Grain Boundary Network Research Articles

Effects of Heat Treatment on Grain-Boundary β-Mg17Al12 and Fracture Properties of Resistance Spot-Welded AZ80 Mg Alloy

The distribution and morphology of β-Mg17Al12 intermetallic phase in resistance spot-welded AZ80 Mg alloy were investigated by means of optical microscopy, scanning electron microscopy, and X-ray diffraction. The influence of intermetallic phase on mechanical strength was studied by tensile shear testing and fractography. The results showed that continuous networks of β-Mg17Al12 formed along grain boundaries in both the nugget and heat-affected zone of the spot-welded AZ80 Mg alloy. Those continuous grain-boundary β-Mg17Al12 networks acted as effective crack propagation paths, which had negative effects on the weld strength. Post-weld solution heat treatment effectively reduced the amount of β-Mg17Al12 and broke the grain-boundary intermetallic networks in both the nugget and heat-affected zone. This significantly increased the weld strength of AZ80 Mg alloy and changed the fracture mode from nugget pull-out in the as-welded condition to through-thickness after heat treatment.

Thermal Cycling Reliability of Sn-Ag-Cu Solder Interconnections—Part 2: Failure Mechanisms

Part 1 of this study focused on identifying the effects of (i) temperature difference (ΔT), (ii) lower dwell temperature and shorter dwell time, (iii) mean temperature, (iv) dwell time, and (v) ramp rate on the lifetime of ball grid array (with 144 solder balls) component boards. Based on the characteristic lifetime, the studied thermal cycling profiles were categorized into three groups: (i) highly accelerated conditions, (ii) moderately accelerated conditions, and (iii) mildly/nonaccelerated conditions. In this work, the observed differences in component board lifetime are explained by studying the failure mechanisms and microstructural changes that take place in the three groups of loading conditions. It was observed that, under the standardized thermal cycling conditions (highly accelerated conditions), the networks of grain boundaries formed by recrystallization provided favorable paths for cracks to propagate intergranularly. It is noteworthy that the coarsening of intermetallic particles was strong in the recrystallized regions (the cellular structure had disappeared completely in the crack region). However, under real-use conditions (mildly/nonaccelerated conditions), recrystallization was not observed in the solder interconnections and cracks had propagated transgranularly in the bulk solder or between the intermetallic compound (IMC) layer and the bulk solder. The real-use conditions showed slight coarsening of the microstructure close to the crack region, but the solder bulk still included finer IMC particles and β-Sn cells characteristic of the as-solidified microstructures. These findings suggest that standardized thermal cycling tests used to assess the solder interconnection reliability of BGA144 component boards create failure mechanisms that differ from those seen in conditions representing real-use operation.

Topological view of the thermal stability of nanotwinned copper

Sputter deposited nanotwinned copper (nt-Cu) foils typically exhibit strong {111} fiber textures and have grain boundary networks (GBN) consisting of high-angle and a small fraction of low-angle columnar boundaries interspersed with crystallographically special boundaries. Using a transmission electron microscope based orientation mapping system with sub-nanometer resolution, we have statistically analyzed the GBN in as-deposited and annealed nt-Cu foils. From the observed grain boundary characteristics and network evolution during thermal annealing, we infer that triple junctions are ineffective pinning sites and that the microstructure readily coarsens through thermal-activated motion of incoherent twin segments followed by lateral motion of high-angle columnar boundaries.

Effect of Distance between Passes in Friction Stir Processing of Magnesium Alloy

Friction stir processing is applied for property improvement of cast alloys for last two decades and many developmental studies were carried out in this topic on various alloys. In the current work, friction stir processing was carried out on rare earth containing magnesium alloy AE42. This alloy was specially developed for automobile application as it has better creep resistance than commercial magnesium alloys. Multi-pass Friction Stir Processing was carried out with varying the distance between passes from 0.5 mm to 2.5 mm using 12 mm shoulder diameter tool. Pin was with conical (tapered) and flat configurations with 3 mm height. After processing, the resultant mechanical and metallurgical properties were evaluated. Microstructure was refined to 5 micron and the secondary phases were made in to tiny pieces of 0.5-1 micron and evenly distributed in the matrix. Continuous network of grain boundary which is reason for poor mechanical properties was eliminated. Mechanical properties were improved by 30%. The variation of mechanical properties of processed material with respect to variation of distance between passes was negligible from 1 mm to 1.5 mm for flat pin tool.

