Habit Plane Research Articles

Precipitate-domain wall topologies in hardened Li-doped NaNbO3

Ferroelectric hardening is an absolute requirement for potential applications of piezoceramics in high-power devices. A very recent work demonstrates that ferroelectric hardening by pinning domain walls with precipitates introduced during processing is feasible, akin to precipitation hardening in metals. With the precipitation of plate-like LiNbO3 in Li-doped NaNbO3 piezoceramics, ferroelectric hardening was achieved, exhibiting an enhanced mechanical quality factor (Qm). The specific morphology and corresponding orientation of the LiNbO3 platelets, exhibiting {110}PC habit planes, were characterized by transmission electron microscopy (TEM). The experimentally revealed crystallographic structures and strain incompatibility were found to correlate well with the thermodynamic simulation of the minimum-energy aspect ratio. In particular, the topology of the precipitate-domain wall assembly was clarified, revealing a full pinning on the domain walls by the incorporated plate-like precipitates. The revealed topology provides a fundamental basis for future optimization strategies for platelet hardening in new lead-free piezoceramics for high-power applications.

Dislocation loop bias and void swelling in irradiated α-iron from mesoscale and atomistic simulations

Dislocation loops are ubiquitous in irradiated materials, and dislocation loop bias plays a critical role in void swelling. However, due to complicated interactions between dislocation loops and point defects, it is challenging to evaluate the bias factors of dislocation loops. Here, we determine the bias of sessile < 100 > loops in α-iron using a recently developed atomistic approach based on the lifetime of point defects. We establish a mechanistic understanding of the loop interaction based on the diffusion tendency of point defects near the loop core region. Mobile self-interstitial atoms tend to be absorbed from the edge of the loop, and a trapping region perpendicular to the habit plane of the loop exists. The dislocation loop bias is found to be substantially lower than those of straight dislocations in α-iron and should be included in swelling rate estimates. With the obtained sink strength and bias values, agreement is achieved with experimental results for both absolute values and temperature dependence.

Multilayer T1 phase precipitated on grain boundary in artificially aged Al–Cu–Li alloy

Precipitation-strengthened Al–Cu–Li alloys are commonly used as structural materials in aerospace equipment. The present work focused on investigating the characteristics of grain boundary T1 precipitate in a rolled Al–Cu–Li alloy. It was found that the T1 phase with a multilayer structure will precipitate at the grain boundary after artificial aging, and it is nearly continuously distributed along grain boundaries in the pre-rolled samples. The habit plane of the intergranular T1 normally parallels a (111) plane of the matrix on one side of the grain boundary. And compared to the preferential growth of platelet edges for the intragranular T1, the grain boundary T1 phase is also prone to thickening via the stacking of successive unit-cell-thick (0.935 nm) layers on the existing broad surface of the plate, resulting in the significant coarsening in the direction perpendicular to its habit plane. In addition, the reason for the obvious thickening of the grain boundary T1 phase was also discussed.

On the Precipitation of the -Phase {111} Al Plates in the Al–Cu–Mg Alloy

The precipitation of Ω-phase {111}α plates in the Al–Cu–Mg alloy has been investigated at a Cu/Mg ratio 10 and low Si content. Unlike Al–Cu–Mg alloys containing Ag, nanoscale plates with an {111}α habit plane has been found for the first time to precipitate in the alloy according to a heterogeneous mechanism, namely, along low-angle boundaries, dislocation lines, and at the θ′-phase/Al-matrix interphaseboundary.

