Annealing Twin Boundaries Research Articles

Achievement of grain boundary engineering by transforming residual stress in selective laser-melted Inconel 718 superalloy

The complicated geometric parts produced by selective laser melting (SLM) are not amenable to grain boundary engineering (GBE), which is primarily implemented via thermomechanical processing. Herein, the high internal residual stress produced by SLM processing is utilized as stored activation energy for direct GBE in a SLM-fabricated Inconel 718 superalloy. The grain boundary distribution characteristics, grain size, texture, and corresponding mechanical properties of the as-built specimens subjected to different heat treatments were investigated. As the annealing temperature increased, the recrystallized fraction increased, accompanied by an initial increase and subsequent decrease in the number of special grain boundaries. The numerous annealing twin boundaries effectively divided the grains, despite grain coarsening induced by grain boundary migration at a higher temperature. Compared to the as-deposited sample, the yield strength and ultimate tensile strength increased from 707 to 1071 MPa and from 947 MPa to 1271 MPa, respectively, and the ductility increased from 17% to 21% in the sample annealed at 1200 °C for 1 h followed by double aging treatment because this sample had the highest fraction of twin boundaries (64.89%). The “Additive Manufacturing + GBE” strategy based on utilizing the residual stress offers a feasible pathway for broadening the application scope of additive manufactured alloys.

Hall-Petch Strengthening by Grain Boundaries and Annealing Twin Boundaries in Non-Equiatomic Ni2FeCr Medium-Entropy Alloy

A novel Co-free non-equiatomic Ni2FeCr medium-entropy alloy (MEA) was designed, and the Hall–Petch strengthening by grain boundaries and annealing twin boundaries was investigated. For this purpose, the alloy was prepared by cold rolling and recrystallization at 873–1323 K for 40 min–6 h. Annealing at different temperatures revealed that Ni2CrFe alloy forms a stable face-centered cubic (FCC) solid solution. Mean grain sizes (excluding annealing twin boundaries) and mean crystallite sizes (including both grain and annealing twin boundaries) were determined using the linear intercept method and the equivalent circle diameter in electron back-scattered diffraction (EBSD) soft. Tensile tests at 293 K indicated that the Hall-Petch slopes of grain sizes and crystallite sizes are 673 and 544 MPa μm1/2, respectively, and this contribution was then subtracted from the overall strength to calculate the intrinsic uniaxial lattice strength (90 MPa). Additionally, tensile tests, performed between 293 K and 873 K, revealed that the Ni2CrFe MEA has a stronger resistance to softening at high temperatures. Transmission electron microscopy of deformed specimens revealed the formation of dislocation pile-ups at annealing twin boundaries, indicating that it is also an obstacle to dislocation slip. Furthermore, the thickening of the annealing twin boundary after deformation was observed and illustrated by the interaction between different dislocations and annealing twin boundaries.

A superior strength-ductility synergy of Al0.1CrFeCoNi high-entropy alloy with fully recrystallized ultrafine grains and annealing twins

• Giant strengthening in Al 0.1 CrFeCoNi by cold rolling and short-time annealing. • Al 0.1 CrFeCoNi possesses yield strength of 885 MPa and uniform elongation of 23.4%. • Ultrafine-grained structure and multiple deformation twins lead to high strength. Grain refinement usually makes the materials stronger, while ductility has a dramatic loss. Here, a superior tensile strength–ductility synergy in a fully recrystallized ultrafine-grained (UFG) Al 0.1 CrFeCoNi with abundant annealing twins was achieved by cold rolling at room temperature and short-time annealing. The microstructure characterization using electron backscattered scattering diffraction demonstrates that abundant geometrically necessary dislocations (GNDs) gather around the grain boundaries and twin boundaries after tensile deformation. Although coarse-grained (CG) samples undergo a larger plastic deformation than UFG samples, the GND density decreases with grain size ranging from UFG to CG. Transmission electron microscopy results reveal that the annealing twin boundary, which effectively hinders the dislocation slip and stores dislocation in grain interior, and the activation of multiple deformation twins are responsible for the superior strength–ductility synergy and work hardening ability. In addition, the yield strength of fully recrystallized Al 0.1 CrFeCoNi follows a Hall–Petch relationship ( σ y = 24 + 676 d –1/2 ), where d takes into account both grain boundaries and annealing twin boundaries. The strengthening effects of grain boundaries and annealing twin boundaries were also evaluated separately.

