A15 Compound Deformation and Secondary Slip in V3Si

The plastic deformation of A15 compounds has been the subject of a number of investigations. Cold working is possible only under high hydrostatic pressure, and Nb₃Sn, V₃Si, and V₃Ga polycrystals have been cold worked under hydrostatic pressures in the 1790 to 6000 MPa range.^[1,2,3] Hot deformation has been more widely evaluated, starting with the work of Greiner and Buehler in 1962.^[4] V₃Si single crystal deformation has been studied in the 1200 to 1800 °C range,^[4–10] and V₃Ga polycrystal deformation has recently been evaluated in the range from 1000 to 1300 °C.^[11,12] The hot deformation of Nb₃Sn polycrystals has been extensively studied in the 1150 to 1650 °C range.^[12–15]     

Low temperature electron microscopy on the cubic-tetragonal transformation of V3Si

The cubic-tetragonal martensitic transition of V₃Si and its precursor have been investigated in the temperature range from 13 K to 80 K using a 1 MV electron microscope with a double-tilting cold stage. Below the transition temperature Tm = 19 K, fine twin lamellae appear parallel to {110} plane traces, being connected continuously with mottled striations having the same orientation. Fine striations along {110} traces or so-called tweeds are observed below 50 K at the precursor of the structural phase transition. The image contrast of mottled striations and tweed patterns is attributed to the presence of lattice distortion owing to the softening of the shear modulus. Reversible changes of the patterns recorded with a TV-VTR system during cooling and heating processes were discussed briefly. This paper is based on a presentation made in the symposium “Pretransformation Behavior Related to Displacive Transformations in Alloys” presented at the 1986 annual AIME meeting in New Orleans, March 2–6, 1986. under the auspices of the ASM-MSD Structures Committee.     

Amorphous Cr5Si3 thin films— morphology and kinetics of crystallization

The changes in morphology and the crystallization kinetics of amorphous Cr₅Si₃ were studied by means ofin situ resistivity measurements and hot stage transmission electron microscopy (TEM). The crystallization process is controlled by nucleation and growth during the continuous heating as well as in the isothermal annealing. Initially the growth is isotropic. In the later stages it becomes anisotropic due to impingement. The growth characteristics and nucleation rates were deduced from the changes in linear dimension of a growing particle with time and from the number of nucleation points as a function of time. Growth rates were found to remain constant for most of the total transformation time while nucleation rates initially increase, subsequently peak, and then rapidly decrease. The determined nucleation and growth rates were used to calculate the transformed volume fractions. The results were compared with data obtained by assuming a linear relationship between instantaneous resistivity and volume fraction and with data based on measurements of projected crystallized areas in transmission (image analysis). Kinetics studies showed that the isothermal crys-tallization follows a sigmoidal curve. The apparent activation energy was found to be ∼2.4 eV. The transformation mode parameter was found to be ∼3 which, given that the crystallization reaction is interface controlled and the nucleation may be approximated as instantaneous, suggests a three-dimensional mode of crystallization.     

Thermal expansion of boron fiber-aluminum composites

The linear thermal expansion of silicon carbide coated boron (BORSIC^®) aluminum composites was measured as a function of volume fraction fiber and angle with respect to the fiber axis. The measurements were made with a standard quartz tube-type dilatometer at a heating rate of 150°C per hr. Measurements were made between 25° and 300°C on 2024 aluminum alloy-BORSIC and 1100 aluminum alloy-BORSIC composites in the 0 and 90 deg fiber orientation as a function of volume fraction fiber. The axial test results are compared with several models found in the literature which predict composite thermal expansion. These predictions yield values which are higher than the measured expansion coefficients of the 0 deg composites. The discrepancy is assumed to be related to yielding in the matrix. The 90 deg composites are found to agree with the transverse thermal expansion coefficient relationship of Schapery (also Levin) which employs the Poisson ratio for each phase and the composite. The expansion coefficients of 2024 aluminum alloy-BORSIC composites containing 54 pct by volume fiber are given for fiber orientations of 0, 15, 30, 45, 60, 75, and 90 deg.     

Thermal expansion of metals reinforced with ceramic particles and microcellular foams

The thermal expansion of three isotropic metal-matrix composites, reinforced with SiC particles or microcellular foam, is measured between 25 °C and 325 °C. All three composites show initial co-efficient of thermal expansion (CTE) values in agreement with the Turner model predictions, and near Schapery’s lower elastic bound for CTE. At higher temperatures, the CTE of foam-reinforced Al decreases, while that of the two particle-reinforced composites increases. These observations are interpreted as resulting from the presence of a very small fraction of microscopic voids within the infiltrated composites. This interpretation is confirmed with finite-element simulations of the influence of voids, cracks, and reinforcement convexity in two-dimensional (2-D) composites featuring an interconnected reinforcement of SiC surrounding isolated Al phase regions, thermally cycled from an elevated processing temperature and deforming in generalized plane strain.