Phase/grain boundary assisted-3D static globularization mechanism of TC17 alloy based on the microstructure reconstruction and in-situ TEM observation

Lamellar globularization in the dual-phase titanium alloy is the key to improving plasticity and strength. However, the mechanism has not been fully elucidated so far. In this work, the role of phase/grain boundary in the static globularization of TC17 alloy was systematically studied by setting different α phase content before annealing through low- and high-temperature deformation. Isothermal compression causes the parallel distribution and fragmentation of 3D α plates and few globular α particles are formed at a strain rate of 1 s–1. Post-deformation annealing promotes the static globularization of α phase while it is affected by initial α phase content. After 730 °C deformation, the development of α/α interface by absorbing dislocations promotes the formation of globular α grains based on the nucleation of separated α particles and pre-recovery α subgrain during subsequent annealing. The α/α/β and α/β/β triple junctions formed due to high α content with about 36% volume fraction are favorable for the further nucleation and growth of globular α grains by reducing interface energy, forming a 3D irregular α plate. Then nucleation and growth of the β phase dominate the microstructure evolution during subsequent annealing, resulting in the local dissolution of the plate and formation of α rods. After 850 °C deformation, the α phase tends to nucleate at the β/β/β triple junctions and grow into a lamellar shape along the high energy β/β grain boundary due to low α content with about 7% volume fraction. The α nucleation that maintains the Burgers orientation relationship (BOR) with the surrounding β phase grows along the habit plane and thickens slowly, resulting in the formation of a precipitated α plate with a flat surface and the suppression of static globularization. The comprehensive investigation of lamellar globularization provides guidance for optimizing the 3D microstructure and properties of dual-phase titanium alloy.

Developing Mg-Gd-Dy-Ag-Zn-Zr Alloy with High Strength via Nano-Precipitation

A high-performance Mg-10Gd-4Dy-1.5Ag-1Zn-0.5Zr (wt.%, EQ142X) alloy was designed by multi-element composite addition in this work, obtaining a high yield strength (~396 MPa) and ultimate tensile strength (~451 MPa) after hot extrusion and ageing. The high strength is mainly related to fine grains and nano-precipitates, especially the latter. β' and γ″ nano-precipitation with high fractions are the main strengthening phases, leading to a strengthening increment of ~277 MPa. Moreover, the multi-element alloying in this study promotes the basal-prismatic network strengthening structure, composed of β' nano-precipitation with (1-210) habit planes, γ″ nano-precipitation with (0001) habit planes, basal plane stacking faults and 14H-long period stacking ordered phase. In addition, the dislocations and fine grains introduced by the hot-extrusion process not only accelerate the precipitation rate of nanostructure and thus improve the ageing hardening efficiency, but also facilitate the formation of more uniform and finer nano-precipitation. Thus, it is proposed that introducing nano-precipitates network into fine-grained structure is an effective strategy for developing high-strength Mg alloys.

Nitride precipitation and formation in IN617 superalloy during creep

Precipitation behaviors of four types of nitrides (including π phase (Cr3.25Ni1.75N), AlN, Cr2N and TiN) were investigated in IN617 after creep rupture at 1000 °C/16 MPa. Nitride formation mechanisms were discussed based on their relative stabilities, crystallographic characteristics, and orientation relationships with the matrix. The influences of nitrides on creep rupture behaviors were discussed. The results indicated that the π phase, which was predicted unstable at 1000 °C by thermodynamical calculations, only precipitated at high-energy grain boundaries and phase boundaries. Thermodynamically stable AlN, Cr2N and TiN could be formed at relatively low-energy sites, such as inside the matrix, at dislocations and twining boundaries. Particularly, AlN and Cr2N are polar crystals and have hcp structures, much different from the fcc matrix. They precipitated from the habit planes with low atomic misfits and grew into a needle shape to reduce the strain energy. Fcc TiN was easy to nucleate and had good coherence with the matrix, so it could either precipitate from the regions with high-density dislocations in a needle shape, or directly from the matrix in spherical or dendritic shapes to minimize the interfacial energy and strain energy. Among those nitrides, the π phase and AlN formed coarse intergranular networks and were difficult to deform and release stress, dominantly responsible for the brittle rupture mode.

