J. Megusar

Plastic flow and fracture in Pd80Si20 near Tg

Above about 0.7Tg, plastic flow in Pd80Si20 becomes homogeneous in the normal ranges of strain rate of short term laboratory experiments. In this range samples exhibit considerable anelasticity before general yield. Load maxima are observed simultaneously with general yield and are followed soon after by a classical behavior of slanted necking, and a mixture of rupture and fracture accompanied by elongations of about 3% typical of a non-hardening rupturing solid of the given thickness and gauge length. This relatively stable behavior has permitted measurement of the stress exponents m of the strain rate for the material at flow at these temperatures. These exponents decrease to unity as the test temperature approaches Tg. The temperature dependence of the flow stress and its strain rate dependence are in good accord with Argons recent theory for plastic flow in metallic glasses.

Shear band induced dilations in metallic glasses

Etude theorique a partir de resultats anterieurs portant sur le changement de volume du a la formation de bandes de cisaillement dans des echantillons cylindriques de verre metallique Pd 77,5 Cu 6 Si 16,5

Low temperature fast-neutron and gamma irradiation of Kapton® polyimide films

Abstract Structure, chemistry and mechanical properties of Kapton® HA and H polyimide films have been studied after their exposure to fast-neutron and gamma irradiation at a target temperature of 4 K. The fast (E>0.1 MeV) neutron fluence was 3.1 × 1022 n/m2. X-ray diffraction showed that irradiation disordered partially crystalline structure of Kapton® H while the structure of Kapton® HA remained amorphous under irradiation. The glass transition temperature (Tg) of Kapton® HA and H increased with irradiation, consistent with the crosslink formation. The increase in Tg was significantly larger in Kapton® H. The FT-IR spectrometry showed no new absorptions in the irradiated Kapton® HA and H. Shifts in some peak positions have been observed, however. Tensile testing showed that irradiation increased modulus and yield strength of Kapton® HA and H, in agreement with the proposed crosslink formation as the predominant mechanism of radiation hardening. The extent of radiation hardening, as evaluated from the total strain at fracture, was significantly larger in Kapton® H.

Internal oxidation of dilute Cu-Ti alloys

The kinetics of internal oxidation of dilute Cu-Ti solid solutions, containing up to 1 wt pct Ti have been investigated over the temperature range 700 to 900°C, and the oxide morphologies produced have been studied by electron metallography. Values of the solubility-diffusivity products for oxygen in copper (No(s)Do) are in good agreement with those obtained for pure copper by electrochemical techniques. The TiO2 particles formed in the Cu-matrix are extremely small (50-200Å) and appear to be ellipsoidal in shape. The oxide particle size has been observed to increase linearly with distance below the specimen surface. In addition, electron metallography reveals TiO2 particles both in the matrix and at grain boundaries which exhibit interesting fringe contrast or “stripes” which are perpendicular to the major axis of the ellipsoids. The origin and possible significance of these regions in terms of coherency of the TiO2/Cu interfaces are discussed. The interfacial energies for incoherent TiO2 /Cu interfaces have been found to be quite low, on the order of 400 to 700 ergs/cm2.

Optimization of structure and properties of path a prime candidate alloy (PCA) by rapid solidification

Abstract A high density of heterogeneous nucleation sites for helium trapping was provided in a Ti-modified 316 stainless steel by reducing the grain size, increasing the TiC content and controlling the dislocation structure by rapid solidification and subsequent thermomechanical treatments. These structural modifications are expected to have an effect on swelling and high-temperature strength and ductility of Path A alloys under irradiation. Thermomechanical treatments were developed which resulted in higher strength and elongation in stress rupture testing at 923 K compared with a 20% cold worked reference state. While rapid solidification allowed for a controlled TiC precipitate size, density and distribution, coarsening may effectively limit the applications of TiC dispersion strengthened stainless steel at higher irradiation temperatures.

Microstructure of rapidly solidified path a prime candidate alloys

Abstract Path A Prime Candidate Alloy (PCA) has been rapidly solidified and its composition has been modified by increasing carbon and titanium content. Microstructural examination showed that rapid solidification allowed for a decrease in over-all segregation, elimination of coarse excess phases, an attainment of fine grain size and an increase in supersaturation of carbon and titanium in the PCA alloy. Structural modifications which include a reduction of grain size, an increase in TiC content and control of dislocation structure provide a high density of heterogeneous nucleation sites for helium trapping and are therefore expected to have an effect on swelling and high temperature strength of Path A alloys under irradiation.

Stabilization and strengthening of Pd80Si20 metallic glass

Abstract The effect of composition on thermal and mechanical stability was studied by adding boron and zirconium to Pd 80 Si 20 . The addition of boron destabilizes Pd 80 Si 20 glass. The addition of boron and zirconium in the amounts of 3 at.% and 1.5 at.% respectively results in an amorphous structure with increased thermal stability (an increase in T g of 73 K). Although Pd 80 Si 20 glass exhibits general yielding, the stabilized (Pd 80 Si 20 ) 0.955 − B 3.0 Zr 1.5 glass behaves substantially elastically when tested under identical conditions at 433 K and slow rates. A controlled decomposition of Pd 80 Si 20 also has an effect on general temperatures. Stabilization may be important in high temperature applications of metallic glasses and in particular under irradation.

Low temperature fast-neutron and gamma irradiation of glass fiber/epoxy composite. Part 1: deformation and fracture

Abstract Deformation and fracture processes, following 4.2 K fast-neutron and gamma irradiation and short-beam shear testing, have been studied in the S2-glass fiber/epoxy composite (a candidate ITER TF coil insulation system). Irradiation to fast-neutron fluences of 0.9 and 1.8 × 10 22 n/m 2 facilitated the glass/epoxy interface debonding by lowering the glass/epoxy interface trength. Scanning electron microscopy (SEM) examination of the composite irradiated to 3.1 × 10 22 n/m 2 revealed glass/epoxy interface debonding and extensive cracking of glass fibers and epoxy. The decrease in shear strength of the composite at this neutron fluence may therefore be attributed to the combined effects of the reduced glass/epoxy interface strength and embrittlement of glass fibers and epoxy.

The potential for iron and copper based rapidly solidified alloys as fusion reactor materials

Rapid quenching from the melt, a new, rapidly evolving field of effort, is revolutionizing alloy design, compositions, processing and the resultant properties. Low alloy segregation (zero for the glassy state), unusual grain size control, down to submicron sizes, sharply increased solid state supersaturation (even of insoluble elements), and controlled size, shape and distribution of excess phases make possible broad control of structure and therefore of properties. Of particular interest as fusion reactor materials are copper alloys and ferritic steels, in part because of excellent thermal conductivity relative to most other alloy systems. Ability to select alloying elements to take into account requirements for activation; useful strength levels up to 400 or 500°C, including thermal stresses; swelling resistance; long term structural stability with increasing temperature are discussed. Strengthening by elemental precipitation, by intermetallic compounds, fine refractory oxides and combinations of these are demonstrated.

Potential for using rapid solidification for improved irradiation performance in the fusion environment

Abstract Rapid solidification has a potential for improving performance in the fusion environment (first-wall materials, limiters, superconductors, 3.) through structural refinements of crystalline materials and the preparation of amorphous materials with selected compositions. Compaction techniques which are used for rapidly solidified particulates allow as well for preparation of graded or layered materials. The following topics are being studied by using rapid solidification under a current M.I.T. alloy development program: swelling resistance and high temperature strength of austenitic stainless steels; DBTT and high-temperature strength of ferritic steels; high strength copper alloys; simulation techniques (boron, lithium doping); irradiation damage in metallic glasses at high fluences.