D. C. Van Aken

Mechanical behavior and interface design of MoSi2-based alloys and composites

The mechanical behavior of hot pressed MoSi2-based composites containing M05Si3, SiO2, CaO and TiC as reinforcing second phases was investigated in the temperature regime 1000-1300 °C. The effects of strain rate on the flow stress for M05Si~-, SiO2- and CaO-containing composites are presented. Effects of several processing routes and microstructural modifications on the mechanical behavior of MoSi2-M05Si ~ composites are given. Of these four composite additions, M05Si 3 and CaO produce strengthening of MoSi 2 in the temperature range investigated. SiO 2 greatly reduces the strength, consistent with the formation of a glassy phase at interface and interphase boundaries. TiC reduces the flow stress of MoSi 2 in a manner that suggests dislocation pumping into the MoSi 2 matrix. The strain rate effects indicate that dislocation creep (glide and climb) processes operate over the temperature range investigated, with some contribution from diffusional processes at the higher temperatures and lower strain rates. Erbium is found to be very effective in refining the microstructures and in increasing the hardness and fracture properties of MoSi2-MosSi 3 eutectics prepared by arc melting. Initial results on microstructural modeling of the deformation and fracture of MoSi2-based composites are also reported.

On the creep of directionally solidified MoSi2-Mo5Si3 eutectics

Abstract The high temperature deformation behaviour of MoSi2-Mo5Si3 eutectics has been investigated as a function of lamellar spacing over the temperature range 1100–1400°C and strain rates (∈) of 1 × 10−4 to 1 × 10−6 s−1. Specimens with lamellar morphologies were produced by directional solidification using the Czochralski method at pull rates of 25–210 mm/h giving lamellar spacings (λ) of 2.6 to 1.09 μm. The measured flow stress was found to increase as the lamellar spacing decreased for a given strain rate. A constitutive model for creep that incorporates reinforcement spacing for creeping fibers in a creeping matrix was found to describe the creep behaviour of the eutectic, i.e. ∈αλ withm = 1. Creep deformation of the eutectic was controlled by ½〈110〉 (001) partial dislocations in the Mo5Si3 phase. The creep behaviour of a [314] oriented Mo5Si3 single crystal was also investigated.

The effect of matrix microstructure on cyclic response and fatigue behavior of particle- reinforced 2219 aluminum: Part I. room temperature behavior

The low-cycle and high-cycle fatigue behavior and cyclic response of naturally aged and overaged 2219/TiC/15p and unreinforced 2219 Al were investigated using plastic strain-controlled and stress-controlled testing. In addition, the influence of grain size on the particle-reinforced materials was examined. In both reinforced and unreinforced materials, the naturally aged conditions were cyclically unstable, exhibiting an initial hardening behavior followed by an extended region of cyclic stability and ultimately a softening region. The overaged reinforced material was cyclically stable for the plastic strains examined, while the overaged unreinforced material exhibited cyclic hardening at plastic strains greater than 2.5 × 10−4. Decreasing grain size of particle-reinforced materials modestly increased the cyclic flow stress of both naturally aged and overaged materials. Reinforced and unreinforced materials exhibited similar fatigue life behaviors; however, the reinforced and unreinforced naturally aged materials had superior fatigue lives in comparison to the overaged materials. Grain size had no effect on the fatigue life behavior of the particle-reinforced materials. The fatigue lives were strongly influenced by the presence of clusters of TiC particles and exogenous Al3Ti intermetallics.

Effect of Microstructure on the Cyclic Response and Fatigue Behavior of an XDTM Aluminum Metal Matrix Composite

Particulate r~,fforced metal matrix composites are being viewed with increasing interest by designers of high performance components. These composites offer many advantages over their unreinfow.ed counterparts and provide the designers with increased flexibility in tailoring the materials properties, e.g. coefficient of thermal expansion and elastic modulus. However, these components are often employed in fatigue limited applications and to date there are only a few studies on the fatigue crack initiation behavior of metal matrix composites (e. g. I-3). In particular, the cyclic stress strain response of composite materials has not been documented nor has the influence of matrix microstructure on fatigue life or cyclic response been explored. Thus the purpose of this work was to investigate the effect of microstructure on the fatigue life, cyclic response, and strain localization of an aluminum.copper based XD TM particulate reinforced metal matrix composite. Since aluminum-based metal matrix composites are often based on conventional precipitation hardened aluminum alloys, an understanding of the fatigue behavior of these alloys provides a foundation for understanding the response of composite materials when subjected to cyclic loading. Of particular interest are comparisons between the fatigue behavior of materials hardened with either shearable or nonshearable precipitates. For comparison with the present work on cyclic response the reader is referred to the works of Laird and coworkers (e.g. 4-6).

On the microstructure and crystallography of directionally solidified MoSi2-Mo5Si3 eutectics

Abstract Directionally solidified MoSi2-Mo5Si3 eutectic rods were produced by the Czochralski method. A script lamellar microstructure was produced and an inverse square root dependence upon the pull rate (39–210 mm/h) was observed for the lamellar spacing. The eutectic rods were textured with [001] of Mo5Si3 and 〈110〉 of MoSi2 parallel to the rod axis and an orientation relationship consisting of [110]MoSi2[110]Mo5Si3 and (1¯10)MoSi2 ||(002)Mo5Si3 was present in all of the samples. The eutectic grows with a lamellar plate morphology inclined at 15° relative to the rod axis and four growth variants are possible. A script lamellar microstructure, as viewed in the transverse plane, was produced by periodic branching of the Mo5Si3 lamellae and this branching was not associated with any change in crystallography. Addition of 0.35 at.% erbium produced a more fibrous microstructure and an erbium-rich compound, Er2Mo3Si4 with space group 21/c, formed congruently with the Mo5Si3-MoSi2 during solidification. A ledge-terrace growth mechanism was proposed to explain the observed microstructures.