Coarsening kinetics of topologically highly correlated grain boundary networks

We apply phase-field simulations in two dimensions to study the thermal coarsening of grain boundary (GB) networks with high fractions of twin and twin-variant boundaries, which for example are seen in grain-boundary-engineered FCC materials. Two types of grain boundary networks with similar starting special boundary fractions but different topological features were considered as initial conditions for the grain growth simulations. A lattice Monte Carlo method creates polycrystalline microstructures (Reed and Kumar (RK)), which exhibit hierarchical organization of random and special coincidence site lattice boundaries. The other type of microstructures (randomly distributed (RD)) contains random distributions of special boundaries subject only to crystallographic constraints. Under the assumption that random boundaries have larger energy and much higher mobility than special boundaries, simulations show that increasing the initial special boundary fraction in both microstructures slows down grain growth. However, the two starting microstructures exhibit very different behavior in the evolution of GB character and triple junction (TJ) distributions. The RD networks coarsened more slowly than the RK networks with comparable initial fractions of special boundaries. The observed trend in the evolution of the RK microstructures is explained by an extended von Neumann-Mullins analysis. This study demonstrates that the special boundary fraction is not a sufficient indicator of the coarsening behavior of twinned GB networks; the network topology must also be considered to correctly predict the grain growth kinetics.

Influence of Deformation on Precipitation Kinetics in Mg-Tb Alloy

Precipitation effects in age-hardenable Mg-13wt.%Tb alloy were investigated in this work. The solution treated alloy was subjected to isochronal annealing and decomposition of the supersaturated solid solution was investigated by positron annihilation spectroscopy combined with transmission electron microscopy, electrical resistometry, differential scanning calorimetry and microhardness measurements. Peak hardening was observed at 200°C due to precipitation of finely dispersed particles of β phase with the D019structure. Vacancy-like defects associated with β phase particles were detected by positron annihilation. At higher temperatures precipitation of β and subsequently β phase takes place. Formation of these phases lead to some additional hardening and introduces open volume defects at precipitate/matrix interfaces. To elucidate the effect of plastic deformation on the precipitation sequence we studied also a Mg-13wt.%Tb alloy with ultra fine grained structure prepared by high pressure torsion. In the ultra fine grained alloy precipitation of the β phase occurs at lower temperature compared to the coarse grained material and the peak hardening is shifted to a lower temperature as well. This effect can be explained by enhanced diffusivity of Mg and Tb atoms due to a dense network of grain boundaries and high density of dislocations introduced by severe plastic deformation. Moreover, dislocations and grain boundaries serve also as nucleation sites for precipitates. Hence, precipitation effects are accelerated in the alloy subjected to severe plastic deformation.

A new approach for DL-EPR testing of thermo-mechanically processed austenitic stainless steel

Standard test methods such as the electrochemical potentiokinetic reactivation test (EPR) and double-loop EPR test (DL-EPR) are commonly used to characterise sensitisation behaviour in austenitic stainless steels and nickel-based alloys. In this study, the DL-EPR test is augmented by large-area image analysis (IA) to characterise and quantify the networks of attacked grain boundaries. A new analysis approach that is based on a grain boundary cluster parameter is proposed to describe the network of corrosion susceptible grain boundaries, which may be estimated from electron backscatter diffraction (EBSD) data. This method may provide a better assessment of the relative DOS of different heats of austenitic stainless steels.