Correlation between shape memory effect and density of annealing twin boundary in an FeMnSiCrNi alloy during special heat treatments with different parameters

It remains unclear whether a negative dependence of shape memory effect (SME) still exists on the density of annealing twin boundary (ATB) in FeMnSi-based alloys during special heat treatments without optimal parameters. To address this issue, the evolutions of grain size and martensite amount in an Fe-14.5Mn-5.5Si-8.2Cr-5.2Ni-0.016C alloy were investigated in addition to the evolution of ATB density during the special heat treatments, i.e. training and thermomechanical treatment with different annealing temperatures. The results showed that the SME still showed a strong negative dependence on the ATB density independent of the special heat treatments. An obvious positive dependence of SME existed on the density of low angle grain boundary concomitant with the reduced ATB density. No obvious correlation between the SME and the grain size existed when the pre-deformation was rather lower (<4%). Larger grain size and less pre-existing martensite were conducive to realizing larger recovery strains.

Effect of Hf and Ta on the tensile properties of PM Ni-based superalloys

In this work, the effects of Hf and Ta on the tensile properties of PM Ni-based superalloys at different temperatures were investigated. From room temperature to 750 °C, Ta significantly enhanced the tensile strength by reducing stacking fault energy, increasing the γ′ volume fraction, and promoting γ′ splitting. However, Ta was detrimental to tensile ductility because it obviously inhibited intragranular deformation. Hf had no obvious effect on tensile strength, but significantly improved high-temperature tensile ductility by promoting intragranular deformation, increasing the equivalent width of the γ channel, reducing the density of the annealing twin boundaries, and increasing the equi-cohesive temperature. We also found that Hf and Ta showed interaction strengthening, which became more significant with increasing temperature. At high temperatures, it fully reflected the advantage of the synergistic effect of Hf and Ta, which not only significantly improved the strength of the alloys but also maintain fair ductility.

Dynamic recrystallization behavior of the equiatomic FeCoCrNi high-entropy alloy during high temperature deformation

The hot deformation behavior of a FeCoCrNi high-entropy alloy was investigated in this work. The tests were carried out at 900–1100 °C, under strain rates of 0.001–0.1 s−1 and the effect of friction between specimen and tool on flow stress curves under different deformation conditions was analyzed and modified. The experimental results were employed to determine the constitutive equation with strain-dependent material constants for modeling and for prediction of flow stress at different deformation conditions, using the data obtained from the Sellars–Tegart–Garofalo equation. Microstructural studies revealed that dynamic recrystallization was the primary softening mechanism, resulting in significant microstructural reconstitution and fine grains formation. A complete-recrystallized microstructure with a high proportion of annealing twin boundary was obtained in the specimen hot compressed at 1000 °C/0.01 s−1, so that the average grain size was 22 μm, compared to the initial grain size of 42 μm. The increase of the size and fraction of the DRXed grains during hot deformation decrease the alloy hardness. The dynamic recrystallization parameters such as critical, peak, and steady-state stress, and strain were determined using the work hardening rate analysis. Moreover, the activation energy for deformation of the alloy was determined to be 417 kJ mol−1. The results also indicated that the critical stress and strain for initiating dynamic recrystallization were inversely proportionate to deformation temperature and the strain rate. Furthermore, increasing the deformation temperature resulted in grain growth, whereas increasing the strain rate led to grain refinement. The critical stress and strain were dependent on the Zener–Hollomon (Z) parameter via a power relation with exponents of 0.14 and 0.12, respectively.

Temperature dependence of tensile deformation behavior and strain hardening of lean duplex stainless steels

Two lean duplex stainless steels with different manganese contents (4 and 8 wt.%) were deformed in the tensile mode in the temperature range of 25–300 °C. The deformation-induced martensitic transformation was characterized as the main austenite deformation mechanism of 4Mn steel at room temperature. In contrast, the strain hardening behavior of 8Mn steel was regulated by the cooperation of twinning induced plasticity and transformation induced plasticity. This was justified considering the lower austenite fraction and austenite stability in 4Mn compared to the steel containing a higher manganese content. Mechanical properties such as ultimate tensile strength and uniform elongation in both steels were significantly changed as the deformation temperature was increased. Differences in chemical composition and deformation temperature led to changing deformation mode due to strain partitioning between austenite and ferrite, austenite stability, and stacking fault energy. The most important factors influencing the mechanical properties are austenite stability and stacking fault energy at elevated temperatures. It was observed that as the austenite stability increases, martensitic transformation occurs directly from austenite grain and annealing twin boundaries but does not significantly improve strength and ductility. The strain partitioning between austenite and ferrite could provide a proper condition for strain-induced martensite transformation and deformation twinning, thereby enhanced mechanical properties at elevated temperatures.

