Instantaneous Creep in Face-centered Cubic Metals at Ultra- low Strain Rates by a High-resolution Strain Measurement

Instantaneous creep in face-centered cubic metals, 5N Al（99.999%）, 2N Al （99%） and 4N Cu （99.99%） with different grain sizes, was firstly investigated by sudden stress-change experiments at ultra- low strain rates ε ≤10-10 s-1 and temperature T 〈 0.32 Tn. The experimental results indicate that the observed instantaneous creep is strongly dependent on grain size, the concentration of impurity, and stacking fault energy. Creep in high-purity aluminum, 5N Al, with a very large grain size, d 〉 1600μm, shows non-viscous behavior, and is controlled by the recovery of dislocations in the boundary of dislocation cells. On the other hand, for 5N A1 with a small grain size, d=30μm, and low-purity aluminum, 2N A1, with d8= 25μm, creep shows viscous behavior and may be related to ＇low temperature grain boundary sliding＇. For high-purity copper, 4N Cu, with d= 40 grn and lower stacking fault energy, creep shows a non-viscous behavior, and is controlled by the recovery process of dislocations. For all of the samples, creep shows anelastic behavior.

The criterion of anomalous slip at OK in body centered cubic metals

It is generally accepted that anomalous slip(AS) takes place by hexagonal dislocation networks(HDNs) in body centered cubic(BCC) metals,but the role of the HDN formation process in AS has rarely been investigated so far.In this work,the critical yield conditions of the HDNs and isolated dislocations were first calculated,respectively,by molecular statics simulations in two BCC metals.Based on these data,a novel mechanism,entitled as the "conjugated dislocation sources"(CDS),to analyze the formation of the HDNs was proposed for the first time and then incorporated into the criterion of the occurrence of AS.Our prediction is in agreement with experimental observations.Contrary to previous study,it has been revealed that the multiplication of isolated screw dislocations involved in AS has to be considered for correctly understanding the AS origin.

Size effect for achieving high mechanical performance body-centered cubic metals and alloys

Submicron and nanostructured body-centered cubic(BCC) metals exhibit unusual mechanical performance compared to their bulk coarse-grained counterparts, including high yield strength and outstanding ductility. These properties are important for their applications in micro-, nano-and even atomic-scale devices as well as for their usages as components for enhancing the performances of structural materials. One aspect of the unusual mechanical properties of small-sized BCC metals is closely related to their dimensional confinement. Decreasing the dimensions of single crystalline metals or the grain sizes of polycrystalline metals contributes significantly to the strengthening of the small-sized BCC metals.In the last decade, significant progress has been achieved in understanding the plasticity and deformation behaviors of small-sized BCC metals. This paper aims to provide a comprehensive review on the current understanding of size effects on the plasticity and deformation mechanisms of small-sized BCC metals. The techniques used for in situ characterization of the deformation behavior and mechanical properties of small-sized samples are also presented.

Mechanism transition of cross slip with stress and temperature in face-centered cubic metals

A<110>/2 screw dislocation is commonly dissociated into two <112>/6 Shockley partial dislocations on{111} planes in face-centered cubic metals.As the two partials are not purely screw,different mechanisms of cross-slip could take place,depending on the stacking fault energy,applied stress and tempe rature.It is crucial to classify the mechanisms of cross-slip because each mechanism possesses its own reaction path with a special activation process.In this work,molecular dynamics simulations have been performed systematically to explore the cross-slip mechanism under different stresses and temperatures in three different metals Ag,Cu and Ni that have different stacking fault energies of 17.8,44.4 and 126.8 mJ/m^2,re spectively.In Ag and Cu with low stacking fault energy,it is observed that the cross-slip mechanism of screw dislocations changes from the Fleischer obtuse angle(FLOA),to the Friedel-Escaig(FE),and then to the FL acute angle(FLAA) at low temperatures,with increasing the applied stress.However,when the temperature increases,the FE mechanism gradually becomes dominant,while the FLAA only occurs at the high stress region.In particular,the FLOA has not been observed in Ni because of its high stacking fault energy.

ISOTROPIC YIELD BEHAVIORS OF BCC AND FCC METALS

Isotropic yield surfaces of bcc and fec metals have been simulated with TBH theory.Using the five-dimensional stress space proposed recently by Lequeu et al.,the yield subsurfaces on any two of the three shear stresses can be considered as the inscribed circles of their corresponding subsurfaces on n plane.Based on this concept,Hershey,Hosford and Hill (HHH) yield func- tion has been transformed into the formula where the parameters are uniquely determined by two crystallographic factors:the mean Taylor factor in plane strain (M_p) and the critical shear stress (τ_) on slip systems.

