Twins!

September 30, 2011

[1] Transformation of dislocations during twin variant reorientation in Ni-Mn-Ga martensite structures

Szczerba and Szczerba

The transformation of perfect dislocation configurations during twin variant reorientation in Ni-Mn-Ga martensites was analyzed by means of the correspondence matrix method. It was found that the dislocations may change remarkably the length of the Burgers vectors and hence the self-energy up to several percent of their initial value. It is suggested that this may affect the mechanical condition of operation of the mechanism responsible for twin variant reorientation.

[2] Strengthening and Toughening of Magnesium Alloy by {10-12} Extension Twins

Y Xin et al

The mechanisms of strengthening and toughening of hot-rolled AZ31 magnesium alloy by {10-12} extension twins during strain path changed recompression are studied in current paper. Under favorite orientation, {10-12} extension twinning can occur in {10-12} twins generated by pre-compression along rolling direction. Grain refinement by extension twins dramatically enhances both yield stress and peak stress of reloading along transverse direction without any degradation in elongations, while the effect is closely related with the level of pre-strain.

Why do crystalline nanoparticles agglomerate with low misorientations?

E Rabkin et al

We considered the role of a “grain boundary- free surface” triple line in adhesive contact formation between spherical nanoparticles and calculated the critical value of the triple line energy above which the nanoparticles do not stick together. Based on this result, we propose a new nanoparticles agglomeration mechanism, which explains the formation of large agglomerates of crystallographically aligned nanoparticles during nano-powder processing.

Is it just me or twins are the new rage in the materials/metallurgical community?

[1] Boundaries and interfaces in ultrafine grain composites

Y Li et al

The present study was motivated by two questions. First, what are the characteristics of grain and phase boundaries in a nanostructured material containing multiple phases? Second, what is the influence of these interfaces on mechanical behavior? Accordingly, a three-constituent Al 5083/B4C ultrafine grain (UFG) composite, consisting of a coarse grain (CG) phase (1–2 μm), an UFG phase (100–200 nm) and B4C particles (∼0.7 μm), was selected for study. Interest in this particular Al 5083/B4C system stems from its hierarchical architecture, which comprises multiple scales, as well as from a reported yield strength of 1145 MPa. The associated grain boundaries (GB) and interfaces were investigated by transmission electron microscopy (TEM), high-resolution TEM, energy dispersive X-ray spectroscopy and electron energy loss spectroscopy methods. The role of high/low-angle GB, equilibrium and non-equilibrium GB within and between the CG and UFG regions, twin boundaries, twist transition boundaries and impurity segregation at GB in strengthening mechanisms is discussed.

[2] Bimodal nanocrystallization of NiTi shape memory alloy by laser shock peening and post-deformation annealing

C Ye et al

In this paper, surface nanocrystallization of NiTi intermetallic alloy by a novel method is reported. The NiTi alloy is processed by laser shock peening (LSP) and controlled annealing. The microstructure of the NiTi alloy after processing is characterized by transmission electron microscopy. At the top surface of the material, a nanostructure with bimodal grains is obtained. The mechanism of the formation of the bimodal microstructure is discussed. At the material subsurface, deformation twins are generated by LSP and retained after controlled annealing. Tensile test results showed that both strength and ductility are significantly improved through LSP and controlled annealing.

[3] Deformation, structural changes and damage evolution in nanotwinned copper under repeated frictional contact sliding

A Singh et al

Nanotwinned metals have the potential for use as structural materials by virtue of having a combination of high strength as well as reasonable ductility and damage tolerance. In the current study, the tribological response of nanotwinned copper has been characterized under conditions of repeated frictional sliding contact with a conical tip diamond indenter. Pure ultrafine-grained copper specimens of fixed grain size (∼450 nm), but with three different structural conditions involving relatively high, medium and negligible concentrations of nanotwins, were studied. The effects of twin density and number of repetitions of sliding cycles on the evolution of friction and material pile-up around the diamond indenter were studied quantitatively by depth-sensing instrumented frictional sliding. Cross-sectional focused ion beam and scanning electron microscopy observations were used to systematically monitor deformation-induced structural changes as a function of the number of passes of repeated frictional sliding. Nanoindentation tests at the base of the sliding tracks coupled with large-deformation finite-element modeling simulations were used to assess local gradients in mechanical properties and deformation around the indenter track. The results indicate that friction evolution as well as local mechanical response is more strongly influenced by local structure evolution during repeated sliding than by the initial structure. An increase in twin density is found to result in smaller pile-up height and friction coefficient. Compared to the low-density nanotwinned metal, high-density nanotwinned copper showed significantly higher resistance to surface damage and structural changes, after the initial scratch. However with an increase in the number of sliding passes, the friction coefficient and rate of increase of pile up for all specimens acquire a steady value which does not change significantly in subsequent scratch passes. The frictional sliding experiments also lead to the striking result that copper specimens with both a high and low density of nanotwins eventually converge to a similar microstructure underneath the indenter after repeated tribological deformation. This trend strongly mirrors the well-known steady-state response of microcrystalline copper subjected to uniaxial cyclic loading. General perspectives on contact fatigue response of nanotwinned copper are developed on the basis of these new findings.

