Y.B. Xu

Shear localization and recrystallization in dynamic deformation of 8090 Al-Li alloy

The microstructural evolution in localized shear deformation was investigated in an 8090 Al-Li alloy by split Hopkinson pressure bar (strain rate of approximately 10(3) s(-1)) at ambient temperature and 77 K. The alloy was tested in the peak-, over-, under-, and natural-aged conditions, that provide a wide range of microstructural parameters and mechanical properties. Two types of localized shear bands were distinguished by optical microscopy: the deformed shear band and the white-etching shear band. They form at different stages of deformation during localization. There are critical strains for the occurrence of deformed and white-etching localized shear deformation, at the imposed strain rate. Observations by transmission electron microscopy reveal that the white-etching bands contain fine equiaxed grains; it is proposed that they are the result of recrystallization occurring during localization. The deformed-type bands are observed after testing at 77 K in all heat treatment conditions, but they are not as well defined as those developed at ambient temperature. Cracking often occurs along the localized shear at ambient temperature. The decrement in temperature is favorable for the nucleation, growth and coalescence of the microcracks along the shear bands, inducing fracture

Dynamic recrystallization induced by plastic deformation at high strain rate in a Monel alloy

An investigation has been made into the plastic deformation behavior of a Monel alloy deformed at high strain rate of 10(5) s(-1) by split Hopkinson bar. The results reveal that there are some equiaxed grains with an average size of 150 nm in diameter in the center of the shear bands, suggesting that this microstructure characteristics be developed by dynamic recrystallization, arising from the deformation and the rapid temperature rise in the band. Analysis shows that the plastic strain rate and the mobile dislocation density play a key role in the new crystallized grain formation and growth. Based on grain boundary energy change and diffusion mechanism, the grain growth kinetics is developed for plastic deformation at a high strain rate.

Cross-sectional electron microscopy observation on the amorphized indentation region in 001 single-crystal silicon

The amorphized region of single-crystal silicon (c-Si) induced by Vickers indentation has been studied cross-sectionally by transmission electron microscopy (TEM) and high-resolution electron microscopy (HREM). A comparison between the V-shaped profile of the amorphous region and the stress isobars under the indenter shows that the deviatoric stress plays a significant role in the formation of amorphous silicon (a-Si). A number of defects near the crystalline/amorphous (c/a) interface, and the refinement and rotation of grains at local regions, are observed by HREM. The distortion of lattice fringes in the c-Si region and the domains characterized by distorted lattice in the a-Si region near the interface as well as continuous transition from the crystalline to the amorphous region at the interface are also observed. A possible mechanism of defect-induced or heavy-deformation-induced amorphization of silicon under indentation is suggested.

Formation, microstructure and development of the localized shear deformation in low-carbon steels

An investigation has been made into the effect of microstructural parameters on the propensity for forming shear localization produced during high speed torsional testing by split Hopkinson bar with different average rates of 610, 650 and 1500 s(-1) in low carbon steels. These steels received the quenched, quenched and tempered as well as normalized treatments that provide wide microstructural parameters and mechanical properties. The results indicate that the occurrence of the shear localization is susceptible to the strength of the steels. In other words, the tendency of the quenched steel to form a shear band is higher than that of the other two steels. It is also found that there is a critical strain at which the shear localization occurs in the steels. The critical strain value is strongly dependent on the strength of the steels. Before arriving at this point, the material undergoes a slow work-hardening. After this point, the material suffers work-softening, corresponding to a process during which the deformation is gradually localized and eventually becomes spatially correlated to form a macroscopic shear band. Examinations by SEM reveal that the shear localization within the band involves a series of sequential crystallographic and non-crystallographic events including the change in crystal orientation, misorientation, generation and even perhaps damage in microstructures such as the initiation, growth and coalescence of the microcracks. It is expected that the sharp drop in the load-carrying capacity is associated with the growth and coalescence of the microcracks rather than the occurrence of the shear localization, but the shear localization is seen to accelerate the growth and coalescence of the microcracks. The thin foil observations by TEM reveal that the density of dislocations in the band is extremely high and the tangled arrangement and cell structure of dislocations tends to align along the shear direction. The multiplication and interaction of dislocations seems to be responsible for work-hardening of the steels. The avalanche of the dislocation cells corresponds to the sharp drop in shear stress at which the deformed specimen is broken. Double shear bands and kink bands are also observed in the present study. The principal band develops first and its width is narrower than that of the secondary band.

Indentation induced amorphization in gallium arsenide

Vickers indentations were carried out on the surface of GaAs single crystal with the load of 0.049 N and were observed using high-resolution electron microscopy in the present experiment. The experimental results reveal that many defects such as dislocation, microtwin and stacking-fault occurred and amorphization took place beneath the indentation. High-pressure induced amorphization and shear deformation induced amorphization were proposed for the transformation from crystalline to amorphous structure.

