Some new papers of interest

February 20, 2012

Three-dimensional phase-field modeling of martensitic microstructure evolution in steels

HK Yeddu et al

Phase-field simulation of solidification morphology in laser powder deposition of Ti–Nb alloys

V Falla et al

Osmotic convection-driven instability and cellular eutectic growth in binary systems

Tegze and Toth

A new approach to dislocation creep

R Oruganti

High strength-ductility of thin nanocrystalline palladium films with nanoscale twins: On-chip testing and grain aggregate model

M-S Colla et al

Reactive diffusion and stresses in spherical geometry

Erdelyi and Schmitz

Some interesting papers!

January 21, 2012

[1] Nanocrystalline Soft Magnetic Materials at High Temperatures: A Perspective

M A Willard et al

[2] Creep and directional coarsening in single crystals of new γ-γ’ cobalt-base alloys

M S Titus et al

[3] Microstructure of free-standing epitaxial Ni-Mn-Ga films before and after variant reorientation

S R Yeduru et al

[4] Work hardening of polycrystalline Cu with nanoscale twins

L Lu et al

[5] Strengthening austenitic steels by using nano-twinned austenitic grains

K Lu et al

[6] Liquid zinc embrittlement of twinning-induced plasticity steel

C Beal et al

[7] Effect of thickness-mediated misfit strain on the heterophase polydomain structure of epitaxial BiFeO3 films

W Zhang et al

[1] Highly mobile twinned interface in 10 M modulated Ni–Mn–Ga martensite: Analysis beyond the tetragonal approximation of lattice

L Straka et al

The huge strains that Ni–Mn–Ga magnetic shape memory alloys can achieve are usually described in a tetragonal unit cell approximation of a five-layered modulated (10 M) crystal structure. Here we analyze the impact of a slight orthorhombic and monoclinic distortion of the 10 M structure in Ni50.2Mn28.3Ga21.5at.% single crystal. Combining dedicated experiments to probe the microstructure, structure and mechanical properties with calculation using elastic continuum theory, we prove the existence of fine a/b-laminates within modulation macrotwins of the order of 100 micrometers in size. This complex twin microstructure containing a Type II macrotwin interface is associated with an extraordinarily low twinning stress of between 0.05 and 0.3 MPa, while Type I twins exhibit twinning stress of about 1 MPa. The findings provide important guidelines for designing the martensitic microstructure for more efficient actuators.

[2] An analytical description of overall phase transformation kinetics applied to the recrystallization of pure iron

B B Rath and C S Pande

Experimental studies of recrystallization in deformed single crystals of pure iron are described. The results are used to analyze various parameters associated with the time evolution of the volume fraction of the growing phase during phase transformations in this system associated with the phenomena of nucleation and growth. In addition, using the experimental results, the phenomenon has been modeled by a new approach which may provide a different, and possibly more precise, description of the kinetics of the process. The proposed analytical approach uses easily measured metallographic parameters, obtained from a systematic two-dimensional surface examination, to provide a detailed description on the time dependence of nucleation, nucleation rate, growth rate and interfacial migration and compared with the classical approach based on Kolomogrov formalism.

[3] Plastic deformation mechanisms of fcc single crystals at small scales

C Zhou et al

Three-dimensional (3-D) dislocation dynamics simulations were employed to examine the fundamental mechanisms of plasticity in small-scale face-centered cubic single crystals. Guided by the simulation results, we examined two distinct modes of behavior that reflect the dominant physical mechanisms of plastic deformation at small scales. We found that the residence lifetimes of internal dislocation sources formed by cross-slip decrease as the system size decreases. Below a critical sample size (which depends on the initial density of dislocations) the dislocation loss rate exceeds the multiplication rate, leading to the loss of internal dislocation sources. In this case nucleation of surface dislocations is required to provide dislocations for deformation and the “starvation hardening” mechanism becomes the dominant deformation process. When the sample is larger than a critical size multiplication of internal dislocation sources provides the dominant mechanism for plastic flow. As the strain is increased the rising dislocation density leads to reactions that shut off these sources, creating “exhaustion hardening”.

[4] Dislocation density evolution and interactions in crystalline materials

P Shanthraj and M A Zikry

Dislocation density-based evolution formulations that are related to a heterogeneous microstructure and are physically representative of different crystalline interactions have been developed. The balance between the generation and annihilation of dislocations, through glissile and forest interactions at the slip system level, is taken as the basis for the evolution of mobile and immobile dislocation densities. The evolution equations are coupled to a multiple slip crystal plasticity formulation, and a framework is established that relates it to a general class of crystallographies and deformation modes. Specialized finite element (FE) methodologies have then been used to investigate how certain dislocation density activities, such as dislocation density interactions and immobilization, are directly related to strain hardening and microstructure evolution. The predictions are validated with channel die compressed (CDC) experiments, and are consistent with inelastic deformation modes of fcc metals.