[4] Dislocation decorrelation and relationship to deformation microtwins during creep of a γ′ precipitate strengthened Ni-based superalloy

R R Unocic et al

The evolution of microtwins during high temperature creep deformation in a γ′ strengthened Ni-based superalloy has been investigated through a combination of creep testing, transmission electron microscopy (TEM), theoretical modeling, and computer simulation. Experimentally, microtwin nucleation sources were identified and their evolution was tracked by characterizing the deformation substructure at different stages of creep deformation. Deformation is highly localized around stress concentrators such as carbides, borides and serrated grain boundaries, which act as sources of a/2〈1 1 0〉 matrix-type dislocations. Due to fine channels between the γ′ particles, coupled with a low γ matrix stacking fault energy, the a/2〈1 1 0〉 matrix dislocations dissociate into a/6 〈1 1 2〉 Shockley partials, which were commonly observed to be decorrelated from one another, creating extended intrinsic stacking faults in the γ matrix. Microtwins are common and form via Shockley partial dislocations, cooperatively shearing both the γ and γ′ phases on adjacent {1 1 1} glide planes. The TEM observations lead directly to an analysis of dislocation–precipitate interactions. The important processes of dislocation dissociation and decorrelation were modeled in detail through phase field simulations and theoretical analyses based on Orowan looping, providing a comprehensive insight into the microstructural features and applied stress conditions that favor the microtwinning deformation mode in γ′ strengthened Ni-based superalloys.

[5] Helium bubble precipitation at dislocation networks

J Hetherly et al

We report on a study of nanoscale He bubble precipitation and growth at a twist grain boundary in two fcc material. Experimentally, the twist boundary in Au captures all He in the sample, forming equal size bubbles at the dislocation intersection junctions. Simulations in Cu reveal a complex structure of the interface and different He pressure in the interface bubbles compared to bulk, providing an explanation to the high efficiency of the boundary to capture all He in the sample.

[6] Formation mechanism of novel two-dimensional single crystalline dendritic copper plates in an aqueous environment

X Xu et al

This paper reports on the creation of a unique form of single crystalline two-dimensional (2-D) copper microdendritic plates and proposes a new crystal growth mechanism in an aqueous environment. The crystals are formed via reduction of CuSO4 with starch in aqueous solution. The 2-D crystals are typically ∼300 nm thick and ∼50 μm wide, and consist of rhombic petals of (1 1 1) planar orientation. The plates are found to nucleate at the centre in polyhedral shapes and grow outwards along zigzag growth paths along the View the MathML source directions. Formation of such a crystal morphology is attributed to three different growth controlling criteria. The formation of polyhedral crystalline nuclei is controlled by the Gibbs–Wulff theorem, driven by the need to minimize the total surface energy for nucleation; growth of the crystal to form a 2-D rosette morphology is controlled by the planar expansion kinetics of low surface energy crystallographic planes; the zigzag dendritic growth pattern is dictated by the Cu2+ concentration gradient at the crystal growth fronts in the solution.

[7] Strain effect on phase transitions of BaTiO3 nanowires

J J Wang et al

The effects of strain on the phase transitions of BaTiO3 nanowires taking into account three components of polarization are studied by thermodynamic analysis based on the Landau theory. Similar to the strain effect on phase transitions in thin films, the mismatch strain between the nanowire and substrate governs the Curie temperature. The complete misfit strain–temperature phase diagram shows six stable ferroelectric phases for BaTiO3 nanowires under different strain and temperature conditions.

[1] Dislocation interactions and low-angle grain boundary strengthening

B liu et al

The transmission of an incoming dislocation through a symmetrical low-angle tilt grain boundary (GB) is studied for {1 1 0}〈1 1 1〉 slip systems in body-centered cubic metals using discrete dislocation dynamics (DD) simulations. The transmission resistance is quantified in terms of the different types of interactions between the incoming and GB dislocations. Five different dislocation interaction types are considered: collinear, mixed-symmetrical junction, mixed-asymmetrical junction, edge junction, and coplanar. Mixed-symmetrical junction formation events are found not only to cause a strong resistance against the incident dislocation penetration, but also to transform the symmetrical low-angle tilt GB into a hexagonal network (a general low-angle GB). The interactions between the incident dislocation and the GB dislocations can form an array of 〈1 0 0〉 dislocations (binary junctions) in non-coplanar interactions, or a single 〈1 0 0〉 dislocation in coplanar interaction. We study how the transmission resistance depends on the mobility of 〈1 0 0〉 dislocations. 〈1 0 0〉 dislocations have usually been treated as immobile in DD simulations. In this work, we discuss and implement the mobility law for 〈1 0 0〉 dislocations. As an example, we report how the mobility of 〈1 0 0〉 dislocations affects the equilibrium configuration of a ternary dislocation interaction.