A microstructure-based three-scale homogenization model for predicting the elasto-viscoplastic behavior of duplex stainless steels

A new microstructure-informed three-scale homogenization scheme for elasto-viscoplastic heterogeneous materials is developed. It is applied to predict the mechanical behavior of cast duplex austenitic-ferritic (AF) steels, which are widely used in primary loop of pressurized water reactors (PWR). At the first scale (microscale), the elasto-viscoplastic behavior of single crystals for both phases is modeled using linear elasticity and a viscoplastic crystal plasticity model. An “affine” type formulation based on the first moments of stresses is applied to the inelastic non-linear part of the deformation. Then, at the second scale (mesoscale), an EBSD-informed two-phase austenite/ferrite laminate structure (LS) model is developed with {110}-type habit planes (HP) in ferrite and a Kurdjumov-Sachs orientation relationship (KS-OR) between both phases. At the third scale (macroscale), the model considers a single ferritic primary grain as an aggregate of spherical two-phase laminate structure domains corresponding to the 24 KS-OR variants. The elasto-viscoplastic self-consistent scheme (EVPSC) is used through the Translated Fields (TF) method to obtain the effective behavior at this scale. The TF-EVPSC scheme is also used for an ensemble of primary ferritic grains to identify materials parameters from experimental tensile curves corresponding to as received and aged specimen. From EBSD measurements, the crystallographic data are thoroughly analyzed to physically feed the three-scale model. The results are discussed in terms of stress/strain responses in ferrite and austenite, which are correlated to the different KS-OR variants and HP orientations. The effect of primary ferritic grain crystallographic orientation and aging is also studied regarding monotonic and cyclic tests to study the possible origins of backstress in this material.

Effect of grain structures and precipitation characteristics on the corrosion behaviour of Sc-containing Al-Cu-Li alloy

ABSTRACT The intergranular corrosion behaviour of Al-Cu-Li-Sc alloy with different microstructures was systematically investigated in this work. The SKPFM characterisation revealed that the AlCuSc (W) phase has more negative potential compared to the Al matrix and undergoes dealloying in the intergranular corrosion solution. The dealloying rate of the W phase in the corrosive solution is relatively slow, which does not lead to serious pitting characteristics. As denser anodic T1 precipitates formed on low-angle grain boundaries of the partially recrystallised sample will reduce the precipitates on the sub-grain interior, resulting in the low-angle boundary being prone to preferential corrosion and reduced intergranular corrosion resistance. The complete recrystallised sample exhibits corrosion of grain interior and propagates along corroded bands parallel to {111}Al habit planes of T1 precipitates, reducing the expansion rate of corrosion along the grain boundaries. Restraining the IGC along the low-angle grain boundary and inducing crystallographic corrosion to improve the intergranular corrosion resistance.

Substructure and crystallography of lath martensite in as-quenched interstitial-free steel and low-carbon steel

In this study, the substructures and crystallographic features of as-quenched lath martensite in interstitial-free (IF) steel and low-carbon steel (0.2C steel) were investigated using focused ion beam-scanning electron microscopy serial sectioning and advanced transmission electron microscopy. A three-dimensional analysis revealed that the morphology of the so-called lath, the smallest structural unit, exhibited a wedge-shaped plate instead of the ideal lath shape. The habit plane orientations were distributed from (5 7 5)A to (1 1 1)A in the IF steel, whereas they were near to that predicted based on the phenomenological theory of martensite crystallography in the 0.2C steel. In the IF steel, well-developed dislocation cells, tangled dislocations, and curved dislocations could be observed. Conversely, straight dislocations and cementite formation were observed in the 0.2C steel. Moreover, the orientation changed significantly inside a given lath in both steels. The lath boundary comprised regularly spaced steps with (0 1 1)M atomistic terrace planes. Interfacial dislocations corresponding to the discontinuity of the (0 1 1)M lattice planes could be observed at steps, and these dislocations were introduced to compensate for the misorientation between adjacent laths. Additionally, carbon segregations were observed at the lath and block boundaries in the 0.2C steel. Our observation results on the dislocation structures, cementite formation, and segregation of carbon atoms on the lath and block boundaries indicated that the substructure changed significantly during quenching. Based on our experimental results, we discuss the origin of the substructures and crystallographic features of lath martensite.

