Dissociation Behavior of Dislocations in Ice

Dislocations in ice behave very differently from those in other materials due to the very low energies of stacking faults in the ice basal plane. As a result, the dislocations dissociate on the basal plane, from a perfect dislocation into two partial dislocations with equilibrium width we ranging from 20 to 500 nm, but what is the timescale to reach this dissociated state? Using physical models, we estimate this timescale by calculating two time-constants: the dissociation-completing time td and the dissociation-beginning time tb. These time constants are calculated for two Burgers vectors as a function of temperature. For perfect dislocations with Burgers vector <c + a>, td is more than one month even at the melting temperature TM, and it exceeds 103 years below −50 ℃, meaning that the dissociation cannot be completed during deformation over laboratory timescales. However, in this case the beginning time tb is less than one second at TM, and it is within several tens of minutes above −50 ℃. These dislocations can glide on non-basal planes until they turn to the dissociated state during deformation, finally resulting in sessile extended dislocations of various widths approaching to the equilibrium value we. In contrast, for perfect dislocations with Burgers vector <a>, td is less than one second above −50 ℃, resulting in glissile extended dislocations with the equilibrium width we on the basal plane. This width is sensitive to the shear stress τ exerted normal to the dislocation line, leading to extension of the intervening stacking fault across the entire crystal grain under commonly accessible stresses. Also, due to the widely dissociated state, dislocations <a> cannot cross-slip to non-basal planes. Such behavior of extended dislocations in ice are notable when compared to those of other materials.

Evolution in Dislocation Configuration of Deformed Fe-40Ni-Ti Alloy during Isothermal Relaxation

The thermo-simulation test and transmission electron microscopy (TEM) were applied to investigate the evolution of dislocation configuration and strain induced precipitation behavior during relaxation at 850°C in a deformed Fe-40Ni-Ti alloy. The stress relaxation curve can be divided into three stages, namely, the process of incubation, nucleation and growth, and the coarsening of strain-induced precipitates. The highly dense and twisted dislocations formed during the deformation develop into dislocation cells and finally, the sub-grains can be observed when relaxing to 1000s. The strain induced precipitates occur both onto the dispersed dislocations and dislocation cells. The precipitates pin the dislocations which results in retarding the progress of dislocation configuration evolution. As precipitates start to coarsen, the pinning effect weakens and the dislocations get rid of the pinning though bypassing mechanism. Adopting the same simulation test to bainitic steel, the optimum refinement could be obtained at 60-200s during relaxation processing, corresponding to the perfect dislocation cells formation of Fe-40Ni-Ti alloy.

Dislocation Generated by Electron Irradiation

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.

Study on the Perfect Dislocation in Cd0.9Zn0.1Te Single Crystals by HRTEM

Two types of 60° perfect dislocation in Cd0.9Zn0.1Te single crystals were observed by HRTEM. The Burger’s vector of the two dislocation were and respectively. With reasonable premise, the possible dissociation mechanisms of the two kinds of dislocations were supposed to be and . There are two kinds of the formation mechanisms, the first one is the accumulation of the point defects and the other kind of dislocation is the lattice gliding caused by the thermal stress.

Stacking Fault Energy of Si Nanocrystals Embedded in SiO2

Si nanocrystals (Si nc) were produced by the implantation of Si+ into a SiO2 film on (100) Si, followed by high-temperature annealing. High-resolution transmission electron microscopy (HRTEM) observation has shown that a perfect dislocation (Burgers vector b=(1/2)〈110〉) can dissociate into two Shockley partials (Burgers vector b=(1/6)〈112〉) bounding a strip of stacking faults (SFs). The width of the SFs has been determined from the HRTEM image, and the stacking fault energy for Si nc has been calculated. The stacking fault energy for Si nc is compared with that for bulk Si, and the formation probability of defects in Si nc is also discussed. The results will shed a light on the dissociation of dislocations in nanoparticles.