Localized recrystallization and cracking of lead-free solder interconnections under thermal cycling

Abstract

EFFECT OF ORIGINAL GRAIN SIZE ON THE BOUNDARY NETWORK IN ALLOY 690 TREATED BY GRAIN BOUNDARY ENGINEERING

Effect of original grain size on the grain boundary character distribution(GBCD) in alloy 690 after treatment by grain boundary engineering(GBE) was studied using electron backscatter diffraction(EBSD) and orientation image microscopy(OIM).The proportion of low-ΣCSL grain boundary and grain boundary network after GBE are obviously influenced by the original grain size. Optimized GBE grain boundary network after the same annealing process can be obtained by altering the cold work degree based on the original grain size.Thus the microstructure after GBE is influenced by the combined effect of original grain size and deformation amount before annealing.Mean strain of grain is used to express the combined effect.A proper value of mean strain of grain is necessary to achieve the desired GBE grain boundary network.

EFFECT OF GRAIN BOUNDARY NETWORK ON THE INTERGRANULAR STRESS CORROSION CRACKING OF 304 STAINLESS STEEL

The grain boundary network in a 304 stainless steel can be controlled by grain bound- ary engineering (GBE). The total length proportion of Σ 3 n coincidence site lattice (CSL) boundaries was increased to more than 70%, and the large size highly twinned grain-cluster microstructure formed through the treatment of GBE. Stress corrosion cracking (SCC) susceptibility of 304 stainless steel was evaluated through C-ring specimen tests conducted in acidified boiling 20%NaCl solution. Based on the characterization by SEM, EBSD and OM, it was found that the large grain-clusters associated with many interconnected Σ 3-Σ 3-Σ 9a ndΣ 3-Σ 9-Σ 27 triple junctions produced by GBE arrest the IGSCC cracks and improve the resistance to IGSCC.

Appling grain boundary engineering to Alloy 690 tube for enhancing intergranular corrosion resistance

The feasibility of applying the grain boundary engineering (GBE) processing to Alloy 690 tube manufacturing for improving the intergranular corrosion resistance was studied. Through small amount of deformation by cold drawing using a draw-bench on a production line and subsequent short time annealing at high temperature, the proportion of low Σ coincidence site lattice (CSL) grain boundaries of the Alloy 690 tube can be enhanced to about 75% which mainly were of Σ3 n ( n = 1, 2, 3, …) type. In this case, the grain boundary network (GBN) was featured by the formation of highly twinned large size grain-clusters produced by multiple twinning during recrystallization. All of the grains inside this kind of cluster had Σ3 n mutual misorientations, and hence all the boundaries inside the cluster were of Σ3 n type and formed many interconnected Σ3 n type triple junctions. The weight losses due to grain dropping during intergranular corrosion for the samples with the modified GBN were much less than that with conventional microstructure. Based on the characterization by scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) technique, it was shown that the highly twinned large size grain-cluster microstructure played a key role in enhancing the intergranular corrosion resistance: (1) the large grain-cluster can arrest the penetration of intergranular corrosion; (2) the large grain-cluster can protect the underlying microstructure.

Relationship between hardness and dislocation processes in a nanocrystalline metal at the atomic scale

(Received 18 February 2011; revised manuscript received 25 March 2011; published 9 June 2011)By combining atomic force microscopy (AFM) and large-scale molecular dynamics (MD) simulations, weexamineatcomparablescalestheatomisticprocessesgoverningnanohardnessinelectrodepositednanocrystallineNi with a mean grain diameter of 18.6 nm under confined contact deformation. Notably, this mean graindiameter represents the “strongest” size for Ni and other nanocrystalline materials where both crystal slipand grain-boundary deformation processes are intertwined to accommodate plastic flow. Accurate hardnessmeasurements were obtained from shallow nanoindentations, less than 10 nm in depth, using an AFM diamondtip.Weshowevidencethatthecontrollingyieldingmechanisminthepeakofhardnessasafunctionofpenetrationdepth corresponds to the emission of partial dislocations from grain boundaries. However, MD simulations alsoreveal for this grain size that the crystalline interfaces must undergo significant sliding at small penetrationdepths in order to initiate crystal slip. The strong interplay between intergranular and intragranular deformationprocesses found in this model nanocrystalline metal is discussed and shown to considerably reduce the localdependenceofnanohardnessontheinitialmicrostructureatthisscale,unlikepastobservationsofnanoindentationinNielectrodepositswithlargergrainsizes.Thesenewfindingsthereforeconstituteanimportantstepforwardtounderstanding the contribution of nanoscale grain-boundary networks on permanent deformation and hardnessrelevant for nanoscale materials and structures.DOI: 10.1103/PhysRevB.83.224101 PACS number(s): 62

