Papers by Keyword: Dislocation Loop

Abstract: A precipitation hardenable Al-Cu-Mg alloy was cryorolled with liquid nitrogen followed solution treatment and then aged at 170 ̊C for different time. The microstructure was characterized by optical microscopy (OM) and transmission electron microscopy (TEM). Hardness and tensile strength were also tested. The dislocation loops in the cryorolled alloy are more than the room temperature rolled alloy. Meanwhile the hardness, yield strength and tensile strength are larger than the room temperature rolled alloy.

Abstract: The study of the role of various factors in plastic behavior of materials is carried out using a mathematical model that takes into account fundamental properties of deformation defects in a crystal lattice based on the continuum theory of dislocations. Calculations were performed for copper, nickel, aluminum, and lead using a specialized software system Dislocation Dynamics of Crystallographic Slip. It has been shown that a decrease in the density of dislocations from 10¹² m^-2 to 10¹¹ m^-2 leads to an increase in the dislocation path in 10−16 times, and the maximum velocity in 1.5−2 times in copper and nickel, by nearly 20% in aluminum, and practically remains unchanged in lead. A decrease in the lattice and impurity friction from 2 MPa to 0.1 MPa leads to a linear increase in the path and the maximum velocity of the dislocation by 10−25%.

Abstract: The damage induced at the cut surface of SiC crystal by slicing were investigated by Raman scattering method and transmission electron microscopy. Electric discharge machining (EDM) predominately forms cracks, silicon, carbon and 3C-SiC by 6H-SiC pyrolysis and wiresawing with loose abrasive (WSLA) induces triangular crystal disordered area, stacking faults and dislocation loop bundles by stressing at the cut surfaces of SiC crystal.

Abstract: Dislocation loop was generated by electron irradiation in nickel aluminum alloy. It is important to know dislocation characteristics obtained from a high energetic electron irradiation. If b is the Burger vector of a dislocation loop and g is the diffraction vector, dislocation loop will appear larger, smaller or disappear for g.b>0, g.b<0 or g.b=0, respectively. Dislocation loop was determined as follows – first, the appearance of dislocation loops is arranged in observation table. Second, based on type of dislocation loop, Burger vector and diffraction vector, appearance of dislocation loop is arranged in calculation table. Third, based on observation and calculation table, Burger vector and type of dislocation loop is determined. The results show that dislocation loops consist of perfect dislocation loops and Frank dislocation loops. The perfect dislocation loops have Burger vectors of ½[0 ] and ½[ 0] while Frank dislocation loops have Burger vectors of ⅓[1 1], ⅓[11 ], ⅓[ 11], ⅓[111], ⅓[1 1], ⅓[11 ] and ⅓[ 11]. All dislocation loops are interstitial types.

Abstract: The behavior of the oxygen-containing precipitate in silicon wafer on different stages of the getter formation process is considered from the mechanical point of view. The precipitate is modeled as a spheroidal inclusion undergoing inelastic eigenstrains in an anisotropic silicon matrix. The stress-strain state in the precipitate and matrix is calculated within the framework of the model. An energetic criterion of breaking the spherical shape by the coherent precipitates is obtained and analyzed. Criteria of the formation and onset of motion of the dislocation loops in the vicinity of the precipitate are also proposed. The obtained results are compared with the available experimental data.

Abstract: To study the removal mechanism of materials during nano cutting, molecular dynamics method is adopted to simulate single crystal copper nanomachining processes, and subsurface defects evolvements and chip forming regulation are analyzed by revised centro-symmetry parameter method and the ratios of the tangential cutting force and the normal cutting force. The results show that there are different defects under different cutting depths. When cutting depths is shallower, there are dislocation loop nucleation in the subsurface of the workpiece beneath the tool; however, when the cutting depths is deeper, there are dislocations nucleation and slipping along {101} plane and (111) plane. In addition, both tangential cutting force and the normal cutting force decrease as the cutting depths decreasing. When the ratios of the normal cutting force and the tangential cutting force is below 0.9, the chip will be formed.

Abstract: We investigated the expansion of single Shockley stacking faults (SSFs) in a 4H-SiC epitaxial layer under high-intensity scanning laser beam during room temperature photoluminescence mapping, which is similar to the degradation of bipolar pin diodes during forward current injection. In an epitaxial layer on an 8 off-axis (0001) substrate, the SSF-related intensity patterns induced by scanning high-intensity laser beam were classified into two types. The first one was a triangular pattern and the second a pattern which expanded in accordance with the motion of the scanning laser beam. The origins of the SSFs responsible for both patterns are presumably due to the preexisting basal plane dislocations and the dislocation-loops on the basal plane in the epitaxial layer, respectively. On the other hand, most of the SSF-expansion in on-axis (11 2 0) epitaxial layers were similar to the second type in the (0001) epitaxial layer. We, therefore, suggest that the dislocation-loops, which were located close to the surface, were dominant nucleation-sites of the SSFs in the (11 2 0) epitaxial layers.

Abstract: We have addressed two issues concerning the relative stabilities of various orienta- tions of interstitial clusters in iron by making a comprehensive comparison between four recent empirical potentials. First, we have investigated the effect of finite temperature on the com- petition between clusters made of a few dumbbells oriented along h111i or h110i. We show by quasi-harmonic calculations that h111i clusters have much larger vibrational formation en- tropies and that they are therefore stabilized with respect to h110i clusters at high temperature. Second, we have compared the formation energies of loops with several hundred atoms with Burgers vector 1 2 h111i or h100i. The 1 2 h111i loops are found to be always more stable, but the energy differences with h100i loops depend strongly on the potential.

Abstract: Extensive simulations based on classical molecular dynamics have shown that small interstitial-type perfect dislocation loops in various metals and alloys have the structure of bundles of crowdions and a loop can easily makes the one-dimensional glide motion due to almost independent motion of crowdions in the loop. However, the experimental knowledge on the motion of loops is not enough. The present study dynamically examined the motion process of loops in pure iron under 1000 keV electron irradiation and thermal annealing by using transmission electron microscopy under which loops could move. Two types of loops were formed by irradiation. Loops of one type possessed the Burgers vector of 1/2<111> and the habit plane of {011}, and loops of the other type were <001> {001}. Loops of the former type made back-andforth glide motion and expansion towards the direction along their Burgers vectors when they were smaller than about a few-ten nanometers in diameter. This strongly suggests that these small 1/2<111> loops have the structure of the bundle of crowdions. Loops of the latter type only rarely moved less frequently when they were smaller than about the same size. When loops of two types grew larger than about 50 nm, the characteristics of the motion of loops changed drastically. Dislocation segments of each large loop made long-distance glide independently of their opposite segments, and the habit plane deviated from the original ones. This kind of motion means that selfinterstitial atoms at the central region of such large loops are no longer the crowdions.

Abstract: Laser welded high purity V-4Cr-4Ti alloy (NIFS-HEAT2), fabricated by National Institute for Fusion Science (NIFS), was used for this study. Copper ion irradiation was carried out with the tandem accelerator at Kyushu University. The TEM samples were sliced from the welded materials and irradiated at 573 and 873K up to the dose of 12 dpa. The microstructure before irradiation showed that relatively large precipitates, which were commonly observed in NIFS-HEAT2, disappeared in the center of the weld metal. After the ion irradiation, fine titanium oxides with {100} habit planes were detected even at the dose of 0.75 dpa. However, less number density of these oxides were observed in the base metal after the same irradiation conditions. This means that the behaviors of oxygen atoms, which dissolved from the large precipitates during the laser welding, is essential to the microstructural evolution of welded V-4Cr-4Ti alloys.