing fine refractory dispersoids. The reactive synthesis step is direct, without intermediate intermetallic products. In a separate processing route, the compound NiMo was first synthesized by hydrogen reduction of nickel oxide and molybdenum oxide. Subsequent reactive mechanical alloying of NiMo powders with metallic nickel and aluminum powders yielded the same phases as the direct route from elemental powders. However, the dispersion step is more efficient with brittle NiMo than with ductile molybdenum, leading to a finer molybdenum dispersion size in the reacted powders. All mechanically alloyed powders are stable on exposure for 1 h at 1000 °C in a reducing atmosphere, but oxidize rapidly in air between 425 °C and 1000 °C.

NiAl-based polyphase in situ composites in the NiAlTaX (X = Cr, Mo, or V) systems

Polyphase in situ composites were generated by directional solidification of ternary eutectics. This work was performed to discover if a balance of properties could be produced by combining the NiAl-Laves phase and the NiAl-refractory metal phase eutectics. The systems investigated were the Ni-Al-Ta-X (X = Cr, Mo, or V) alloys. Ternary eutectics were found in each of these systems and the eutectic composition, temperature, and morphology were determined. The ternary eutectic systems examined were the NiAl-NiAlTa-(Mo, Ta), NiAl-(Cr, Al) NiTa-Cr, and the NiAl-NiAlTa-V systems. Each eutectic consists of NiAl, a C14 Laves phase, and a refractory metal phase. Directional solidification was performed by containerless processing techniques in a levitation zone refiner to minimize alloy contamination. Room temperature fracture toughness of these materials was determined by a four-point bend test. Preliminary creep behavior was determined by compression tests at elevated temperatures, 1100-l400 K. Of the ternary eutectics, the one in the NiAl-Ta-Cr system was found to be the most promising. The fracture toughness of the NiAl-(Cr, Al)NiTa-Cr eutectic was intermediate between the values of the NiAl-NiAlTa eutectic and the NiAl-Cr eutectic. The creep strength of this ternary eutectic was similar to or greater than that of the NiAl-Cr eutectic.

Microstructural Effects on Ductile Phase Toughening of Nb-Nb Silicide Composites

AbstractIn the Nb-Si system, the terminal Nb5 Si3 phase and the Nb Si phase are virtually immiscible up to approximately 2033K. This system offers the potential of producing composites consisting of a ductile refractory metal phase and a strong intermetallic phase. In-situ composites containing different volume fractions of the ductile Nb phase were produced via vacuum arc-casting. Microhardness testing as well as smooth bend bar testing was conducted at temperatures ranging from 298K to 1673K in an attempt to determine microstructural effects on the yield strength and smooth bar fracture strength. Notched bend specimens were similarly tested to determine the effects of the ductile phase (i.e. Nb) on enhancing the notched bend toughness. It is shown that the Nb phase often behaves in a ductile manner during testing, thereby toughening the in-situ composite. The mechanism of toughening appears to be due to crack bridging.