Study on Ultra-Short Laser Pulse Ablation of Metals by Molecular Dynamics Simulation

The dynamical progresses involved in ultra-short laser pulse ablation of face-centered cubic metals under stress confinement condition are described completely using molecular dynamics method. The laser beam absorption and thermal energy turning into kinetics energy of. atoms are taken into account to give a detailed picture of laser metal interaction. Superheating phenomenon is observed, and the phase change from solid to liquid is characterized by a destroyed atom configuration and a decreased number density. The steep velocity gradients are found in the systems of Cu and Ni after pulse in consequence of located heating and exponential decrease of fluences following the Lambert-Beer expression. The shock wave velocities are predicted to be about 5 000 m/s in Cu and 7 200 m/s in Ni. The higher ablation rates are obtained from simulations compared with experimental data as a result of a well-defined crystalline surface irradiated by a single pulse. Simulation results show that the main mechanisms of ablation are evaporation and thermoelastic stress due to located heating.

First-principles calculation of the structural,electronic,and magnetic properties of cubic perovskite RbXF3(X = Mn,V,Co,Fe)

First-principles calculations by means of the full-potential linearized augmented plane wave method using the generalized gradient approximation with correlation effect correction（GGA＋U） within the framework of spin polarized density functional theory（DFT＋U） are used to study the structural,electronic,and magnetic properties of cubic perovskite compounds RbXF3（X = Mn,V,Co,and Fe）.It is found that the calculated structural parameters,i.e.,lattice constant,bulk modulus,and its pressure derivative are in good agreement with the previous results.Our results reveal that the strong spin polarization of the 3d states of the X atoms is the origin of ferromagnetism in RbXF3.Cohesive energies and the magnetic moments of RbXF3 have also been calculated.The calculated electronic properties show the half-metallic nature of RbCoF3 and RbFeF3,making these materials suitable for spintronic applications.

Local inhomogeneity of mechanical properties in stir zone of friction stir welded AA1050 aluminum alloy

The local inhomogeneity of the stir zone in friction stir welded face-centered cubic metal was investigated,which has multiple activated slip systems during plastic deformation,by selecting commercial AA1050 aluminum alloy as an ideal experimental material.The local inhomogeneity was evaluated by uniaxial tensile tests using small samples with a 1 mm gauge length.The corresponding microstructural parameters such as grain size,misorientation angle distribution,and micro-texture,were quantified by the backscattered electron diffraction technique.A comprehensive model was used to reveal the microstructure−mechanical property relationship.The experimental results showed that the uniaxial tensile property changes significantly across the weld.The maximum ultimate tensile strength(UTS)occurred in the center of the stir zone,which was 99.0 MPa.The weakest regions were located at the two sides of the stir zone.The largest difference value in UTS reached 14.9 MPa,accounting for 15%of the maximum UTS.The analysis on the structure−mechanical property relationship suggests that the micro-texture change with the location formed during the rotational material flow is the main reason for the local inhomogeneity.

Brazed joints of CBN grains and AISI 1045 steel with AgCuTi-TiC mixed powder as filler materials

The brazing process of cubic boron nitride （CBN） grains and AISI 1045 steel with AgCuTi-TiC mixed powder as a filler material was carried out.The joining strength and the interfacial microstructure were investigated.The experimental results indicate that the spreading of the molten filler material on AISI 1045 steel is decreased with the increase of TiC content.A good interface is formed between the TiC particulates and AgCuTi alloy through the wetting behavior.In the case of AgCuTi＋16wt% TiC,the strength of the brazed steel-to-steel joints reached the highest value of 95MPa dependent upon the reinforcement effect of TiC particles within the filler layer.Brazing resultants of TiB2,TiB,and TiN are produced at the interface of the CBN grains and the AgCuTi-TiC filler layer by virtue of the interdiffusion of B,N,and Ti atoms.

STUDY ON FRACTOGRAPHY OF TRANSGRANULAR SCC OF 70Cu-3OZn IN AN AMMONIACAL SOLUTION

The microfractography of transgranular stress corrosion cracking (TSCC) of 70Cu-30Zn a-brass in ammoniacal solution was studied. The observations indicate that on a very microscale, the crack path of TSCC of or-brass follows {111} planes. The crack path very often alternates between {111} Planes to result in 'cleavage-like'facet, the usual average orientation of which is {110} with preferential microscopic crack propagation in (100) and (112) directions. The average orientation of wide secondary facets is often close to {100}. The size of {111} microfacets increases with incrmsing stress intensity K, which indicates that the microscopic crack path follows {111} planes on which some localized slip has occurred. Possible TSCC mechanisms which appear to be consistent with the microfraphic features observed in the present study are also discussed.