Re-examination of the effect of reducing annealing twin boundary density on the shape memory effect in Fe-Mn-Si-based alloys

Reducing annealing twin boundary (ATB) density was proposed to substantially improve the shape memory effect (SME) of Fe–Mn–Si-based alloys, irrespective of their composition, deformation temperature and thermomechanical treatments. Here, we re-examined the effect of reducing ATB density on the SME of a Fe20Mn5.5Si9Cr5Ni (wt%) alloy. We set off with three starting states, i.e., the as-cast state, the hot forged and solution-treated (ST) state as well as the subsequently δ-annealed (DA) state. Our results reveal that the as-cast sample with the lowest ATB density exhibits the lowest shape recovery ratios, while the shape recovery ratios of the DA sample are close to those of the ST sample although the ATB density in the former is only ~1/3 of that in the latter. In other words, reducing ATB density does not necessarily lead to the improvement of SME. Compared to the ST sample, the lower ATB densities in the as-cast and DA samples are actually concomitant with higher densities of low angle grain boundaries (LAGBs), larger γ grain sizes and a limited amount of χ precipitates (volume fraction < 1.6%). The increase of LAGB density and the precipitation of elongated χ precipitates are deleterious to the SME, while the increase of γ grain size improves the SME. Thus, reducing ATB density cannot result in a monotonic change to the SME, given that the decrease of ATB density is concomitant with other microstructural changes.

Homogeneously introducing more and thinner nanotwins by engineering annealing twin boundaries: A TWIP steel as an example

It is a challenge to homogeneously introduce more and thinner nanotwins into bulk materials. Our results showed that more and thinner deformation nanotwins could be more homogeneously introduced in a bulk solution-treated FeMnSiC twinning-induced plasticity (TWIP) steel after significantly reducing boundaries of annealing twins by thermomechanical processing that consisted of three cycles of deformation at room temperature (RT) and subsequent annealing. After introducing the recovery-annealed deformation nanotwins by 30% deformation at RT and annealing at 673 K, the three-time-trained steel showed a similar ductility comparable to the solution-treated steel but obtained 125 MPa higher in yield strength. The engineering annealing twins will open a promising pathway for strengthening bulk TWIP steels by homogeneously introducing thinner recovery-annealed deformation nanotwins.

Novel Reconstruction Approaches of Austenitic Annealing Twin Boundaries and Grain Boundaries of Ultrafine Grained Prior Austenite

Novel Reconstruction Approaches of Austenitic Annealing Twin Boundaries and Grain Boundaries of Ultrafine Grained Prior Austenite

Microstructure evolution of 316L stainless steel during solid-state additive friction stir deposition

ABSTRACT Solid-state additive friction stir deposition was employed to three dimensionally print a 316L stainless steel. The microstructure, texture and grain boundary distribution were characterised using five-parameter boundary analysis. The bulk microstructure was exceptionally fine (∼0.7 µm) throughout the thickness of the deposited layers, consisting of equiaxed grains along with sub-grains delineated by high and low angle boundary segments. The misorientation angle distribution of the deposited layers was considerably different from the as-received microstructure, exhibiting a significant reduction in the relative areas of ∑3 annealing twin boundaries due to their distortion by severe plastic deformation. The overall texture also exhibited strong shear components associated with FCC metals, suggesting that the microstructure refinement largely took place through progressive subgrain rotation, known as continuous dynamic recrystallisation. The grain boundary plane distribution revealed significantly lower anisotropy compared with the as-received material, showing two moderate peaks at the (111) and (110) orientations. The migration of high-angle mobile boundaries, which may have been induced by thermal cycle upon subsequent deposition along with the dislocation gradients between adjacent grains, slightly enhanced the intensity of low energy (111) orientations at the expense of (110) high-energy boundaries.

An EBSD based investigation on the deformation mechanisms under HCF-creep interaction in a Ni-based superalloy (alloy 617M)

The study presents an insight into the deformation mechanisms under high cycle fatigue-creep interaction (HCF-creep interaction) in a Ni-based superalloy (Alloy 617 M), using electron backscatter diffraction (EBSD) as an important tool. HCF-creep interaction tests are carried out at 700 °C by imposing periodic holds at mean stress (σm) during HCF cycling. Detailed EBSD investigation using kernel average misorientation (KAM) map indicates a non-uniform distribution of local misorientation especially at the grain boundaries, with increase in σm, which is attributed to the increased creep damage therein. Creep cavitation through grain boundary sliding is found to increase with an increase in the hold temperature to 750 °C owing to a higher dislocation mobility triggered by precipitate coarsening and dislocation climb. Change in twin boundary character from Σ3 to other higher angle boundaries caused by dislocation-twin interaction is noticed under HCF-creep interaction, leading to partial detwinning. A decrease in the fraction of Σ3 annealing twin boundaries through partial detwinning is observed, which is found to be more pronounced when the creep hold is maintained at 750 °C. This caused a loss of strength at the above temperature, through coalescence of secondary intergranular cracks, in turn leading to intergranular failure. In contrast, the hold imposed at 700 °C with similar loading parameters resulted in run out.