On the effect of matrix relaxation during the melting of embedded indium particles

Abstract The melting transformation of micron-sized (1–20 pm in diameter) indium particles embedded in an aluminium matrix has been studied using differential scanning calorimetry. An average elevation in the melting-temperature of 4°C was observed for particles embedded in the grain interiors, whereas particles situated at high-angle boundaries melted at the equilibrium temperature. The relative contributions from interfacial surface energy and strain energies resulting from thermal expansion mismatch and a volume change upon transformation to the melting-temperature were calculated. It was shown that the volume transformation stress upon melting was the major factor in causing the melting-temperature elevation. The difference between the melting behaviour of the embedded and the grain-boundary inclusions was related to differences in the rate of stress relaxation associated with the formation of the liquid nuclei. Volume changes at the grain-boundaries were rapidly accommodated by diffusional processes,...

On the application of magnetomechanical models to explain damping in an antiferromagnetic copper-manganese alloy

The Smith-Birchak model for magnetoelastic damping was successfully applied to model the damping observed in an antiferromagnetic Cu-48Mn-1.5Al (wt pct) alloy. Antiferromagnetic domains were developed by solution treatment at 820 ‡C and subsequent aging at 400 ‡C for 4, 10, and 16 hours. Damping capacity and dynamic elastic modulus were measured as a function of strain amplitude and temperature. A maximum in the strain-amplitude-dependent damping was obtained for the 4-hour-aged sample for which a magnetostriction constant, λ, equal to 4.65 × 10-4, was derived. An exact fit for the Smith-Birchak model was obtained at low strains, whereas the model predicted lower damping than was observed for strains greater than 1.1 × 10-3. This discrepancy was attributed to an additional damping mechanism at high strain amplitudes,i.e., dislocation damping. A magnetostriction constant equal to 3.23 × 10-4 was also calculated based upon the Néel temperature and the observed microstructure.

The effect of matrix microstructure on cyclic response and fatigue behavior of particle-reinforced 2219 aluminum: Part II. Behavior at 150 °C

The 150 °C cyclic response of peak-aged and overaged 2219/TiC/15p and 2219 Al was examined using fully reversed plastic strain-controlled testing. The cyclic response of peak-aged and overaged particle-reinforced materials showed extensive cyclic softening. This softening began at the commencement of cycling and continued until failure. At a plastic strain below 5 × 103, the unreinforced materials did not show evidence of cyclic softening until approximately 30 pct of the life was consumed. In addition, the degree of cyclic softening (†σ) was significantly lower in the unreinforced microstructures. The cyclic softening in both reinforced and unreinforced materials was attributed to the decomposition of the θ′ strengthening precipitates. The extent of the precipitate decomposition was much greater in the composite materials due to the increased levels of local plastic strain in the matrix caused by constrained deformation near the TiC particles.

THE EFFECT OF MICROSTRUCTURAL SCALE ON HARDNESS OF MoSi2-Mo5Si3 EUTECTICS

Recently there has been a renewed interest in molybdenum disilicide (MoSi2) as a possible high temperature structural material. Extensive research on the mechanical behavior of monolithic and MoSi2-based materials is being performed and structure property relationships are beginning to be addressed (e.g. 1-4). Of particular interest to this study is the relationship between hardness and the microstructural scale of the MoSi2. It has previously been shown that some intermetallic compounds, like FeCo, FeCo-V and Ni3Mn, (5) obey the Hall-Petch relationship and Tiwari et al (6) have suggested the same for MoSi2. In the case of MoSi2, the reported data cover a range of grain diameters between 7 and 30 Bm (1,2,7-12) and this information is tabulated with the reported hardness values in Table I. It is apparent that a smaller grain size is associated with higher hardness values, but the amount of data is too limited to suggest a specific structure-property relationship. Although the hardness and grain diameter database for monolithic MoSi2 is small, a wide range of values have been reported for MoSi2-SiC particulate composites (13). This data shows that as the SiC content is increased the hardness also increased and the grain size of the MoSi2 decreased. It is interesting to speculate that some of the increased hardness may have resulted from a decreasing grain size.

The role of bulk nucleation in the formation of crystalline cordierite coatings produced by air plasma spraying

Abstract Near stoichiometric cordierite (2MgO–2Al 2 O 3 –5SiO 2 ) glasses doped with 6.3, 8.3 and 10.1 weight percent (wt.%) TiO 2 were plasma sprayed onto SiO 2 -based refractory concrete substrates. Substrate temperatures were controlled such that the maximum surface temperature exceeded the crystallization temperature during plasma spraying. Only the cordierite powders containing 8.3 and 10.1 wt.% TiO 2 produced crystalline coatings. For compositions with less than 8 wt.% TiO 2 the crystallization is believed to be controlled by surface nucleation at imperfections. Surface nucleation became more difficult with increasing preheat temperature and is related to improved wetting between coating splats. Bulk nucleation-controlled crystallization was observed at compositions above 8 wt.% TiO 2 and the presence of Al 2 TiO 5 was detected in these compositions.