Characterization of superdislocation dissociations in Al66Ti25Cr9 with transmission electron microscopy observations and image simulations

The nature of dissociated superlattice dislocation cores in Al66Ti25Cr9, deformed at room temperature, has been characterized by weak-beam transmission electron microscopy (TEM) and comparison of experimental images with computer-simulated images. The displacement fields associated with narrowly dissociated APB- and SISF-dissociated ‹110› superdislocations were calculated to account for the asymmetry in dislocation contrast and led to a better understanding of the formation of images. Such calculations are a powerful aid, when coupled with image simulations, in distinguishing the “real” intensity peaks from the supplementary peaks that can be generated under experimental imaging conditions. While both APB- and SISF-dissociated superdislocations were identified, the vast majority of superdislocations were determined to be APB-dissociated. Corrected values of the fault energies (γAPB and γSISF ) have been measured for this alloy. These energies and the observed dissociations are shown to be self-consistent.

In-Situ Heating Observations on Superlattice Dislocation in A Ni3(Al, Zr) Single Crystal

The TEM weak-beam technique has been used to investigate the behavior of dissociated superlattice dislocation in Ni3Al single crystal as a function of temperature. The observed dislocation with the Burgers vector of [110] partly dissociated on the (001) plane forming Kear-Wilsdorf (KW) lock. The dissociated pair did not indicate significant variation of separation in the temperature range from room temperature to 773K. but turned to form a jog at 773K. At 898K, which is near the peak temperature, the dissociated segment constricted completely. The experimental observations are discussed.

Weak beam image of a superlattice dislocation in deformed Ni3 (AlHfB)

A number of intermetallic compounds with the L12 structure exhibit a strange increase in the flow stress and work-hardening rate with increasing temperature. Despite the success of some model in explaining macroscopic properties, the detailed dislocation processes of model that are assumed to take place have not been observed in microscope. This work is an attempt to determine how the dislocation fine structure is related to the deformation behaviour of L12.Single crystal Ni-23Al-1Hf-0.1B(at%) was used in the present study. direction was chosen as compression axis. Samples were deformed to plastic strain of 6%, specimens were cut parallel to by spark erosion. Fined electropolishing was donein solution of 1% perchloric acid in methanol at −50°c and 30V. The g/3g diffraction condition used in weak beam observation.Fig1 shows the dislocation structure in foil. Long fairly straight screw dislocations with b=a[011] are imaged. The formation of dipoles is regarded as a characteristic and unusual feature of the dislocation structure. This indicates that annihilation is difficult at room temperature deformation. Weak beam images of superlattice dislocations are shown in fig.2 by using different reflections. The dislocation CC is long straight screw dislocation.

A SISF formation mechanism in Ni3Al

In the ordered Ll2 structure, the superlattice intrinsic stacking fault (SISF) has often been observed to be formed through two steps, the formation of antiphase boundary (APB) by dissociation of superlattice dislocation at first and then the conversion of the APB to the SISF(1,2). The present paper studied the SISF in NijAl by TEM and found that the SISF can also be formed by superlattice dislocation dipoles.The tensile deformation of the directionally solidified Ni3Al (of composition in wt% of 82.88Ni, 8.5A1, 0.8Zr, 7.8Cr, and 0.02B) specimens was performed at room temperature. Thin foils were made normal to the specimen axis (parallel to [001]) and studied at 200 kv in H-800.As shown in Fig.l, a SISF is formed at the end of a superlattice dislocation dipole, according to the SISF-type dissociation [10] =1/3 [11]+1/3[2l] on (111) plane. In Fig.2, a superlattic dislocation dipole is moving away from a SISF.

Characterization of GaAs and Ge Films on (100) Silicon

ABSTRACTA UHV MBE apparatus in which the deposition of both group IV and group III-V components is possible without breaking vacuum has been utilized to compare the growth of GaAs epilayers on non-polar Si(100) and Ge coated Si(100) substrates. In addition, a comparison of GaAs epilayers grown on substrates cleaned by ex-situ techniques and on substrates given all UHV in-situ surface preparation was made. Defect reduction by the incorporation of strained-layer superlattice dislocation filters and by post-growth rapid thermal anneal (RTA) thermal cycles was also investigated. Optical and material properties comparable to MBE grown GaAs/GaAs were obtained for GaAs grown on Ge coated Si(100) substrates.

Ge and Gesl Heteroepitaxy on Si(100) by MBE

ABSTRACTWe have grown a series of Ge and graded Si1-xGex epilayers on (100)Si substrates by MBE under different conditions. The quality of the layers has been characterized by cross-sectional TEM, Rutherford backscattering/ channeling and x-ray diffraction. This work addresses the optimization of growth temperature, (300–700°C) an evaluation of compositional grading, the effect of the incorporation of strained layer superlattice dislocation filters and post growth anneal cycles. Particular attention has been paid to grading GexSi1-x, x = 0 to 1 and the growth morphology of intermediate alloy epilayers.