Papers by Keyword: Dislocation Motion

Abstract: The relaxation properties of GH4169 alloy were studied contrastively at temperatures ranging from 600 ^oC to 700 °C and initial stress ranging from 550 MPa to 850 MPa. The relationship between the microstructure and relaxation behavior was evaluated using transmission electron microscopy techniques. It was found that the relaxation limit and relaxation stability of the alloy decreased obviously with the increase of temperature. Further investigations show that the relaxation behavior is mainly depend on both precipitate characteristics and its interaction with dislocations. The alloy with higher strength lever has more excellent stress relaxation stability, because of the inhibition of a large number subgrains on dislocations motion.

Abstract: In the paper the temperature and amplitude dependence of internal friction in the AZ31, AZ63 and AZ91 alloys is reported. A rapid increase in internal friction with increasing temperature was estimated. Internal friction peaks were observed in AZ63 and AZ91 alloys. Position and height of the peaks in the temperature scale depends on the heating rate. Simultaneously, microstructural observations were performed in the selected points of the temperature scale. The amplitude dependence of internal friction in an AZ31 alloy, prepared by rolling, was measured. A significant influence of the rolling texture has been estimated. Main mechanisms of internal friction are connected with precipitation, dislocation motion and twinning.

Abstract: The accurate prediction of springback is a subject of major concern during the sheet metal forming. In this study, the recovery behavior of Hastelloy C-276 sheet under different amounts of pre-strain has been investigated by uniaxial cyclic tensile tests. The total springback during unloading could be separated into a linear springback and a nonlinear springback. The percentage of nonlinear recovery to the total recovery increased as the pre-strain increased. Both unloading and reloading elastic moduli decreased as the pre-strain increased, which affected the springback phenomenon significantly. Nonlinear recovery could be explained by the movement of dislocations in the reverse direction. The decrease of unloading elastic modulus is mainly related with the dislocation motion and the mobile dislocation density.

Abstract: Mean free path of dislocation motion have a significant effect on mechanical properties because dislocation motion is blocked by eutectics and intermetallics in Al-Si casting alloys. In this study, we proposed a statistical method to evaluate mean free path of dislocation motion by using image processing, and investigated relation between the measured mean free path and mechanical properties of Al-Si casting alloys.

Abstract: Effects of prior rapid thermal processing (RTP) under different atmospheres on the motion of dislocations initiated from indentations in Czochralski (CZ) silicon have been investigated. It is found that the maximum gliding distances of dislocations in the specimens with the prior RTP under nitrogen (N₂) atmosphere are much smaller than those in the specimens with the prior RTP under argon (Ar) atmosphere. This is also the case when the specimens received annealing for oxygen precipitation (OP) subsequent to the RTP at 1250 °C under N₂ and Ar atmospheres, respectively. It is believed that the nitrogen atoms introduced during the RTP under nitrogen atmosphere or the oxygen precipitates facilitated by the RTP-introduced nitrogen atoms can exhibit pinning effect on the dislocation motion, which increases the critical resolved shear stress for dislocation glide.

Abstract: The microstructure and creep property of as-cast Mg-6Al-xSr(x=0, 2,3) alloys were studied. Results showed that the branch-like Al₄Sr phase is the main precipitating phase in these alloys containing Sr. Their secondary creep rates under the applied stress of 70MPa at 175°Cdecreased with the increasing in the content of Sr and reached the minimum value of 1.4×10^-8s^-1 in the Mg-6Al-3Sr alloy, which is one thirtieth of secondary creep rates of the Mg-6Al alloy. It was mainly attributed to Al₄Sr, which effectively hindered the grain boundaries sliding and dislocation motion in the creep.

Abstract: The microstructure and creep property of as-cast Mg-6Al-xNd(x=2,4,6) alloys were studied in this paper. Results showed that the needle-like Al11Nd3 phase and polygonal Al2Nd phase are the main precipitates in these alloys containing Nd, and the weight fraction of the former is much more than that of the latter. Their secondary creep rates under the applied stress of 70MPa at 175°C decreased with the content of Nd increasing and reached the minimum value of 5.0×10-8s-1 in the Mg-6Al-6Nd alloy, which is one ninth of that of the Mg-6Al alloy. The improvement in creep property of these alloys containing Nd was mainly attributed to the needle-like Al11Nd3 phase, which hindered the grain boundaries sliding and dislocation motion effectively in the creep.

Abstract: The present study investigates stability and motion of low angle dislocation boundaries in an array of precipitates. The model considers discrete dislocations and precipitates that are treated as impenetrable particles. Peach-Koehler forces, which originate due to the combined effect of dislocation-dislocation interactions and the applied stress, act the individual dislocations on. Both, the dislocation glide and the dislocation climb at elevated temperatures are taken into account. Results of the numerical study suggest that a critical applied shear stress (CASS) always exists which separates stable and unstable low angle boundary configurations. Varying particle size, interparticle spacing and density of dislocations in the boundary cause changes of the CASS that are systematically investigated. It is shown that the CASSs can considerably differ from the standard Orowan stress controlling the equilibrium of an isolated dislocation in a given microstructure. This result underlines the importance of long-range dislocation interactions that influence the high temperature strength of the precipitation-hardened alloys.

Abstract: Nondestructive methods may help to detect changes in the internal structure of a material and to explain the behaviour of the material. This paper describes a series of nondestructive tests performed on magnesium composites with a variety of matrices: commercial pure Mg and three magnesium alloys AZ91, ZC63 and ZE41. Short fibres of δ-Al2O3 (Saffil®) were used as the reinforcement. Internal friction measurements and joint dilatation and acoustic emission studies were used to demonstrate how thermal cycling influences the deformation behaviour of Mg based metal matrix composites. The values of the logarithmic decrement are influenced by the upper temperature of the cycle. The acoustic emission activity and the residual strain increase with increasing upper temperature. The results may be explained assuming that internal thermal stresses are generated.