Cooperative Grain Boundary Sliding and Shear Banding at High Strains in Ultrafine Grained and Nanocrystalline Pd Alloys

2010 ◽  
Vol 667-669 ◽  
pp. 649-656
Author(s):  
Yulia Ivanisenko ◽  
Hans Jörg Fecht
Keyword(s):  
High Pressure ◽  
Grain Boundary ◽  
Shear Strain ◽  
High Pressure Torsion ◽  
Mechanical Test ◽  
Shear Bands ◽  
Grain Boundary Sliding ◽  
Materials Testing ◽  
Ultrafine Grained ◽  
Boundary Sliding

Instrumented high pressure torsion, i.e. mechanical test in a torsion mode under high pressure, allows interesting possibility of materials testing, because materials mechanical response can be studied in a practically unlimited shear strain range. We have studied microstructures formed in initially coarse crystalline and nanocrystalline (nc) Pd and its alloys after instrumented HPT up to shear strain 300, and revealed signatures of similar processes occurring in all these materials. In particular, we found traces of cooperative grain boundary sliding in the form of aligned in parallel segments of boundaries of several grains with straightened triple points. Fracture surfaces contained shear bands. Texture measurements revealed lower dislocation activity in nanocrystalline state as compared with coarse crystalline one. Therefore we argue that cooperative grain boundary sliding is an important deformation mechanism at large strain which develops in both ultrafine grained (ufg) and nanocrystalline materials. In nc and ufg materials planes of cooperative grain boundary sliding act as precursors of shear bands and shear occurs along planes formed by numerous grain boundaries.

scholarly journals Using High-Pressure Torsion to Achieve Superplasticity in an AZ91 Magnesium Alloy

Metals ◽  
2020 ◽  
Vol 10 (5) ◽  
pp. 681 ◽  
Author(s):  
Roberto B. Figueiredo ◽  
Terence G. Langdon
Keyword(s):  
High Pressure ◽  
Magnesium Alloy ◽  
Grain Boundary ◽  
High Pressure Torsion ◽  
Grain Boundary Sliding ◽  
Grain Structure ◽  
Strain Rates ◽  
Az91 Magnesium Alloy ◽  
Az91 Magnesium ◽  
Boundary Sliding

An AZ91 magnesium alloy (Mg-9%, Al-1% Zn) was processed by high-pressure torsion (HPT) after solution-heat treatment. Tensile tests were carried out at 423, 523, and 623 K in the strain rate range of 10−5−10−1 s−1 to evaluate the occurrence of superplasticity. Results showed that HPT processing refined the grain structure in the alloy, and grain sizes smaller than 10 µm were retained up to 623 K. Superplastic elongations were observed at low strain rates at 423 K and at all strain rates at 523 K. An examination of the experiment data showed good agreement with the theoretical prediction for grain-boundary sliding, the rate-controlling mechanism for superplasticity. Elongations in the range of 300–400% were observed at 623 K, attributed to a combination of grain-boundary-sliding and dislocation-climb mechanisms.

Structure Evolution and Deformation Mechanisms in Ultrafine-Grained Aluminum under Tension at Room Temperature

2010 ◽  
Vol 667-669 ◽  
pp. 915-920
Author(s):  
Konstantin Ivanov ◽  
Evgeny V. Naydenkin
Keyword(s):  
Grain Boundary ◽  
Room Temperature ◽  
Grain Boundary Sliding ◽  
Deformation Mechanisms ◽  
Structure Evolution ◽  
Dislocation Slip ◽  
Polished Surface ◽  
Angular Pressing ◽  
Ultrafine Grained ◽  
Boundary Sliding

Deformation mechanisms occurring by tension of ultrafine-grained aluminum processed by equal-channel angular pressing at room temperature are investigated using comparative study of the microstructure before and after tensile testing as well as deformation relief on the pre-polished surface of the sample tested. Deformation behavior and structure evolution during tension suggest development of grain boundary sliding in addition to intragrain dislocation slip. Contribution grain boundary sliding to the overall deformation calculated using the magnitude of shift of grains relative to each other is found to be ~40%.

The Role of Diffusion Accommodation and Phase Boundary Wetting in the Deformation Behaviour of Ultrafine Grained Sn-Pb Eutectic

2010 ◽  
Vol 297-301 ◽  
pp. 1002-1009 ◽  
Author(s):  
Faina Muktepavela ◽  
R. Zabels
Keyword(s):  
Grain Boundary ◽  
Diffusion Processes ◽  
Deformation Behaviour ◽  
Grain Boundary Sliding ◽  
Ultrafine Grained ◽  
Interphase Boundaries ◽  
Boundary Sliding ◽  
High Homologous Temperature ◽  
Microscopy Techniques

Mechanical properties, microstructure of the Sn–38wt. %Pb eutectic and the development of deformation - induced diffusion processes on interphase boundaries (IB) were investigated. Experiments were carried out both in deformed and annealed states of eutectic using micro- and nanoindentation, SEM, AFM and optical microscopy techniques. It was found that the deformation of the annealed alloy is localized at the Pb/Sn interphase boundaries and occurs by grain boundary sliding (GBS) accompanied by sintering micropore processes under the action of the capillary forces on the Pb/Sn IB. During severe plastic deformation of Sn-Pb eutectic phase transition in the Sn grain boundary occurs. This deformation-induced process takes place due to the wetting of tin with Pb. These diffusion accommodation processes (sintering and wetting) are facilitated by the low values of the Pb/Sn interphase energy (0.07 J/m2). Wetting is thermodynamically favourable because the condition γgb > 2 γib is satisfied and it is also kinetically allowed due to the relatively high homologous temperature (> 0.5•Tm). The obtained values of the nanohardness and elastic modulus evidence that the IBs in the Sn–Pb eutectic have to be considered as a separate quasi-phase with its own properties.