Grain boundary engineering of austenitic steel PNC316 for use in nuclear reactors

Austenitic stainless steel PNC316 was subjected to grain boundary engineering (GBE). It was found that the grain boundary engineered PNC316 (PNC316-GBEM) had a coincidence site lattice (CSL) fraction of 86% and that the network of random grain boundaries was perfectly divided by the CSL boundaries. The thermal stability and the void swelling behavior of PNC316-GBEM were investigated by means of SEM and TEM analyses. After thermal aging at 973 K for 100 h, structural changes were observed neither in the grain boundary networks of PNC316-GBEM nor in another sample of PNC316-GBEM subjected to 20% additional cold rolling, PNC316-GBEM20%CW. PNC316-GBEM showed a higher void swelling rate than as-received PNC316 (PNC316-AS). However, with additional 20% cold rolling after GBE, the void swelling rate decreased to as low as that of PNC316-AS.

Grain boundary networks in high-performance, heteroepitaxial, YBCO films on polycrystalline, cube-textured metals

Grain boundaries (GBs) in high-temperature superconductors suppress the critical current density (J c) dramatically, with the J c decreasing exponentially with GB angle, especially when GB misorientation exceeds 4°. To reduce the number of high-angle GBs, fabrication of biaxially textured, superconducting wires via heteroepitaxial growth on cube-textured metals has been widely investigated worldwide. Such wires exhibit very high J c in applied magnetic fields despite having a majority of GBs with total misorientations in the range 4–8°. Here, we show that GB networks in these wires have numerous GBs with out-of-plane misorientations greater than 4° but few boundaries having in-plane misorientations greater than 4°. Transport measurements on bicrystal GBs show that GBs with out-of-plane tilts between 4° and 8° are well linked. Together, these results explain the high performance of superconducting films on cube-textured metals.

Improving the intergranular corrosion resistance of 304 stainless steel by grain boundary network control

The grain boundary network (GBN) was controlled by grain boundary engineering (GBE) in a 304 stainless steel. The total length proportion of Σ3 n coincidence site lattice (CSL) boundaries was increased to more than 70% associating with the formation of large size highly twinned grain-cluster microstructure. Only coherent twin boundaries (Σ3 c) were found to be resistant to intergranular corrosion (IGC) and only such boundaries could be termed “special” ones. The improvement of resistance to IGC of the GBE specimen can be attributed to the large size grain-clusters associated with high proportion of the Σ3 n boundaries and the interconnected Σ3 n -type triple junctions.

Stress concentrations, diffusionally accommodated grain boundary sliding and the viscoelasticity of polycrystals

Using analytical and numerical methods, we analyse the Raj–Ashby bicrystal model of diffusionally accommodated grain-boundary sliding for finite interface slopes. Two perfectly elastic layers of finite thickness are separated by a given fixed spatially periodic interface. Dissipation occurs by time-periodic shearing of the viscous interfacial region, and by time-periodic grain-boundary diffusion. Although two time scales govern these processes, of particular interest is the characteristic time t D for grain-boundary diffusion to occur over distances of order of the grain size. For seismic frequencies ωt D ≫1, we find that the spectrum of mechanical loss Q −1 is controlled by the local stress field near corners. For a simple piecewise linear interface having identical corners, this localization leads to a simple asymptotic form for the loss spectrum: for ωt D ≫1, Q −1 ∼const. ω − α . The positive exponent α is determined by the structure of the stress field near the corners, but depends both on the angle subtended by the corner and on the orientation of the interface; the value of α for a sawtooth interface having 120 ° angles differs from that for a truncated sawtooth interface whose corners subtend the same 120 ° angle. When corners on an interface are not all identical, the behaviour is even more complex. Our analysis suggests that the loss spectrum of a finely grained solid results from volume averaging of the dissipation occurring in the neighbourhood of a randomly oriented three-dimensional network of grain boundaries and edges.

