Formation Of Microtwins Research Articles

Tailoring the deformation mechanisms in a Ni-Co-based superalloy by controlling γ′ precipitate size

The role of micro-twins in the high-temperature deformation of superalloys is crucial and influenced by precipitates and deformation conditions. This study investigates the deformation mechanisms in a Ni-Co-based superalloy with γ′ precipitates varying from 31 to 122 nm under two compression conditions at 760 °C. The deformation mechanism exhibits a close correlation with the size of γ′ precipitates under 5 % stain, while the formation of micro-twins is relatively limited. Microtwinning becomes the predominant deformation mode when compressed to 20 %. The micro-twins reach the peak intensity for the alloy with 47 nm-sized γ′ precipitate. The optimizing of γ′ precipitate size can significantly enhance the formation of micro-twins in Ni-Co-based superalloys. This strategy of modulating the deformation mechanism through precipitate size regulation offers valuable insights for the design of advanced alloys.

Effects of Oxygen Content on Microstructure and Creep Property of Powder Metallurgy Superalloy

The effects of oxygen content on the microstructure and creep properties of the FGH96 superalloy were investigated. When oxygen content increased from 135 ppm to 341 ppm, the prior particle boundary (PPB) rose from degree 2 to degree 3, the size of the γ′ phase on PPB enlarged from 1.07 μm to 1.27 μm, and the MC carbide size grew from 77.4 nm to 104.0 nm. Meanwhile, the steady creep rate accelerated from 4.34 × 10−3 h−1 to 1.87 × 10−2 h−1, and the creep rupture life shortened from 176 h to 94 h, the creep rupture mode transferred from intergranular and transgranular mixed fracture to along PPB fracture. During creep, the micro-twin formation and gliding will be restrained by ∑3 boundaries. FGH96 superalloy with higher oxygen content contains less ∑3 boundaries, and its micro-twins cross-slipped instead of single-direction slip in lower oxygen content superalloy. Consequently, samples with a higher oxygen content crept faster and ruptured earlier.

Characteristics of negative creep aging and its microstructure-oriented tensile behavior

UNS S17400 steel was creep-aged under a constant stress of 300 MPa at 480 °C and 600 °C for 1, 3, and 5 h, and its microstructural phase evolution was investigated. During the initial creep aging at 480 °C for 15 min, negative creep strain occurred because of the stress-assisted precipitation, which resulted in Cu-rich precipitates impeding the movement of dislocations. By contrast, the sample creep-aged at 600 °C had a typical creep-strain curve comprising a primary creep stage and secondary steady-state stage. The creep stress accelerated the growth of Cu-rich precipitates and austenite γ phase reversion, which were clearly observed in samples creep-aged at 600 °C. The reverted γ phase formed due to the stress-assisted aging at 480 °C for 3 h, as well as at 600 °C due to the elevated temperature aging. The samples with the longer creep aging times decreased in tensile yield strength but increased in strain hardening rate. Strain hardening after strain softening was observed in the sample creep-aged at 600 °C for 5 h, which had the highest strain hardening rate. Strain hardening was attributed to the dislocations becoming entangled with nanoscale Cu-rich precipitates, the formation of microtwins, and the presence of deformation-induced martensite.

Modeling the Rheological Behavior of a Novel Hot Isostatic Pressed Powder Metallurgy Superalloy

The rheological behavior and deformation mechanisms of a new powder metallurgy (P/M) superalloy at various deformation conditions are researched. The deformation conditions have significant influence on the rheological stress. Increasing the deformation temperature or decreasing the strain rate can decrease the rheological stress. The discontinuous hardening and softening phenomena are observed at the strain rate of 1 s−1, resulting from the complex phase transformation and dynamic recrystallization. Besides, the deformation activity energy (Q) declines with increasing the strain. The phenomenon is attributed to the spheroidization of γ′ phase and the decreased content/aspect ratio of γ′ phase. The deformation mechanisms of the researched superalloy are the accumulation of dislocation, stacking faults shearing, dislocations pinned by γ′ phase, and the formation of microtwins during hot deformation. The strain‐compensated Arrhenius and particle swarm optimization‐based backpropagation artificial neural network (PSO‐BP ANN) models are established to predict the rheological behavior. Compared to the strain‐compensated Arrhenius equation, the developed PSO‐BP ANN model presents the higher accuracy in predicting the rheological behavior of the researched alloy. Furthermore, for the developed PSO‐BP ANN model, the correlation coefficient is 0.9995, and the root mean square error is 1.224 MPa. So, the forecasted rheological stresses are consistent with the measured ones.

