Structural Defects in the 18R Martensite of CuZnAi Shape Memory Alloys
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ABSTRACT The lattice relaxation at the type I faults in the 18R martensite was studied by high resolution TEM. Local atomic plane rotations and small changes of the interplanar distances were observed over a distance of 1.5 nm. It is shown that this distorted region can be rationalized as an intermediate structure between the martensite and the second variant of the parent phase. INTRODUCTION A large density of non-basal plane defects, called type I faults, are commonly observed in the 18R type of martensites. They have been studied by several authors either with conventional transmission electron microscopy (TEM) [1,2] or high resolution TEM [3,4]. Since the type I faults are connected to the basal plane stacking faults they would have the same displacement vector. However, it has been found [2] that a further relaxation occurs along the non-basal faults because of the mismatch of the atomic planes joining at the defects. Such relaxation could have important consequences on the mechanical properties of the 18R martensites serving as obstacles for dislocation gliding [5], or could be related in some way to the rubber-like behavior [6]. In this work the lattice relaxation at the type I faults was studied by high resolution TEM. Atomic plane rotations and local changes of the lattice parameters are reported. The results suggest that the atomic relaxation gives an intermediate structure approaching the second variant of the parent 03phase. EXPERIMENT A single crystal of 18R martensite with an alloy composition of Cu -17.52 at% Al -12.96 at% Zn has been used. Thin foils with the zone axis along the [210] direction were polished as explained elsewhere [2]. The high resolution TEM images were taken with a Philips CM200 ultra twin microscope, operating at 200 kV, with an objective lens spherical aberration of 0.52 mm. The suitable underfocus to obtain the image of the projected structure was estimated to be around 63 nm, following the procedure outlined in [7]. An objective aperture with a diameter of 14 nmnf was used. RESULTS A typical high resolution TEM image containing part of a type I defect is shown in Fig. 1. The basal plane stacking sequences at both sides of the defect are indicated. A careful observation shows that the (0 0 18) and the (1 2 10) planes are shifted upon crossing the defect, giving rise to an additional displacement vector in agreement with the previous finding [2].
445 Mat. Res. Soc. Symp. Proc. Vol. 459 ©1997 Materials Research Society
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Figure 1. High resolution image of a type I fault, from [210] zone axis. The stacking sequences at both sides of the defect are indicated. A shift of the (0 0 18) and (1 2 10) planes upon crossing the defect can be noted. Negligible shift along the (T12 8) plane is observed.
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In order to study in more detail the nature of this relaxation, an area centered at the defect, in Fig. 1, was analyzed using the Fourier transform method [8]. The corresponding diffractogram is shown in Fig. 2a. Figure 2b corresponds to the perfect 18R structure. It can be note
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