[2] Nucleation and growth of the γ′(AlAg2) precipitate in Al–Ag(–Cu) alloys

J M Rosalie et al

Precipitation of the γ′(AlAg2) phase was investigated in Al–Ag(–Cu) alloys using high-resolution transmission electron microscopy and scanning transmission electron microscopy. Precipitation commenced with segregation of Ag to stacking faults, followed by thickening in steps corresponding to single unit cell height ledges. In conjunction with gradual segregation of Ag and Al into ordered layers, this yielded γ′ phase platelets with a thickness of either 2 or 3 × the AlAg2c-lattice parameter. Plates with a thickness of 2c(AlAg2) could not achieve self-accommodation of the shape strain for transformation. Further thickening of the precipitates was slow, despite considerable Ag segregation around the precipitates. Growth by the addition of single unit cell height ledges is expected to lead to an additional shear strain energy barrier to ledge nucleation and this may contribute to a process of nucleation-limited growth.

[3] Bulk eutectic Cu-Ag alloys with abundant twin boundaries

Y Z Tian and Z F Zhang

Abundant growth twin boundaries are found and characterized in two bulk eutectic Cu-Ag alloys which can be obtained conveniently. The statistical electron backscattering diffraction (EBSD) results show that both hetero-twin and cube-on-cube orientation relationships coexist in the eutectic Cu-Ag alloy. The tensile strength of the eutectic alloy increases with decreasing the layer thickness of the Cu/Ag phase. This study provides a potential way to produce bulk eutectic Cu-Ag alloy with abundant twin boundaries for achieving high strength and high ductility.

[4] Effect of Cu addition on the mechanical behavior of austenitic twinning-induced plasticity steel

S Lee et al

The effect of copper additions on the mechanical behavior of a Fe-12%Mn-0.7%C-1.0%Al twinning-induced plasticity steel was investigated by analysis of the mechanical properties obtained in uniaxial tensile tests by means of a physically-based constitutive model. The addition of copper was found to retard the kinetics of twin formation and influenced the type of the serrations on the stress-strain curve. The copper additions also resulted in a remarkable increase in the total elongation without a loss of strength.

Migration of Σ7 tilt grain boundary in Al under an applied external stress

Molodov et al

Stress driven migration of symmetrical tilt grain boundaries with misorientations close to 38.2°, 73.4° and 135.6° rotations related to the same special Σ7{132} CSL boundary was measured by in-situ observations in a scanning electron microscope. Contrary to expectations and theoretical predictions, the investigated boundaries moved under an applied stress, but their motion did not produce shear. The three crystallographically equivalent Σ7 boundaries were found to behave different with respect to migration rate and its temperature dependence.

Micropores in Al-Cu alloys

September 4, 2011

Curvature of micropores in Al–Cu alloys: An X-ray tomography study

Felberbaum and Rappaz

Micropores formed in Al–Cu alloys cast under controlled conditions have been analyzed using high-resolution X-ray tomography. The influence of inoculation conditions, copper content, cooling rate and initial hydrogen content on the morphology of pores has been investigated. Based on the three-dimensional reconstructed shape of the pores, the distribution of curvature was estimated. It is shown that the mean curvature of pores in either non-inoculated or inoculated Al–4.5 wt.%Cu alloys can be as large as 0.35 μm−1 near the end of solidification and can be fairly well approximated by a set of interconnected cylinders growing in between the primary phase dendrites. The so-called “pinching” effect, i.e. the restriction of the pore curvature by the solid network, is a function of the volume fraction of the primary phase and of the secondary dendrite arm spacing. If the fraction of porosity is highly dependent on the initial hydrogen content, the curvature itself is only weakly influenced by this parameter. Based on these results, it is concluded that curvature plays a major role in porosity models and that the analytical pinching model developed by Couturier et al. [1] offers a fairly good and simple approximation of this contribution.

[1] Microstructural and crystallographic characteristics of interpenetrating and non-interpenetrating multiply twinned nanostructure in a Ni–Mn–Ga ferromagnetic shape memory alloy

Cong et al

[2] Insight into the Deformation Mechanisms of α-Fe at Nano-scale

Xie et al