T. C. Chou

Mechanism of MoSi2 pest during low temperature oxidation

The pest of monolithic poly- and single-crystalline molybdenum disilicide, as well as its composites, has been investigated by low temperature oxidation in air at temperatures ranging from 350 to 700 °C. Pest phenomenon (i.e., disintegration from bulk into powders) was consistently observed from samples oxidized at temperatures between 375 and 500 °C. In contrast, samples oxidized at 550 °C exhibited only severe cracking, and samples oxidized at 350 °C or at/above 600 °C were intact. The pested samples resulted in powdery products consisting of MoO 3 whiskers, SiO 2 clusters, and residual MoSi 2 . The MoO 3 whiskers exhibited protruding characteristics, and were highly concentrated at microstructural heterogeneous sites, such as interparticle boundaries, grain boundaries, and cracks. Blisters were observed and found to be formed predominantly on the grain boundary and interparticle boundary interfaces of tested samples. These blisters were often found to be erupted, indicating that significant vapor pressure was built up underneath the sample surface. Interrupted oxidation tests at 500 °C revealed that a substantial volume change also occurred. The development of the pest of MoSi 2 was found, macroscopically, to consist of nucleation and growth. In most cases, it initiated from some local microstructural inhomogeneities, and propagated throughout the samples. Based on the morphologies of disintegrated powders, two different kinetic processes were identified to be responsible for the pest reaction. According to the results from single crystals, the pest of MoSi 2 appeared to be kinetically controlled by the formation and volatilization of MoO 3 . The mechanism of MoSi 2 pest is proposed and discussed. Thermodynamic analyses are also presented to substantiate the experimental observations.

PESTING OF THE HIGH-TEMPERATURE INTERMETALLIC MOSI2

Degradation resulting from environmental effects on the properties of high-temperature intermetallics has recently stimulated much interest in the materials science community. Most notably, iron, nickel, and titanium aluminides were found to be more ductile at room temperature when tested in vacuum or dry oxygen as compared to laboratory air. Environmental oxidation can also degrade materials to a measurable, sometimes catastrophic, extent. For example, an important oxidation-induced degradation phenomenon observed in intermetallics is pest disintegration. It was first observed in molybdenum disilicide in 1955. Since then, pest disintegration has been reported in many intermetallics, including silicides, aluminides, and beryllides. This article examines the pesting of MoSi2 and presents kinetic processes responsible for pesting.

Kinetics of MoSi2 pest during low-temperature oxidation

The kinetics of MoSi[sub 2] pest, caused by oxidation in air, has been studied. Experimental results indicated that pest disintegration occurred at temperatures between 375 and 500 [degree]C. The volumes of test samples increased with oxidation duration. Analysis of change in sample volume versus oxidation duration revealed that the pest disintegration consisted of two stages, namely nucleation (or incubation) and growth.The onset of growth stage depended on the test temperature. More importantly, changes in sample volume were found to obey a linear relationship with time during the growth stage. Equations were formulated to demonstrate that the growth kinetics of pest disintegration was proportional to the rates of change in sample volume. The rates of volume change during MoSi[sub 2] pest were calculated to be 4.9[times]10[sup [minus]6], 2.8[times]10[sup [minus]5], 3.7[times]10[sup [minus]5], and 5.4[times]10[sup [minus]5] cm[sup 3]/s at 375, 400, 425, and 450 [degree]C, respectively; the growth kinetics increased with oxidation temperature. The activation energy for the growth stage of pest disintegration was determined to be 27.6 kcal/mole, which agrees well with the activation energy for the low-temperature oxidation of MoSi[sub 2].