Solidification Microstructure and Mechanical Properties of B1-type TiC in Fe-Ti-C Ternary Alloys

The microstructure of the B1-type TiC formed during solidification and its mechanical properties were investigated using arc-melted Fe-Ti-C ternary alloys. The TiC formed at relatively high temperatures in the liquid as the primary phase exhibited a dendritic shape. With decreasing temperature and/or decreasing Ti and C content in the liquid, its morphology changed to a cubic shape with the {001}TiC habit plane, a plate shape with the {011}TiC habit plane, and a needle shape with the <001>TiC preferential growth direction. The morphology of the TiC was characterized by the anisotropy of its surface energy and its growth rate. The cubic shape with the {001}TiC habit plane was formed as a result of the minimum surface energy of the {001}TiC. However, the plate shape with the {011}TiC habit plane and the needle shape with the <001>TiC preferential growth direction should be formed due to the slowest growth rate of <011>TiC and the fastest growth rate of <001>TiC, respectively. At room temperature, alloy with the dendritic TiC was fractured in the elastic deformation region because the TiC exhibited no plastic deformation. On the other hand, at 800ºC, it was suggested that the TiC has plastic deformability and alloy with the dendritic TiC was also plastically deformed.

Precipitation and evolution behaviors of grain boundary α and α side plates for ultra-high strength titanium alloy during continuous cooling

In this paper, precipitation behavior and evolution mechanism of grain boundary α (αGB) and α side plates (αSP) of ultra-high strength titanium alloy are investigated. Precipitation of αGB can change original morphology of β grain boundary into irregular shapes, and it is much more obvious at slower cooling rate. The λ, which denotes the minimum value of misorientations between 12 α variants of βnBOR and αGB, is introduced to quantitatively evaluate morphology of αGB. The β phase that maintains Burgers orientation relationship (BOR) with αGB is labeled as βBOR, conversely, it is labeled as βnBOR. The deformation degree of αGB increases with the increasing λ. However, the λ is not the only factor that determines morphology of αGB, and it is also related to the angle between habit plane and grain boundary plane. Orientation of αGB is affected by β phase on both sides. The αGB maintains BOR with βBOR and the s-BOR with βnBOR. The s-BOR is defined as (0 0 0 1) plane of αGB is not parallel to (110) plane of βnBOR, but one of 〈1 1 –2 0〉 directions of αGB is parallel or close to one of βnBOR’s 〈111〉 directions.

Interaction of face-centered cubic phase and twin in Zr–1.0Sn–1.0Nb–0.3Fe alloy

The thermally induced face-centered cubic (FCC) phase and its interactive interface with {101¯1} twin in Zr–1.0Sn–1.0Nb–0.3Fe alloy were characterized via transmission electron microscopy. The FCC-Zr phase and the matrix have a B-type orientation relationship, however, the interface deviates from the (0001)HCP habit plane. Moreover, the FCC-Zr phase and the twin follow Py-type orientation relationship, namely <12¯10>HCP//<11¯0>FCC and {101¯1}HCP//{111}FCC, which may be attributed to expansion and shuffling adjustment. Although the Py-type orientation relationship between the hexagonal close-packed and the FCC phases in Zr is rarely observed, due to the large crystal plane mismatch, this study provides intuitive evidence of this relationship.

Interface structures and thickening mechanisms of the Q′ and QP2 phases for Al-Mg-Si-Cu alloys