Evolution and analysis of γ' rafting during creep of single crystal nickel base superalloy

Abstract By means of TEM observation and finite element analysis, an investigation has been made into the directional coarsening of the γ′ phase for a single crystal nickel base superalloy with [001] orientation during creep at 1040°C. The results show that the strain energy change related to the elastic strain is to be the driving force for γ′ rafting. The extruded strain of the lattice in the cuboidal γ′ interfaces results in a supersaturation of the elements Ta and Al of larger atomic radius. The extrusion or expansion strain in the lattice of the cuboidal γ′ planes may repel or trap these atoms to promote the directional growth of the γ′ phase into a needle-like raft structure along the direction parallel to the stress axis under an applied compression stress, or into a meshlike raft structure along the direction perpendicular to stress axis under applied tensile stress. The normal direction of the expanding lattice is supposed to be the one in which the γ′ rafts grow. The rate of γ′ rafting is enhanced by increasing viscoplastic flow in the γ matrix and elastic strain in the γ′ phase. Therefore, there is a smaller rate of growth under compressive than under tensile stress as a result of the smaller expansion strain and viscoplastic flow occurring in the former.

Directional coarsening of γ′ phase in single crystal nickel based superalloys during tensile creep

Abstract The γ′ precipitate rafting kinetics and morphological evolution for two model single crystal superalloys have been studied. The microstructure of the alloys after different stages of tensile creep at 1040°C and under a range of stresses are examined using TEM and SEM. The chemical compositions of both γ and γ′ phases are analysed by energy dispersive spectrometry. Results show that a meshlike γ′ raft structure is formed along the direction normal to the stress axis during primary creep. The applied stress causes a decrease in the coherent strain energy at the γ′/γ interfaces in the (001) crystal plane. The released energy is the driving force for the diffusion of elements, leading to the formation of the γ′ rafts. A longer time is required for the formation of γ′rafts in alloy 2 owing to its higher content of the refractory element W which obstructs the migration of the other elements in the diffusion field of the γ′ rafts.

Guinier-preston zone, quasicrystal and long-period stacking ordered structure in Mg-based alloys, a review

Both the solid solution and precipitation are mainly strengthening mechanism for the magnesium-based alloys. A great number of alloying elements can be dissolved into the Mg matrix to form the solutes and precipitates. Moreover, the type of precipitates varies with different alloying elements and heat treatments, which makes it quite difficult to understand the formation mechanism of the precipitates in Mg-based alloys in depth. Thus, it is very hard to give a systematical regularity in precipitation process for the Mg-based alloys. This review is mainly focused on the formation and microstructural evolution of the precipitates, as a hot topic for the past few years, including Guinier-Preston Zones, quasicrystals and long-period stacking ordered phases formed in a number of Mg-TM-RE alloy systems, where TM = Al, Zn, Zr and RE = Y, Gd, Hd, Ce and La.

Shear-activated indentation crack in GaAs single crystal

Cracks induced by 0.049N load indentation on the surface of gallium arsenide single crystal were investigated before and after annealing. The results revealed that the crack initiation essentially relates to the shear deformation during indentation. There were dislocation generation, lattice distortion, and the transformation from crystalline to disordered structure, leading, to the occurrence of an amorphous band in front of the crack tip. After being annealed at 500 degreesC for 60 min, the amorphous band disappeared, and instead, recrystallized grains appeared along the crack-propagation direction. It is reasonable to propose that crack propagation is the result of the decohesion of the amorphous band rather than the direct debonding along a certain atomic plane.

Plastic deformation inhomogeneity in a single crystal nickel-base superalloy

Extension and compression tests at different temperatures with various strain rates have been carried out on DD8 single crystal superalloys with [0 0 1] [1 1 0] and [1 1 1] orientations. The results show that plastic deformation heterogeneity (P-L effect) occurs in the temperature range from 300 to 700 degreesC at the strain rate of 3.3 x 10(-4) S-1 and also in the strain rate range of 1.3 x 10(-4)-5.3 x 10(-4) S-1 at 700 degreesC for the [0 0 1]-oriented single crystal nickel-base superalloy during extension. The dependence of the plastic deformation inhomogeneity on the crystallographic orientations during compression under the strain rate of 1.0 x 10(-4) S-1 at 550 degreesC were investigated, and the results indicate that the tendency to the plastic deformation inhomogeneity decreases in the order of [0 1 1], [0 0 1] and [1 1 1]. The mechanical tests display that this plastic instability may lead to the catastrophic drop in yield strength of the alloys, but can be removed by a new heat treatment, with which the yield strength can be improved