Improving surface quality of oxygen-free copper tube via grain growth inhibition with La microalloying

The effect of La microalloying on microstructure evolution of oxygen-free copper tube annealed at elevated temperature was investigated. Different with the significant grain growth in La-free copper tube, growth behavior of La microalloyed tube is effectively inhibited. Meanwhile, Cu6La particles are detected with mainly segregating near annealing twin boundaries but less at grain boundaries. With such experimental observations, a new mechanism for constrained growth of recrystallized grain is proposed with pinning of Cu6La particle coupled with annealing twin dragging. Moreover, refined microstructure is maintained for La microalloyed tube after elevated temperature, which greatly improves the surface quality during secondary bending and flattening processes.

The microstructure and crystallography of lath martensite with Greninger-Troiano orientation relationship in a Fe-12.8Ni-1.5Si-0.22%C steel

The excellent matching of strength and toughness with respect to lath martensite is closely related to its hierarchical configuration and crystallographic features. Recently, a few literatures reported that the Greninger-Troiano (G-T) relationship, instead of the generally accepted Kurdjumov-Sachs (K S) relation, was determined in some lath martensite steels. However, the underlying reason is still waiting to be explored. In the present study, systematic characterizations were performed in water-quenched Fe-12Ni-1.5Si-0.2C steel firstly, then a modified version of crystallographic model developed by Morris et al. was proposed to clarify the microstructure and the crystallography with G-T relationship. As the main crystallographic features, the orientation relationship between prior austenite and martensitic variants is close to G-T rather than K S relation. Further characterization shows that the block interface is parallel to the annealing twin boundary of prior austenite, indicating the {111} γ habit plane of blocks. Naturally, three crystallographically distinct blocks with {111} γ interfaces can be configured into a packet with self-accommodation manner to minimize macro strain. In fact, such a hierarchical configuration with G-T relationship can be understood clearly by the modified version of crystallographic model. Firstly, regarding the bivariant block as the basic element which undertakes the invariant plane shear, the total shear strain within each packet region can thus be effectively cancelled out on the common habit planes of {111} γ due to the self-accommodation effect, leading to an essentially shear strain-free pattern; Secondly, a higher hierarchy is accompanied with a significant decrease of strain and only a homogenous dilation of 0.02 remains in a prior austenite grain, which is closely related to the unique hierarchy; Thirdly, the G-T relationship is natural preferred for the present steel, which satisfies two conditions in this crystallographic model: (i) a minimum of angular deviation between the {111} γ and the habit plane of bivariant blocks; (ii) three slip systems of dislocations with 〈111〉 α’ Burgers vectors involved during the martensitic transformation, which is also universally observed in the TEM examinations. Besides that, compared with the case of K S relationship, a potential influence of G-T relationship on mechanical properties may embody in inducing a lower ductile-brittle transition temperature and better toughness of lath martensite steels. • G-T relation was determined by a newly-developed scheme. • Microstructure and crystallography were clarified by a modified model. • A higher hierarchy is accompanied with a significant decrease of strain.

Evolution of annealing twins in Inconel 625 alloy during tensile loading

In situ electron backscatter diffraction (EBSD) has been demonstrated as a powerful tool for tracking the microstructural evolution of alloys; however, it has not yet been applied to analyse the evolution of annealing twin boundaries (TBs) during loading. In this study, we investigated the evolution of annealing TBs and their interaction with dislocations in Inconel 625 alloy under room-temperature uniaxial tension using in situ EBSD and ex situ transmission electron microscopy (TEM) techniques, which provided experimental evidence and detailed insights into the interaction between TBs and dislocations during plastic deformation. The results showed that annealing TBs remained stable during the initial deformation period (strain of 5%). With increasing strain, high-angle grain boundaries (HAGBs) changed to low-angle grain boundaries (LAGBs), the fraction of TBs decreased, and the TBs reacted with dislocations. As a result, the TBs gradually lost coherence and transformed into random HAGBs, which then partially transformed into LAGBs. The tensile yield strength of 348 MPa, ultimate tensile strength of 634 MPa, and elongation of 50.7% of the Inconel 625 alloy were attributable to the reaction of grain boundaries and pre-existing annealing TBs with dislocations as well as to the formation of stacking faults and deformation nano-twins during tensile loading. In addition, the deformation of the Inconel 625 alloy was mainly caused by dislocation slip during the initial deformation period, whereas deformation twinning occurred in the later period.

