Dislocation Storage Capacity Research Articles

Microstructure, Texture and Mechanical Properties Anisotropy of Ni-16Cr and Ni-16Cr-16Mo Solid Solution Alloys in Hot Rolled and Annealed Condition

Present work describes microstructure-texture-mechanical property anisotropy correlation of Ni-16Cr and Ni-16Cr-16Mo alloys in hot rolled and annealed condition. The high and low SFE values associated with Ni-16Cr and Ni-16Cr-16Mo alloys result in partial recrystallization after recovery and presence of twins in recrystallized grains, respectively. Both the alloys display two-slopes in the true plastic stress-strain curves and follow Ludwigson relation. However, the deformation mechanisms of both the alloys are different. Sample orientation dependent ductility of these alloys has been explained based on dislocation storage capacity and dynamic recovery coefficient using Kock-Mecking-Estrin analysis.

Mechanical Properties Anisotropy of Isothermally Forged and Precipitation Hardened Inconel 718 Disk

The present work describes the tensile and cyclic flow behavior of the as-received disk of Inconel 718 in solution treated and precipitation hardened condition at different locations and orientations. The disk shows moderately high values of anisotropy index indicating significant difference in uniform true strain along radial and tangential orientations. The tensile true stress–plastic strain curves exhibit two slopes defined by Ludwigson relation [\( \sigma = K_{1} \varepsilon^{{n_{1} }} + \exp (K_{2} + n_{2} \varepsilon ) \)]. The low-strain regime during tensile test is associated with low-strain localization between broad annealing twins and slips, while high-strain regime is related to the presence of large volume fraction of deformation twins and high-strain localization between narrow deformation twins. It appears that both the γ′ and γ″ play a critical role during low deformation regime while the role of γ″ precipitates becomes significant in high-strain regime. The stabilized cyclic true stress–plastic strain curves follow Ludwik relationship (σ = Ke n ) similar to that of high-strain regime of two-slope tensile curves. The true stress–strain curves show softening during cyclic test in comparison to that of monotonic condition and are independent of sample orientations and locations. The lower degree of cyclic softening associated with radial-oriented sample can be attributed to the alignment of δ-phase precipitates normal to the loading direction. The low ductility and low work-hardening exponent of radial-oriented sample in web region have been explained based on the dislocation storage capacity and dynamic recovery coefficient using Kock–Mecking–Estrin analysis.

Mechanical Properties Anisotropy of Cold-Rolled and Solution-Annealed Ni-Based Hastelloy C-276 Alloy

This work describes a correlation among texture, in-plane anisotropy in tensile properties, and yield locus in Ni-based Hastelloy C-276 alloy. The alloy exhibits moderate values of in-plane anisotropy and anisotropy index, which has been attributed to the presence of moderate overall intensity of texture. The alloy displays two slopes in true plastic stress–strain curve and follows a Ludwigson relation. At low plastic strains, the sample displays the presence of annealing twins and less strain localization at grain boundaries, while the formation of deformation twins and high strain localization within the deformation twins and at the grain boundaries are observed in a high-strained region. The 45-deg and 67.5-deg orientation samples show relatively low ductility and low work-hardening exponent. This has been explained based on dislocation storage capacity and dynamic recovery coefficient using Kock–Mecking–Estrin analysis.

Combination of in situ straining and ACOM TEM: A novel method for analysis of plastic deformation of nanocrystalline metals

Nanocrystalline metals are expected to exhibit different deformation mechanisms when compared to their coarse grained counterparts because the dislocation storage capacity decreases and the grain boundary mediated processes become more pronounced with decreasing grain size. As a new approach to directly image and quantify the plastic deformation processes in nanocrystalline thin films, a combination of automated crystal orientation mapping in microprobe STEM mode with in situ straining inside a TEM was developed. ACOM-TEM closes the gap between EBSD and BF/DFTEM by providing full orientation maps with nanometer resolution. The novel combination with in situ straining provided for the first time the possibility to directly image and quantify the structural changes of all crystallites in the ensemble of a thin film at the nanometer scale during mechanical deformation. It was used to characterize the metallographic changes during tensile deformation of a nanocrystalline Au thin film prepared by magnetron sputtering. The investigation of the grain size, grain orientation and twinning on a global (grain average over a micron sized area) and local (assembly of selected grains) scale allowed for the development of an in depth picture of the deformation processes. Grain boundary motion and local grain rotation were two of the processes acting to dissipate the applied stress. Additionally, twinning/detwinning occurred simultaneously during straining. These processes, which occurred locally already in the micro-plastic regime, led to global grain growth starting at the transition to the macro-plastic deformation regime.