[5] Growth of dislocation clusters during directional solidification of multicrystalline silicon ingots

B Ryningen et al

Highly detrimental dislocation clusters are frequently observed in lab-scale as well as industrially produced multicrystalline silicon ingots for solar cell applications. This paper presents an investigation of dislocation clusters and how they develop over the whole height of a pilot-scale ingot. A 12-kg ingot, cast in a pilot-scale directional solidification furnace using a standard slip cast silica crucible and standard coating containing silicon nitride powder, was studied with respect to dislocation clusters. Dislocation clusters originating from grain boundaries were identified and followed from an early stage to the top of the ingot. One possible model for growth and multiplication of the dislocations in the clusters during solidification where slip on the View the MathML source〈1 1 0〉 system must be allowed is described in detail. Another possible mechanism is also discussed.

[6] Appearance of dislocation-mediated and twinning-induced plasticity in an engineering-grade FeMnNiCr alloy

A Geissler et al

By comparing the microstructural and texture evolution with tensile stress–strain response of an Fe–24Mn–7Ni–8Cr (mass%) alloy, a slip-dominated deformation process and, at a later stage of deformation, twinning-induced plasticity are observed. The occurrence of deformation twinning is texture sensitive and occurs only in the 〈1 1 1〉 fibre texture component. Based on these experimental observations, a model is presented, which reflects an orientational and configurational peculiarity of face-centred cubic stacking faults bordered by two Shockley partials. With this model, the onset point of stacking fault growth, i.e. movement of the leading partial and stopping of the trailing partial, is evaluated. This point reflects the formation of twins in the sense that a twin is regarded as an arrangement of stacking faults on every consecutive slip plane. Furthermore, based on the tensile test results, a model-compatible description of the mechanical behaviour is shown and a reasonable stacking fault energy of about 8 mJ m−2 is calculated for the onset of partial dislocation breakaway, i.e. the onset of deformation twinning.

[1] Significance of mechanical twinning in hexagonal metals at high pressure

W Kanitpanyacharoen et al

Diamond anvil cells (DAC) in radial synchrotron X-ray diffraction geometry were used to investigate texture development and identify deformation mechanisms in zinc and osmium at the Advanced Photon Source (APS) and the Advanced Light Source (ALS), respectively. Further experiments on cadmium and hafnium wires were carried out in the Deformation-DIA (D-DIA) multi-anvil press at APS to study the simultaneous effects of pressure, temperature and strain. At room temperature and increasing pressure the c-axis aligns near the compression direction in all hexagonal metals, but with considerable differences. Texture in zinc evolves gradually between 10 and 15 GPa and strengthens as pressure is increased to 25 GPa. In osmium, texture development starts very early (4 GPa). At ambient temperature cadmium and hafnium develop a similar textures as zinc and osmium, respectively. Texture in cadmium evolves gradually with axial shortening to 34%, whereas in hafnium texturing develops immediately after small strains. When hafnium is simultaneously heated to 700 K and deformed in compression, a texture develops with compression axes near View the MathML source. Simulations from a visco-plastic self-consistent (VPSC) polycrystal plasticity model suggest that the gradual texture evolution observed in zinc and cadmium is controlled primarily by View the MathML source basal slip and later accompanied by View the MathML source tensile twinning when the c/a ratio is below View the MathML source. Conversely, early texture development in osmium and hafnium at room temperature is contributed mainly by View the MathML source tensile twinning. However, the View the MathML source texture in hafnium at high temperature is attributed to basal and prismatic slip.

[2] Particle strengthening in fcc crystals with prolate- and oblate-shaped precipitates

B Sonderegger and E Kozeschnik

The prediction of precipitation hardening is a key factor for optimizing strength or creep performance of advanced structural materials. An essential part of this task is a reliable assessment of precipitate distances based on arbitrary discrete size distributions. Up to now, models are available for spherical precipitate shape. The present work advances these approaches to all kinds of spheroids. The results are expressed in compact form as a correction factor to the spherical case, only depending on the precipitate shape.

[3] Continuum modeling of dislocation starvation and subsequent nucleation in nano-pillar compression

A Jerusalem et al

The mechanical behavior of single crystalline aluminum nano-pillars under uniaxial compression differs from coarse-grained Al in that the former is characterized by a smoother transition from elasticity to plasticity. We propose an extension of the phenomenological model of dislocation starvation originally proposed in [Greer and Nix, Phys Rev B, 73:245410 (2006)] additionally accounting for dislocation nucleation. The calibrated and validated continuum model successfully captures the intrinsic mechanisms leading to the transition from dislocation starvation to dislocation nucleation in fcc nano-pillars.

[4] A Crystal-Plasticity Finite Element Method Study on Effect of Abnormally Large Grain on Mesoscopic Plasticity of Polycrystal

Y S Choi and T A Parthasarathy

In order to investigate the effect of an abnormally-large grain on plastic responses of polycrystal, elasto-viscoplastic crystal plasticity finite element simulations were performed using synthetic three dimensional microstructures with and without an abnormally-large grain. Results indicated that abnormally-large grain can be an important feature to cause the instability of mesoscopic plasticity by drawing hot spots of plastic shear. In particular, the maximum slip system shear hot spots are pronounced in the presence of the abnormally-large grain oriented in single slip.

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] 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

[1] Cluster type grain interaction model including twinning for texture prediction: Application to magnesium alloys

Mu et al

[2] Twinning and grain subdivision during dynamic deformation of a Mg AZ31 sheet alloy at room temperature

Dudamell et al

[3] Self-energy of elliptical dislocation loops in anisotropic crystals and its application for defect-free core/shell nanowires

Chu et al