Study of the relationship between intergranular stress corrosion cracking and grain boundary characteristics in brass

The purpose of this study is to investigate the correlation between intergranular stress corrosion cracking (IGSCC) and grain boundary characteristics in brass. The Volta potential and nature of the grain boundaries in H62 brass were studied by scanning Kelvin probe force microscopy and Electron Backscattered Diffraction, respectively. The α/β phase boundaries and random grain boundaries with low work functions can act as anodic areas to promote IGSCC propagation. Moreover, due to a combination of the inherent characteristics and high work function of Σ3 annealing twin boundaries (ATBs), IGSCC is successfully halted at a triple junction containing Σ3 ATB.

High-resolution digital image correlation study of the strain localization during loading of a shot-peened RR1000 nickel-based superalloy

The improved crack nucleation resistance of shot-peened materials has been usually studied from the point of view of the residual stresses. However, experimental limitations have made difficult to quantify strain localization during further deformation. In this study, we have used high-resolution digital image correlation (HRDIC) and electron backscatter diffraction (EBSD) in a large area covering shot-peened material and bulk to study quantitatively the strain distribution at the submicron scale. We found that, at the earlier deformation steps (e < 2.5%), the slip intensity and distribution at the shot-peened region differ from that observed at the bulk. In the former, well-spaced, sharp and homogeneously distributed slip bands are observed. On the other hand, in the shot-peened region, slip is less evident, and a more homogeneous diffuse deformation is observed along the majority of the grains. However, this region also contains a small number of slip bands that accumulate high strains, mainly along Σ3 annealing twin boundaries. Additionally, the border between both regions, at a depth with no residual stress, present the minimum strain values. During further deformation (e > 2.5%), the strain distributions become similar along the material simultaneously to the residual stresses relaxation. A further analysis of the HRDIC maps allowed for the first time to quantify strain heterogeneity. This study reveals the importance of the local stress state and the strain history on the strain localization during loading of peened materials.

Role of the annealing twin boundary on the athermal α′-martensite formation in a 304 austenitic stainless steel

The preferential formation of α′-martensite at the annealing twin boundary (TB) was identified in a 304 austenitic stainless steel. The α′-martensite holds the N-W orientation relationship with austenite (matrix or twin). Atomic-level analysis reveals that the unidirectional shear into the <112>γ forms α′ in the annealing TB-containing austenite matrix. Formation of athermal martensite leaves stacking fault and ε-martensite at the surrounding matrix. The role of the annealing TB on the α′ formation was further discussed.

Crystallography and elastic anisotropy in fatigue crack nucleation at nickel alloy twin boundaries

Fatigue crack nucleation at annealing twin boundaries (TBs) within polycrystal nickel-based superalloy René88DT is investigated with a microstructure-sensitive crystal plasticity (CP) model, digital image correlation strain measurements and experimental SEM crack nucleation observations. Strong slip localizations at TBs were experimentally observed and predicted by the CP model, which also showed high predicted geometrically necessary dislocation and corresponding stored energy densities, capturing experimental observations of crack nucleation. In a systematic study, elastic anisotropy was found to drive local elastic constraint and hence resolved shear stress, slip activation, GND density and stored energy density, demonstrating for this reason that TBs are preferential sites for crack nucleation in this alloy. The parent grain / twin pair crystallographic orientation with respect to remote loading was also demonstrated to be key to slip activation parallel to TBs and hence to stored energy density and fatigue crack nucleation, and the range of most damaging parent grain orientations has been identified.

Stress-induced martensitic transformation in metastable austenite grains during nanoindentation investigation

ABSTRACT In general, the formation of martensite in the metastable austenite grains in steels during nanoindentation investigations is believed to be induced by strain as the plastic deformation of austenite takes place prior to the martensitic transformation. However, it is not clear whether the formation of martensite occurs without the prior initiation of plasticity (stress-induced martensitic transformation) during nanoindentation measurement. The present work demonstrates that the martensitic transformation can be triggered during elastic deformation of austenite under an ultralow load when the indenter is close to the annealing twin boundaries. The indentation pressure interacts with the martensite transformation strain, providing the mechanical interaction energy to overcome the nucleation barrier of the martensite embryo. The present work suggests that the annealing twin boundaries can also serve as the nucleation sites of martensite and the stress-induced martensitic transformation is possible during nanoindentation investigation.