Pest disintegration of thin MoSi2 films by oxidation at 500° C

Thin molybdenum disilicide (MoSi2) films have been produced by magnetron sputter deposition, and subjected to oxidation tests for the study of “MoSi2 pest”-a phenomenon showing disintegration of a solid piece of MoSi2 into powdery products. The as-prepared films were of an amorphous structure. Oxidation of the films in air at 500° C led first to cracking of the films, and then the cracked pieces eventually evolved into disintegrated powders with a yellowish appearance. Secondary electron microscopy and Auger electron spectroscopy revealed that the reaction products consisted of MoO3 whiskers (platelets), Si-Mo-O fibres, SiO2 clusters, and some residual MoSi2. The disintegration of MoSi2 films appeared to be independent of their crystal structure; a similar phenomenon was also observed in crystallized films, with a metastable hexagonal structure, oxidized under the same conditions. The disintegration of the MoSi2 films is compared to and correlated with the “pest reaction” of bulk MoSi2.

Creep of a niobium beryllide, Nb2Be17

A niobium beryllide, Nb[sub 2]Be[sub 17], has been prepared by powder-metallurgy techniques and the mechanical properties characterized both at room and elevated temperatures. Microhardness and fracture toughness were measured at room-temperature. Hardness and hot-hardness test results indicated that, although the material was brittle at low temperatures, it became plastic at elevated temperatures ([gt]1000 [degree]C). Creep properties of Nb[sub 2]Be[sub 17] were studied at temperatures from 1250 to 1350 [degree]C and applied stresses from 10 to 90 MPa. The stress exponent, determined from stress-change tests, was about 3, and the activation energy, determined from temperature-change tests, was about 575 kJ/mol. The creep of Nb[sub 2]Be[sub 17] at high temperature is apparently controlled by dislocation glide; this proposal was supported by transient creep experiments. Comparisons have been made between the creep properties of Nb[sub 2]Be[sub 17] and other intermetallics.

Explosive anisotropic grain growth of delta‐NiMo by solid‐state diffusion

Anomalous, anisotropic grain growth has been observed in delta(δ)‐NiMo intermetallic compound during the annealings of Mo/Ni thin‐film diffusion couples at 700 and 800 °C. Two layered microstructures showing median‐sized, equiaxed grains and large columnar single crystalline grains were generated. The growth direction of the columnar grains was parallel to the direction of Ni diffusion flux. Electron diffraction indicated that both the median‐sized and the columnar grains were δ‐NiMo. The composition of δ‐NiMo was determined to be Ni48‐Mo52 (at.%). According to the thickness of reaction‐formed δ‐NiMo, the apparent interdiffusion coefficient was measured to be about 10−10 cm2/s which is 4 to 5 orders of magnitude greater than literature data. The enhanced diffusion rate in Ni‐Mo, and the anomalous anisotropic grain growth of δ‐NiMo compound are discussed on the basis of exothermic reactions between Ni and Mo during diffusional intermixing. The enthalpy of the formation of δ‐NiMo is calculated and demonstra...

Structural evolution in niobium beryllides during mechanical alloying

Abstract Mechanical alloying of 1Nb+12Be and 2Nb+17Be powder mixtures has been conducted. Both of the powders, in a premixed condition, exhibited agglomeration and compositional inhomogeneity. Upon MA for 6 h, broadening of Nb peaks was observed, whereas most Be peaks lost their identity. Significant lattice expansion ( e =∼2.5 %), manifested by peak position shift, was observed in the Nb unit cell in the 1Nb+12Be powders, but no measurable peak position change was noted in the 2Nb+17Be powders. In both cases, contamination from the WC tooling became noticeable. On the basis of AES analyses, the lattice expansion of Nb was caused by the incorporation of Be, which has a wide range of solid solubility in Nb. As the duration of MA increased, increasing degrees of amorphization of the powders were generated; concurrently, contamination from WC also became more extensive. While extensive MA improves the composition uniformity of the 1Nb+12Be mixtures, macroscopic inhomogeneity is still present in the 2Nb+17Be mixtures. In both cases, the average particle size decreased from a few tens of microns (in the premixed conditions) to submicron levels. Complete amorphization was achieved after MA for 24 h; no significant change was noted in the XRD spectra from the powders after MA for increased periods of time. Vacuum annealing of the 72 h, MA powders resulted in massive crystallization; the 1Nb+12Be powders turned into a mixture of NbBe 12 and Nb 3 Be 2 , and the 2Nb+17Be powders evolved into mainly Nb 2 Be 17 and some unidentified minor second phases.