Effect of carbon nanotube substrate temperature on the evolution mechanism of microstructure in FeCoNiCrCu coatings

The microstructure and substrate temperature dependence of FeCoNiCrCu high-entropy alloys (HEAs) coatings on carbon nanotube (CNT) surfaces were investigated using experimental and molecular dynamics simulations. It was found that the crystal structure of FeCoNiCrCu underwent the evolution of amorphous, stacking faults (SFs), multiple-fold twins, and columnar grains with an increase in the CNT substrate temperature. When the CNT substrate temperature was less than 800 K, the SFs and microtwin formation was induced by the mismatch stress at the interface of HEAs and CNT. Meanwhile, the refinement of SFs was caused by continuous crossing and cutting of dislocations. When the CNT substrate temperature was greater than 800 K, multiple-fold twin were induced by thermal stress at the surface of HEAs coating. During the evolution of the fivefold twin, the first microtwin was formed from the surface of the HEAs, and the remaining four microtwin growth from the center successively. These results provide a new method to design and control the interface structure between HEAs and CNTs.

Effect of cold rolling on microstructure, texture, and tensile properties of a Ni-Fe-based superalloy

In this work, the effect of cold rolling on microstructure, texture, and tensile properties of a Ni-Fe-based superalloy for Generation Ⅳ nuclear reactor application was systematically investigated. The results show that different deformed microstructures were observed during cold rolling, including dislocation tangles, slip bands, and microtwins (MTs). The dominant cold rolling deformation mechanism of the alloy gradually changed from planar slipping to deformation twinning, which was a little different from Ni-/Co-based superalloys. A texture transition from Copper {112}< 111 > to Brass {110}< 112 > and Goss {110}< 001 > at 25 % rolling reduction was detected due to the changed deformation mechanism. The standard heat treatment alloy exhibited a yield strength of 213 MPa and good elongation of 54 %. By increasing the rolling reduction up to 55 %, the yield strength was significantly enhanced to 872 MPa, which was attributed to the combined strengthening from both MTs and dislocations. Fine-spaced MTs with coherent interfaces can impede dislocation motion effectively and play a critical role in the strengthening, contributing 75–235 MPa to the yield strength. MTs can also provide ample room for dislocation accumulation, which maintained a reasonable elongation of 12 %. Moreover, due to the texture evolution during cold rolling, the averaged Schmid factor (SF) values for {111}< 110 > slip system exhibited a decreasing trend with the increased rolling reduction, contributing to the progressive increase in yield strength. The present work demonstrates that the formation of MTs at high rolling reductions is an essential reason for the improvement of mechanical properties of the alloy, which also provides a new strategy for designing and optimizing high-performance solid-solution strengthened Ni-Fe-based superalloys.

Deformation micro-twinning arising at high temperatures in a Ni-Co-based superalloy

• Deformation twinning discovered in Ni-Co-based superalloys above 1000 °C. • Low SFE and poor dislocation-driven form LAGBs and ITBs. • Suppression of cross-slip event affects the formation mechanism of MTs at high temperatures. • Deformation mechanisms at high temperatures related to the loading direction of columnar grains. Deformation twinning is an important deformation mechanism in nickel-based superalloys. For superalloys, deformation twins are generally observed at low or intermediate temperatures and high strain rates; however, the appearance of microtwins (MTs) at high temperatures has rarely been reported. In this study, transmission electron microscopy (TEM) was used to study MT formation in Ni-Co-based superalloys following compression at 1120 °C/1 s −1 . The deformation behavior was discussed in detail to reveal the mechanism of MT formation. The twinning mechanism at elevated temperatures was theoretically attributed to the low stacking fault energy (SFE) and poor dislocation-driven deformations caused by the high strain rate in specific directions.

