Parameters Of Dislocation Structure Research Articles

The quantitative relationship between non-linear stress-strain behaviour and dislocation structure in martensitic stainless steel

The reversible behaviour of metals at low applied stresses is more complex than the generally assumed linear behaviour. This is primarily because of the reversible nature of dislocation motion leading to a strain contribution known as anelasticity. This work aims to investigate (a) quantification of dislocation structures in industrial grade stainless steels, (b) unloading behaviour, and (c) the fundamentals of reversible and mechanical behaviour occurring below the yield stress. Mechanical testing of martensitic stainless steel (Stavax ESR) was performed in two different modes: incremental plastic deformation and cyclic loading-unloading below the yield stress with a focus on the measurement of small strains and corresponding stresses, occurring in the pre-yield regime. The non-linear reversible behaviour was quantitatively analysed, as opposed to the common approximation of an empirical determination of apparent Young's modulus. The recently proposed pre-yield model has been refined and, for the first time, successfully applied to a complex microstructure such as stainless steel. The quantification of dislocation structure parameters is shown to be an efficient alternative to the conventional experimental methods of quantifying dislocation structure. Further, a unique representation and quantification of the unloading and hysteresis behaviour provides more insight into the material behaviour. Lastly, the little studied microplasticity occurring below the yield stress upon cyclic loading-unloading was determined. Importantly, the physical basis of the model will allow quantification of plastic deformations in the pre-yield region, large enough to be significant in industrial processes.

Evolution of Dislocation Structure Parameters in Deformed Polycrystalline FCC Solid Solutions

The paper presents the research results of the strain hardening and evolution of the dislocation structure parameters in Cu–Mn polycrystalline solid solutions having the average grain size of 20, 120 and 240 μm. The dislocation structure is studied on a transmission electron microscope. The density of scalar dislocations, geometrically necessary dislocations, statistically stored dislocations and misoriented deformation boundaries is measured. It is shown that these parameters change with deformation and depend on the grain size. It is found that the highest degree of accumulation of geometrically necessary dislocations occurs during the formation of the deformation boundaries.

Effect of annealing on dislocation structure parameters of ultrafine-grained copper: comparative study by acoustic and x-ray methods

The acoustic spectroscopy method and X-ray diffraction analysis were used to study the effect of severe plastic deformation (SPD) and subsequent annealing on the parameters of the dislocation structure of ultrafine-grained (UFG) FRTP (Fire Refining Tough Pitch) copper. To this end, the effect of SPD and subsequent annealing on the main parameters of the low-temperature Bordoni acoustic relaxation was studied in detail. Annealing was performed in the temperature range 90 – 430 C, including the primary crystallization temperature of highly deformed copper Tann  135 C. For all states of the samples, the estimates of the density of mobile dislocations m, which contribute to the low-temperature dynamic relaxation, are obtained. Annealing leads to the decrease in the dislocation density from m  1.7·1012 m-2 to m  1.5·1011 m-2. In addition to the dislocation density, an estimate of the 1st order Peierls’ stress 4 P  19 MPa 2.5·10 G    (G – shear modulus) was also obtained from the acoustic measurements. In parallel with the acoustic measurements, the parameters of the dislocation structure of the samples were studied by X-ray diffraction. The values of the lattice parameter, the values of microdeformation, the average size of the coherent scattering region (CSR), and also the average density of dislocations were obtained. It turned out that the X-ray density of dislocations is two orders of magnitude higher than the values obtained from acoustic measurements. At the same time, the data on the effect of annealing obtained by both methods are in qualitative agreement with each other; in particular, a decrease in the dislocation density of the same order of magnitude is observed.

Parameters of Dislocation Structure and Work Hardening of Ni3Ge

ABSTRACTOrientation dependence of the yield stress temperature anomaly in Ni3Ge single crystals with the L12 structure was investigated during compression tests. The measurements were carried out in the 4.2 K-1000 K temperature interval for two orientations of single crystals, [001] and [234]. The dislocation structure was studied by TEM. Quantitative measurements of different parameters of dislocation structure were carried out. The values of the scalar dislocation density, ρ, were determined for different temperatures in the deformation interval from the yield stress up to fracture. Temperature dependence of the friction stress τF (T) and the interdislocation interaction parameter α(T) were also obtained. The change in the fraction of straight dislocations as a function of temperature was analyzed.

Evolution of dislocation structures having various orientations in strained single crystals of the alloy Ni3Ge

The dislocation structure of strained single crystals of Ni3Ge with various orientations is investigated by electron microscopy. The evolution of the dislocation structure parameters is studied as a function of the degree of strain, temperature, and orientation of the single crystals. Analysis of the experimental dependences of the yield stress on the density of dislocations leads to certain conclusions about how various mechanisms for dislocation drag make temperature-dependent contributions to the deforming stress, and about the nature of the thermal hardening of Ni3Ge.

Microstructural study of an irradiated high-nickel alloy by X-ray line profile analysis and TEM observations

The microstructure was investigated in previously deformed specimens of a high-nickel alloy with four levels of initial dislocation density, both before and after irradiation in BOR-60 reactor at 370–400°C and three displacement rates. The network dislocation density, dislocation loop radius and loop number density were determined by both X-ray line profile analysis and TEM observations. The dependence of dislocation structure parameters on irradiation conditions and initial network density was obtained.

Investigation of the dislocation structure parameters in single crystals by X-ray method

Investigation of the dislocation structure parameters in single crystals by X-ray method

Determination of Dislocation Structure Parameters from Data on the Amplitude Dependence of Internal Friction

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Evolution of the dislocation structure and the hardening mechanisms of multiple slip oriented Ni3Ge single crystals

We studied the evolution of the dislocation structure of Ni3Ge single crystals deformed by compression at room temperature. It is shown that the distribution of dislocations is spatially uniform in the studied alloy. The uniformity in the dislocation distribution is produced by relatively high amounts of the frictional stresses of the dislocations. On the basis of the obtained values of the dislocation structure parameters, the contributions of the various mechanisms in the dislocation drag are determined. It is shown that the resistance to deformation is determined primarily by overcoming reactive and unreactive “forest” dislocations, the total contribution of which is 0.9 of the applied stress.