In situ atomic scale mechanisms of strain-induced twin boundary shear to high angle grain boundary in nanocrystalline Pt

Twin boundary can both strengthen and soften nanocrystalline metals and has been an important path for improving the strength and ductility of nano materials. Here, using in-lab developed double-tilt tensile stage in the transmission electron microscope, the atomic scale twin boundary shearing process was in situ observed in a twin-structured nanocrystalline Pt. It was revealed that the twin boundary shear was resulted from partial dislocation emissions on the intersected {111} planes, which accommodate as large as 47% shear strain. It is uncovered that the partial dislocations nucleated and glided on the two intersecting {111} slip planes lead to a transition of the original symmetric tilt ∑3/(111) coherent twin boundary into a symmetric tilt ∑9/(114) high angle grain boundary. These results provide insight of twin boundary strengthening mechanisms for accommodating plasticity strains in nanocrystalline metals

Modification of Eutectic Si in Al-Si-(Ba) Alloy by Inducing a Novel 9R Structure in Twins

The change of twinning morphology plays an important role in the modification of Al-Si alloys, which are widely used in industrial applications. However, the interpretation of this change is still insufficient. In this work, the microstructure of twins was investigated in two kinds of Al-Si alloys with different additions of Ba using high-resolution transmission electron microscopy (HRTEM). Unlike the normal {111} twin that exists in Ba-free alloy, discontinuous twins and multiple twins were observed in the Ba-containing alloy. In addition, the 9R structure formed by the dissociation of twins was firstly observed at the turning of discontinuous twins and the intersection of multiple twins in Al-Si alloys

Plastic deformation through dislocation saturation in ultrasmall Pt nanocrystals and its in situ atomistic mechanisms

The atomic-scale deformation dynamic behaviors of Pt nanocrystals with size of similar to 18 nm were in situ investigated using our homemade device in a high-resolution transmission electron microscope. It was discovered that the plastic deformation of the nanosized single crystalline Pt commenced with dislocation "appreciation" first, then followed by a dislocation "saturation" phenomenon. The magnitude of strain plays a key role on dislocation behaviors. At the early to medium stage of deformation, the plastic deformation was controlled by the full dislocation activities accompanied by the formation of Lomer dislocation locks from reaction of full dislocations. When the strain increased to a significant level, stacking faults and extended dislocations as well as Lomer-Cottrell locks appeared. The Lomer-Cottrell locks can unlock through transferring into Lomer dislocation locks first, and then Lomer dislocation locks were destructed under high stresses. The very high density dislocations and the frequent dislocation reactions through Lomer dislocations and Lomer-Cottrell locks may lead to work hardening in nanosized Pt

Atomic structure of gamma '' phase in Mg-Gd-Y-Ag alloy induced by Ag addition

Mg-Gd based alloys are typical high strength magnesium alloys owing to their outstanding age hardening behavior. The mechanical strength of aged Mg-Gd alloys is enhanced by high density prismatic-shaped precipitates in Mg matrix. Moreover, the addition of Ag has been found to enhance the strength of Mg-Gd based alloys further by forming a plate-shaped precipitate, named as gamma '', on basal planes of Mg. However, the structure of this precipitate is not well understood due to the complex alloying system. In this work, the atomic structure of gamma '' phase is investigated using atomic-resolution high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM). Based on the accurate atomic structure from three independent directions, e.g. in [0001](Mg), [110](Mg) and [010](Mg) zone axes, the unit cell of gamma '' is well established. The gamma '' phase has a hexagonal structure, and its lattice parameters are a = 0.55 nm and c = 0.42 nm. The orientation relationship between gamma '' phase and Mg matrix is (0001)(gamma '')//(0001)(Mg) and (gamma '')//(Mg)

Modification of Eutectic Si in Al-Si-(Ba) Alloy by Inducing a Novel 9R Structure in Twins

The change of twinning morphology plays an important role in the modification of Al-Si alloys, which are widely used in industrial applications. However, the interpretation of this change is still insufficient. In this work, the microstructure of twins was investigated in two kinds of Al-Si alloys with different additions of Ba using high-resolution transmission electron microscopy (HRTEM). Unlike the normal {111} twin that exists in Ba-free alloy, discontinuous twins and multiple twins were observed in the Ba-containing alloy. In addition, the 9R structure formed by the dissociation of twins was firstly observed at the turning of discontinuous twins and the intersection of multiple twins in Al-Si alloys

Mechanical Properties and Microstructure Evolution of Mg-Gd Alloy during Aging Treatment

Rare-earth-containing Mg alloys are a group of widely investigated alloys due to the disperse nano-sized precipitations formed during heat treatment. The underlying formation and strengthening mechanisms of precipitation is critical for their industrial applications. In this work, we systematically studied the evolution of precipitations in a Mg-10Gd alloy, based on the atomic-scaled TEM and HAADF-STEM observations. Especially, the in-depth transition mechanism from G.P. Zone to β”, β’, βT and βM is proposed, as well as their relationships with mechanical properties. It is found that blocking effect of precipitations improves the strength significantly, according to the Orowan mechanism. The elliptic cylinder shaped β’ phase, with a base-centered orthorhombic lattice structure, provides significant strengthening effects, which enhance the hardness and ultimate tensile strength from 72 HV and 170 MPa to 120 HV and 300 MPa

Mechanical Properties and Microstructure Evolution of Mg-Gd Alloy during Aging Treatment

Rare-earth-containing Mg alloys are a group of widely investigated alloys due to the disperse nano-sized precipitations formed during heat treatment. The underlying formation and strengthening mechanisms of precipitation is critical for their industrial applications. In this work, we systematically studied the evolution of precipitations in a Mg-10Gd alloy, based on the atomic-scaled TEM and HAADF-STEM observations. Especially, the in-depth transition mechanism from G.P. Zone to β”, β’, βT and βM is proposed, as well as their relationships with mechanical properties. It is found that blocking effect of precipitations improves the strength significantly, according to the Orowan mechanism. The elliptic cylinder shaped β’ phase, with a base-centered orthorhombic lattice structure, provides significant strengthening effects, which enhance the hardness and ultimate tensile strength from 72 HV and 170 MPa to 120 HV and 300 MPa

Reveal the size effect on the plasticity of ultra-small sized Ag nanowires with in situ atomic-scale microscopy

Revealing the atomic-scale deformation mechanisms of metallic nanowires (NWs) is important for their practical application. However, there are few reports providing direct atomic-scale experimental elucidation on those metallic NWs. Here, we conduct serial in situ deformation tests on silver (Ag) nanowires with diameters of 3-11 nm. The in situ atomic-scale observations reveal a transition in the deformation mechanism with a decrease in the diameter of Ag NWs. For the [5 (5) over bar4] and [001] oriented NWs with diameters of similar to 11 nm, the plastic deformation is dominated by full dislocation that involves leading and trailing partial dislocations, whereas the full or extended dislocations are rarely observed in the NWs with diameters in the range of similar to 5-8 nm, and their plastic deformation is governed by SF generation and annihilation. Moreover, for the [(1) over bar 11] oriented NWs, 60 degrees mixed and pure edge dislocations are frequently observed when the diameter is approximately 5 nm and the plastic deformation is accommodated by relative slip between two adjacent {111} planes for NWs with diameters below similar to 3 nm. These results indicate that the plastic deformation not only depends on the size of NWs but also can be significantly impacted by the loading orientation. (C) 2016 Elsevier B.V. All rights reserved