F re ti c r i c k Sc I t M at c I-i ;i I s Re sea rc h Laboratory , Univcrsity of Illinois at U~-bana-Cliampaign, IJrbana IL 0 I801 ' Materials Science and 'I'czhliology I)i\.ision, La wrcncc L i vel-in ox N ;i I i on ;I 1 Laboi.a t o 1-y , L i v c m ore C' A 04 5 5 0 ABSTRACT Deformation experiments performed iii-situ in the transmission electron microscope have led to an increased understanding of dislocation dynamics. To illustrate the capability of this technique twoexamples will be presented. I n the first cxample, thc processes of work hardening in Mo at room temperature will be presented. These studies have improved our understanding of dislocation mobility, dislocation generation, and dislocation-obstacle interactions. In the second example, the interaction of matrix dislocations with grain boundaries will be described. From such studies predictive criteria for slip transfer through grain boundaries have been developed.

To determine the influence of interface type on the accumulation of damage and ion mixing in GaAs/AlxGa1−xAs heterostructures, the damage produced by ion implantation at 77 K in single-layer (GaAs/AlxGa1−xAs/GaAs) and double-layer (GaAs/AlxGa1−xAs/GaAs/ AlxGa1−xAs/GaAs) heterostructures has been investigated by using a combination of Rutherford backscattering spectrometry and transmission electron microscopy. In the single-layer geometry, the degree of disorder increases with depth and the mixing is greater at the AlxGa1−xAs on GaAs interface than at the GaAs on AlxGa1−xAs one. The damage distribution in the sample with the double-layer geometry was different in the two layers, but overall it was similar to that in the single-layer geometry. These trends were observed in samples with x=0.6 and 0.2. These results indicate that migration of charged defects due to the presence of an implantation-induced electric field is not responsible for the asymmetry in the damage accumulation across the layer, the interface disorder and ion mixing, and the initiation of amorphization at interfaces. Instead, these effects can be better understood in terms of the depth dependence of the density of cascade events.

Materials selection criteria in gas turbine engine design are reviewed, and several design challenges are introduced where selection of low coefficient of thermal expansion (CTE) materials can help improve engine performance and operability. This is followed by a review of the types of low CTE materials that are suitable for gas turbine engine applications, and discussion of their advantages and disadvantages. The primary limitation of low CTE materials is their maximum use temperature; if higher temperature materials could be developed, this could result in novel turbine system designs for gas turbine engines.

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