Superfluid density in a two-dimensional model of supersolid

We study in 2-dimensions the superfluid density of periodically modulated states in the frame- work of the mean-field Gross-Pitaevskiˇ i model of a quantum solid. We obtain a full agreement for the su- perfluid fraction between a semi-theoretical approach and direct numerical simulations. As in 1-dimension, the superfluid density decreases exponentially with the amplitude of the particle interaction. We discuss the case when defects are present in this modulated structure. In the case of isolated defects (e.g. dislocations) the superfluid density only shows small changes. Finally, we report an increase of the superfluid fraction up to 50% in the case of extended macroscopical defects. We show also that this excess of superfluid fraction depends on the length of the complex network of grain boundaries in the system.

Ferromagnetic properties of the Mn-doped nanograined ZnO films

Dense nanograined pure and Mn-doped Zn1−xMnxO polycrystals with x ranging between 0.1–34 at. % were synthesized by the wet chemistry method from butanoate precursors. Pure and Mn-doped ZnO possesses ferromagnetic properties only if the ratio of grain boundary (GB) area to grain volume sGB exceeds a certain threshold value sth. The polycrystals in this work satisfy these conditions and, therefore, reveal ferromagnetic properties. The observed dependence of saturation magnetization on the Mn concentration shows an unexpected nonmonotonous behavior. The increase in saturation magnetization at low Mn concentration is explained by the injection of divalent Mn2+ ions and charge carriers into pure ZnO. The decrease in saturation magnetization between 0.1 and 5 at. % Mn can be explained by the increase in the portion of Mn3+ and Mn4+ ions. The second increase in saturation magnetization above 5 at. % Mn is explained by the formation of multilayer Mn segregation layer in ZnO GBs. The shape of the dependence of saturation magnetization on Mn concentration is different for the Mn-doped nanograined ZnO manufactured by different methods. It is most probably controlled by the topology of GB network (ferromagnetic GB foam) in the ZnO polycrystals.

Characterisation of grain boundary cluster compactness in austenitic stainless steel

The distribution of grain boundaries of particular crystallographic character can provide descriptive information on the properties of engineering materials. For example, the fraction and connectivity of corrosion susceptible grain boundaries typically correlates with the extent of intergranular corrosion and stress corrosion cracking resistance in sensitised austenitic stainless steels. A parameter defining the cluster compactness is proposed to describe the breakup of the network of corrosion susceptible grain boundaries. It may therefore provide a measure of intergranular stress corrosion cracking resistance. The cluster compactness of the network of random grain boundaries (>Σ29) in electron backscatter diffraction assessments of microstructure is shown to decrease with increasing fraction of Σ3 boundaries. However, the cluster compactness of the network of corroded grain boundaries identified after electrochemical testing is less sensitive to changes in microstructure obtained by thermomechanical processing.

On the Grain-Boundary Diffusion in Polycrystalline Materials and Thin Films

In this paper, examples of some of the most challenging features of GB diffusion are considered covering selected problems, strongly related to the research activity at our Laboratories and to the scientific interest of Boris Bokstein too. The following problems (and still open questions related to them) are addressed: i) Diffusion in a random network of grain boundaries with different structures and diffusion coefficients in polycrystalline materials; ii) Segregation effects; iii) Stress effects and iv) Effect of the presence of moving and/or non-equilibrium grain boundaries.