Precipitate/matrix interfacial structure is a key issue for controlling the nucleation and growth of precipitates, affecting their morphologies, sizes and thus determining the hardening potential and thermal stability of the alloy. The interfacial structures and thickening mechanisms of two typical precipitates (Q′ and QP2 phases) in the Al-Mg-Si-Cu alloys were systematically studied by atomic-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) in this article. It was revealed that the different thickening rates of the precipitates are mainly determined by their interfacial structures. Two different interfaces are detected for both the two precipitates: the interface I constituted by a transition layer arranged of Mg-Si-Mg-Si( Al) chain, while the interface II comprised by a row of modified Cu sub-unit clusters embedded in the Al matrix. The thickening of the two phases is supposed to be achieved by alternate transformation between the two interfaces. The thickening of the Q′ phase is controlled by the Cu replacing certain site at the habit plane and its evolution to Cu sub-unit clusters, while the thickening of the QP2 phase is controlled by the nucleation of the ledges at its habit plane. The limited supply of ledges at the habit plane of the QP2 phase is a main reason for its high resistance of thickening. • Different thickening rates of precipitates are determined by their interfacial structures. • Two different interfaces are detected for both the Q' and QP2 precipitates. • The limited supply of ledges in the QP2 phase is responsible for its high thickening resistance.

Electron irradiation enhanced precipitation in a Mg-6 wt% Sn alloy in TEM

The precipitation in a Mg-6 wt% Sn alloy under electron irradiation has been investigated by in-situ heat treatment at 200 °C in a transmission electron microscope. Electron irradiation produced a high number density (9.78 ± 1.27 × 1021 m−3) of fine β-Mg2Sn precipitates with the average length close to 15 nm whereas the β-Mg2Sn precipitates formed in the region without electron irradiation have a low number density of 2.55 ± 0.68 × 1019 m−3 and a coarse size averaging around 200 nm. The β-Mg2Sn precipitates formed with and without electron irradiation have the same orientation relationship with the Mg matrix:11¯00Mg//112β,0002Mg//22¯0β with the habit planes parallel to the basal (0001)Mg plane. Experimental results clearly demonstrate that electron irradiation is effective in refining the precipitate distribution in the Mg-6 wt% Sn alloy and the possible underlying mechanisms are discussed in relation to the supersaturation of vacancies and interstitials produced by electron irradiation.

The deformation behavior of the {111}Al plates in an Al-Cu-Mg-Ag alloy

The deformation behavior of {111}Al Ω plates has been analyzed in an Al-Cu-Mg-Ag alloy after creep tests to fracture by atomic-resolution scanning transmission electron microscopy (STEM). A transition from particle shearing to Orowan dislocation loop formation was found to occur at critical plate thicknesses of 3 and 3.5 cθ-o (i.e., 2.54-2.97 nm) at elevated temperature if the Ω phase is considered to compose of the orthorhombic θ lattice (θ-o). For the plates thinner than the critical value, a full dislocation provides the particle segments to shift relative to each other in the planes orthogonal to the particle habit plane. These discrete particle shifts/shears are proportional to ¼ cθ-o(1 d111Al) normal to the broad plate surfaces or the plate habit plane. A full dislocation with Burgers vector b→ = [001]θ-o can dissociate to a few partials inside the θ-o lattice. For the thick plates, a dislocation loop was found to consist of the two most extent segments with jogs making its line somewhat parallel to the broad plate surfaces. Interposition of these two segments belonging to the same dislocation loop along the broad interfaces, depends on elastic strain fields, e.g., originating from the ledges formed after particle shearing, broad coherent interface structures and the precipitate/matrix volumetric incompatibility in <111>Al//<001>θ-o. The dislocation loops cause particle segmentation and deviation by lattice rotation around <110>Al axes from the well-known Al/Ω orientation relationship.