Exploring the correlation between microscopic mechanisms and macroscopic behaviour in creep of a directionally solidified tungsten-free γ/γ′ CoNi-base superalloy

We describe in detail the deformation micromechanisms operative during the compressive creep of a directionally solidified (DS) tungsten-free γ/γ' Cobalt-base superalloy across a range of stresses and temperatures and their manifestation on the macroscopic creep behaviour. Creep experiments were carried out at 800 °C, 850 °C, and 900 °C under different stress levels between 200 MPa and 500 MPa. We observe two low and high-stress domains with stress exponent values of 2.3–3.2 and 6.8–9.5, and the estimated activation enthalpies of ∼352 KJ/mol (at 200 MPa) and ∼657 KJ/mol (at 400 MPa), respectively. Transmission electron microscopy and atom probe tomography were used to understand operative micromechanisms. In the low-stress regime, dislocations are confined to the γ matrix channels and form γ/γ' interface networks with enrichment of Mo solute at the dislocation cores implying that a viscous drag may influence dislocation bow out processes from the interface network leading to a stress exponent of 2–3. The high-stress regime is characterized by γ′ precipitate shearing along with the formation of superlattice-extrinsic stacking faults and micro-twins at 800 °C. Compositional analysis reveals segregation at these faults and twin boundaries. At higher temperatures (850 °C and 900 °C), γ' shearing is dominated by the formation of antiphase boundaries rich in Co and depleted in Mo. Accordingly, we identify three distinct deformation mode domains within the stress versus temperature space associated with interface network formation and low stress exponent creep, γ′ precipitate shearing by SESFs/micro-twinning, and shearing by APB.

Microstructure and mechanical properties of difficult to weld Rene 77 superalloy produced by laser powder bed fusion

Fabrication of γ′ precipitation strengthened nickel-based superalloys via laser powder bed fusion still remains a challenge. In this study, Rene 77, a high γ′ containing superalloy that is considered as difficult to weld, was processed by laser powder bed fusion. Crack-free parts with high density were fabricated without any compositional modifications or preheating of the built plate. This defect-free structure was maintained upon solutionizing and aging heat treatment. The microstructure of the samples has been characterized in detail following the fabrication and after the heat treatment. Scanning electron microscopy analysis revealed that the as-built microstructure consists of columnar grains mainly aligned in the <100> direction along with extremely fine γ′ precipitates and spherical cell boundary carbides. The grain structure and texture were unaffected by the applied heat treatment due to the pinning effect exerted by the carbide particles. Development of a bimodal γ′ distribution including cuboidal primary and spherical secondary precipitates was observed in the heat-treated sample. Additional carbide formation as a discontinuous grain boundary film was also seen. Tensile deformation behavior for both conditions was also tested at room temperature and 810 °C. Measured strength values for all test conditions were higher compared to a wrought and heat-treated alloy tested at room temperature. The as-built sample showed hardening and loss of ductility during elevated temperature testing due to γ’ precipitation at the test temperature. The microstructure of the heat-treated sample was not altered during testing at 810 °C. However, improved elongation behavior and transition in fracture mode from cleavage to ductile fracture were observed due to microtwin formation at elevated temperatures.

Deformation Effect on Microtwinning and Crystal Lattice Distortion of Polycrystalline Cu–Al Alloys

The paper considers the dynamics of changing of misoriented dislocation substructures appearing at a stage of developed plastic deformation. The types of defects, which induce the distortion of the crystal lattice are studied in detail. The quantitative parameters are obtained for the lattice distortion in alloys at a different grain size. It is found that the lattice bending and torsion parameters in the misoriented cellular-network and microtwin substructures depend on the degree of deformation. The lattice distortion is investigated both in the vicinity and far from the microtwins. It is shown that the microtwin formation occurs with increasing deformation at a different grain size.

Effect of Orientation on Stress-Rupture Property and Related Deformation Microstructure of a Ni-Base Re-containing Single-Crystal Superalloy at 900 °C

Stress-rupture properties of a Ni-base Re-containing single-crystal superalloy with three orientations have been tested under 900 °C/445 MPa. An obvious anisotropy of stress-rupture property is attributed to orientation reliant deformation microstructure. The good strength in [001] orientation is attributed to the rapid multiplication of dislocations active in horizontal channels and later γ’ cutting via dislocations pair coupled with anti-phase boundary. The microtwin formation largely limits the strength and plasticity as a result of the continuous shearing across γ/γ’ microstructure by {111}〈112〉 slip activated in [011] orientation. The property in [111] orientation results mainly from the lateral cross-slip movements of the screw dislocations within connected matrix channels as well as the precipitate shearing by coplanar dislocations. Microcracks all initially originate from the interdendritic micropores in three orientations. The critical temperature of stress-rupture anisotropy could be increased by a high level of refractory solutes especially Re.