Tensile and fatigue properties of a cast aluminum alloy with Ti, Zr and V additions

The development of aluminum alloys for automotive powertrain applications is in high demand due to the required weight reduction and fuel efficiency. The aim of this study was to evaluate the microstructure and mechanical properties of a newly developed Al–7%Si–1%Cu–0.5%Mg cast alloy with further additions of Ti, Zr and V. The microstructure of the alloys consisted of Al dendrites surrounded by Al–Si eutectic structures with Mg/Cu/Fe-containing Si particles, and contained nano-sized trialuminide precipitates in the Ti/Zr/V added alloys. The alloys had a significantly (60–87%) higher yield strength but lower ductility than A356-T6 and 319-T6 alloys. With the addition of Ti/Zr/V both monotonic and cyclic yield strengths increased, but ductility and hardening capacity decreased due to reduced dislocation storage capacity caused by stronger interactions between dislocations and trialuminide precipitates. The Zr/V-modified alloy had a longer fatigue life, and all the alloys exhibited cyclic stabilization at low strain amplitudes and cyclic hardening at higher strain amplitudes. With increasing strain amplitude, the extent of cyclic hardening increased. Both cyclic yield strength and cyclic strain hardening exponent were higher than the corresponding monotonic yield strength and strain hardening exponent, indicating that a stronger cyclic hardening ability of the alloys developed. Fatigue cracks were observed to initiate at near-surface defects, and crack propagation was mainly characterized by the formation of fatigue striations together with secondary cracks.

Tensile Properties and Strain Hardening Behavior of a Friction Stir Welded AA2219 Al Alloy

Microstructures, tensile properties and work hardening behavior of friction stir welded (FSWed) AA2219-T62 aluminum alloy (in its one-third bottom slice of a 20 mm thick plate) were evaluated at different strain rates. While the yield strength was lower in the FSWed joint than in the base metal, the ultimate tensile strength of the FSWed joint approached that of the base metal. In particular the FSW resulted in a significant improvement in the ductility of the alloy due to the prevention of premature failure caused by intergranular cracking along the second-phase boundary related to the presence of the network-like grain boundary phase in the base metal. While stage III and IV hardening occurred after yielding in both base metal and FSWed samples, the FSW led to stronger hardening capacity and higher strain hardening exponent and rate due to the enhanced dislocation storage capacity associated with the microstructural change after FSW. The fracture surface of the FSWed joint was mainly characterized by dimples and tearing ridges along with micropores.

Grain growth and dislocation density evolution in a nanocrystalline Ni–Fe alloy induced by high-pressure torsion

The structural evolution of a nanocrystalline Ni–Fe alloy induced by high-pressure torsion (HPT) was investigated. HPT-induced grain growth occurred via grain rotation and coalescence, forming three-dimensional small-angle sub-grain boundaries. Further deformation eliminates the sub-grain boundaries from which dislocations glide away on different {1 1 1} planes. A significant number of these dislocations come together to form Lomer–Cottrell locks that effectively increase the dislocation storage capacity of the nanocrystalline material . These observations may help with developing strong and ductile nanocrystalline materials.

Influence of stacking fault energy on deformation mechanism and dislocation storage capacity in ultrafine-grained materials

Partial dislocation emission from grain boundaries in metals with medium-to-high stacking fault energies is observed primarily in the grain size range of a few tens of nanometers. Here we report that a reduction in the stacking fault energy permits the emission of partial dislocations from grain boundaries in ultrafine-grained materials with grain sizes significantly larger than 100 nm and this produces twinning. Such twins are effective in increasing the dislocation storage capacity, which may be used to improve the ductility.

Deformation behavior and mechanisms of a nanocrystalline multi-phase aluminum alloy

A nanocrystalline (nc) Al–Fe–Cr–Ti alloy containing 30 vol.% nc intermetallic particles has been used to investigate deformation behavior and mechanisms of nc multi-phase alloys. High compressive strengths at room and elevated temperatures have been demonstrated. However, tensile fracture strengths below 300 °C are lower than the corresponding maximum strengths in compression. Creep flow of the nc fcc-Al grains is suppressed even though rapid dynamic recovery has occurred. It is argued that the compressive strength at ambient temperature is controlled by propagation of dislocations into nc fcc-Al grains, whereas the compressive strength at elevated temperature is determined by dislocation propagation as well as dynamic recovery. The low tensile fracture strengths and lack of ductility at temperatures below 300 °C are attributed to the limited dislocation storage capacity of nanoscale grains. Since the deformation of the nc Al-alloy is controlled by dislocation propagation into nc fcc-Al grains, the smaller the grain size, the higher the strength. This new microstructural design methodology coupled with ductility-improving approaches could present opportunities for exploiting nc materials in structural applications at both ambient and elevated temperatures.