γ″ variant-sensitive deformation behaviour of Inconel 718 superalloy

• This study reveals, for the first time, the micro-mechanism of deformation of γ″ strengthened superalloys using neutron diffraction. • Inconel 718 samples with different γ″ variant distributions were prepared by different stress ageing heat treatments. • The γ″ variant-sensitive deformation behavior was studied by interpreting the stress-lattice strain curves. • Compressive-aged sample shows larger yield strength compared to the tensile-aged sample • The difference in strengthening arises from the interactions between dislocations and different γ″ variants. Strengthening in Inconel 718 superalloy is derived from dislocation interaction with γ″ precipitates, which exist in disk-shaped three possible orientation variants with their {100} habit plane normal to each other. The interactions between dislocations and γ″ precipitates vary according to the γ″ orientation variants, which makes the deformation behaviour complicated and difficult to reveal experimentally. In this work, γ″ variant distributions of Inconel 718 samples were tailored by ageing heat treatment under either tensile or compressive stress. The γ″ variant-sensitive deformation behaviours were then studied by in situ tensile tests via neutron diffraction at room temperature. It is demonstrated that yielding first takes place in grains oriented with <110> parallel to the loading direction. An identical lattice strain response to applied stress of both the γ matrix and the γ″ precipitates was observed during yielding, suggesting that dislocations shearing through the γ″ precipitates is predominant at this stage. Variations in yield strength for samples with different γ″ variant distributions were observed, which can be attributed to different strengthening that arises from interactions between dislocation and different γ″ variants.

Formation mechanism of faulted bands and its effect on α′-martensitic transformation

Understanding the martensitic transformation mechanism is of critical importance to control martensitic transformations and hence retained austenite. Linear defects reported in a previous study can provide nucleation sites for martensitic transformation, but their nature, formation mechanism and effect on martensitic transformation were not revealed. In this study, detailed evidence was provided that linear defects were formed by the motion of Shockley partial dislocation on a close-packed plane and usually could be in the form of faulted austenite, twins and ɛ-martensite, defined as faulted bands. Faulted bands were formed prior to martensitic transformation at a starting transition temperature higher than Ms. They can provide nucleation sites for α′-martensitic transformation but can act as an effective barrier to limit the growth of α′-martensite laths. In addition, the same plane was found between the formation plane of the faulted band and the habit plane of α′-martensite, which indicates that the variant types of α′-martensite were decided by the variant types of faulted bands.

Comparative Study on the Crystallography of Isothermal and Athermal Precipitations in HCP-BCC System.

Due to the diversity of the lattice parameter ratio c/a of hexagonal structure and precipitation mechanism, a systematic overview of this transformation has not been fully established, which draws back the attempt to control crystallographic features of the precipitates and microstructures of applied metals and alloys. Here, a comparative investigation to the crystallography of isothermal and athermal precipitations occurring in the HCP-BCC system was demonstrated in a full range of the lattice parameter ratio by using an invariant deformation element (IDE) model. It was then proposed that a precipitation in the HCP-BCC system could be either of the isothermal type if the observed habit plane Miller index falls into a zone axis of BCC &lt;11w&gt;(w ≠ 0) or HCP 211¯0, or the athermal type when it is found to locate in a zone axis of BCC &lt;11w](w = 0) or HCP [0001]. The crystallographic investigation on the precipitations in the HCP-BCC system in a full range of the lattice parameter may be a practical guide for computing material science when building a crystallographic interface model under an optimised orientation relationship, which is necessary to minimise the transformation system energy.

An investigation on reorientation and textural evolution in a martensitic NiTi rolled sheet using EBSD

Variant reorientation is a ubiquitous phenomenon in shape memory alloys and has a great influence on their texture and mechanical response. In the present study, a martensitic NiTi rolled sheet is investigated by the electron backscattering diffraction (EBSD) technique; the variant reorientation and texture evolution are followed during bending. The initial weak texture components of martensite evolve by reorientation toward a strong texture around [1¯04]M and a weak one around [1¯12]M. The initial laths are transformed into blocks made of one or two variants. The EBSD maps were analyzed with the Interaction Work (IW) criterion associated with (a) the individual variant and (b) habit plane variants (a combination of two distortion variants). Compared with the experimental results, the prediction given with individual variant fits better than with the habit plane variants. An empirical quantification of the threshold on interaction work (IW) permits to rationalize and predict the variant pairs formed by the reorientation and their numbers in the blocks. A critical influence of the pre-texture of the parent B2 grains on the final texture and on the number of variants per block was revealed. This study could give insights into the mechanical response of the NiTi alloys with various textures during tensile and fatigue tests.