A Model for the Effects of Strain Rate and Temperature on the Deformation Behavior of Ultrafine-Grained Metals

2011 ◽  
Vol 291-294 ◽  
pp. 1173-1177
Author(s):  
Zi Ling Xie ◽  
Lin Zhu Sun ◽  
Fang Yang
Keyword(s):  
Grain Boundary ◽  
Strain Rate ◽  
Deformation Behavior ◽  
Boundary Diffusion ◽  
Rate Sensitivity ◽  
Grain Boundary Sliding ◽  
Dislocation Slip ◽  
Deformation Response ◽  
Ultrafine Grained ◽  
Boundary Sliding

A theoretical model is developed to account for the effects of strain rate and temperature on the deformation behavior of ultrafine-grained fcc Cu. Three mechanisms, including dislocation slip, grain boundary diffusion, and grain boundary sliding are considered to contribute to the deformation response simultaneously. Numerical simulations show that the strain rate sensitivity increases with decreasing grain size and strain rate, and that the flow stress and tensile ductility increase with either increasing strain rate or decreasing deformation temperature.

The Effect of Grain Boundary Sliding and Strain Rate Sensitivity on the Ductility of Ultrafine-Grained Materials

2010 ◽  
Vol 667-669 ◽  
pp. 677-682 ◽  
Author(s):  
Nguyen Q. Chinh ◽  
Tamás Csanádi ◽  
Jenő Gubicza ◽  
Ruslan Valiev ◽  
Boris Straumal ◽  
...  
Keyword(s):  
Grain Boundary ◽  
Strain Rate ◽  
Strain Rate Sensitivity ◽  
Rate Sensitivity ◽  
Grain Boundary Sliding ◽  
Room Temperature Ductility ◽  
Ultrafine Grained ◽  
Boundary Sliding ◽  
Ultrafine Grained Materials ◽  
High Strain Rate Sensitivity

Most ultrafine-grained (UFG) materials produced by severe plastic deformation (SPD) exibit only limited ductility which is correlated with the low strain rate sensitivity (SRS) of these materials. Recently, it was demonstrated that SPD is capable of increasing the room temperature ductility of aluminum-based alloys attaining elongations up to 150%, together with relatively high strain rate sensitivity. In the present work, additional results and discussions are presented on the effect of grain boundary sliding (GBS) and SRS on the ductility of some UFG metals and alloys. The characteristics of constitutive equations describing the steady-state deformation process are quantitatively analyzed for a better understanding of the effects of grain boundaries and strain rate sensitivity.

Compressive Behavior of Ultrafine-Grained Mg-Zn-Y-Zr Alloy Containing Quasicrystalline Phase

2008 ◽  
Vol 584-586 ◽  
pp. 287-292
Author(s):  
Ming Yi Zheng ◽  
S.W. Xu ◽  
Wei Min Gan ◽  
Kun Wu ◽  
Shigeharu Kamado ◽  
...  
Keyword(s):  
Grain Boundary ◽  
Grain Boundary Sliding ◽  
Quasicrystalline Phase ◽  
Strain Rates ◽  
Temperature Range ◽  
Angular Pressing ◽  
Ultrafine Grained ◽  
Boundary Sliding ◽  
Compressive Testing ◽  
Zr Alloy

An ultrafine-grained (UFG) Mg-5.0wt%Zn-0.9wt%Y-0.2wt%Zr magnesium alloy with a grain size of about 0.8 µm was produced by subjecting the extruded alloy to equal channel angular pressing (ECAP) for 8 passes at 473 K. Compressive testing was performed on the ECAPed alloy in a temperature range from 423 K to 523 K and under strain rates from 1.67×10-3 to 1.67×10-1 s-1. The ultrafine grains of the ECAPed alloy were stable during compression because of the presence of the dispersion of a fine quasicrystal I-phase and of precipitates in the alloy, which restricted grain growth. The activation energy for the compression at the temperature range from 423 K to 523 K is close to the value for grain boundary diffusion in magnesium, indicating that the compressive deformation is mainly controlled by grain-boundary sliding.

Emission Nuclear Gamma-Resonance Spectroscopy of Grain Boundaries in Coarse-Grained and Ultrafine-Grained Polycrystalline Mo

2015 ◽  
Vol 364 ◽  
pp. 147-156 ◽  
Author(s):  
Vladimir V. Popov ◽  
A.V. Sergeev ◽  
A.V. Stolbovsky
Keyword(s):  
High Pressure ◽  
Grain Boundary ◽  
Grain Boundaries ◽  
High Pressure Torsion ◽  
Boundary Segregation ◽  
Grain Boundary Segregation ◽  
Coarse Grained ◽  
Resonance Spectroscopy ◽  
Nuclear Gamma Resonance ◽  
Ultrafine Grained

Grain boundaries in coarse-grained Mo with grain boundaries of recrystallization origin and in ultrafine-grained (UFG) Mo obtained by high pressure torsion have been studied by the emission Mössbauer spectroscopy on the57Co (57Fe) nuclei. It is demonstrated that Co atoms diffuse along grain boundaries by interstitials. The temperature dependence of grain-boundary segregation factor of Co in coarse-grained Mo has been determined. It is shown that the state of Co atoms in grain boundaries and near-boundary areas in UFG Mo differs from that in coarse-grained Mo.

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