Влияние деформации на эволюцию микродвойникования и кривизну-кручение кристаллической решетки поликристаллических сплавов Cu-Al

Development of misoriented dislocation substructures (DSS) appearing during large plastic deformation in polycrystalline Cu-Al alloys was studied. A qualitative analysis of defect types, which are sources of curvature-torsion (χ) of the crystal lattice, was carried out. The value of χ was measured in alloys with different grain sizes &lt; d &gt;. Changes in curvature-torsion of the crystal lattice in misoriented cellular-network and microtwin substructures were studied as a function of the deformation degree. The values of χ in the vicinity of microtwins and further away from themwere determined. Examples of microtwin formation with an increase of the deformation degree and grain sizes were provided.

Enhanced mechanical properties of wrought γ′-strengthened Co-base superalloys by adjusting the relative content of Al and Ti

With the goals of exploring the feasibility of improving the yield strength without decreasing the hot workability, four novel Co–30Ni-(7-x)Al–2W-1.5Mo–1Ta-(3.5 + y)Ti–12Cr-0.1Hf-0.08C-0.08B-0.4Si-0.01Zr (x = 0, 1, 1.5, 3, y = 0, 0, 0.7, 1.4 at%) γ′-strengthened wrought Co-base superalloys were designed, and the effects of Al and Ti on microstructure and properties of γ′-strengthened wrought Co-base superalloys were analyzed. Comparing the alloys 7Al-3.5Ti, 5.5Al-4.2Ti and 4Al-4.9Ti with the similar hot workability, the Al(at%)/Ti(at%) in the γ′ decreased with the decrease of Al(at%)/Ti(at%) in the alloys, this composition change in the γ′ made the alloys’ yield strength at room temperature and 800 °C increase from 915 MPa to 1025 MPa and from 764 MPa to 826 MPa, respectively. Based on the 6Al-3.5Ti alloy, the addition of 1 at% Al changed the γ′ compositions, which resulted in a decrease of yield strength at room temperature by 45 MPa but a significant increase of yield strength by 108 MPa at 800 °C, and this significant increase of yield strength at 800 °C may also be associated with the formation of numerous microtwins in 7Al-3.5Ti alloy; In the four experimental alloys, Mo preferentially partitions to the γ matrix, the relative content of Al and Ti has a negligible influence on the hot deformation resistance at 1150 °C, and there is no harmful phase after a heat treatment of 900 °C at 8 h + 750 °C at 12 h.

Influence of microstructure on phase transformation behavior and mechanical properties of plasma arc deposited shape memory alloy

Recently, additive manufacturing have gradually become an attractive processing technology for TiNi alloy. In this work, the Ti50Ni50 alloy was successfully prepared by plasma arc deposition (PAD) technology. The microstructures, phase transformation characteristics and mechanical properties were investigated in detail. The PAD-TiNi alloy mainly comprised of TiNi (B2), TiNi (B19’) and Ti2Ni phases. The TiNi cellular crystals, TiNi dendrite crystals and TiNi equiaxed crystals were presented at the bottom, middle and top area of PAD-TiNi alloys, respectively. Some irregular Ti2Ni phases were distributed in the inter-dendritic areas. The peritectic reaction of L (Ti) + TiNi →Ti2Ni led to the formation of irregular Ti2Ni phases. The coherence of Ti2Ni phases and TiNi matrix phases drove the two-step phase transformation (B2→ R → B19’) during the cooling process. The as-deposited samples showed favorable hardness and high strength, especially possessing special quasi-linear superelasticity (up to 3.8%) with narrow hysteresis, which was derived from the quick appearance and disappearance of deformation microtwins under loading and unloading condition. The newly formed interface between Ti2Ni precipitates and TiNi matrix phase offered the nucleation sites for deformation microtwins and promoted the formation of microtwins.

Molecular beam epitaxy and defect structure of Ge (111)/epi-Gd2O3 (111)/Si (111) heterostructures

Molecular beam epitaxy of Ge (111) thin films on epitaxial-Gd2O3/Si(111) substrates is reported, along with a systematic investigation of the evolution of Ge growth and structural defects in the grown epilayer. While Ge growth begins in the Volmer-Weber growth mode, the resultant islands coalesce within the first ∼10 nm of growth, beyond which a smooth two-dimensional surface evolves. Coalescence of the initially formed islands results in the formation of rotation and reflection microtwins, which constitute a volume fraction of less than 1%. It is also observed that while the stacking sequence of the (111) planes in the Ge epilayer is similar to that of the Si substrate, the (111) planes of the Gd2O3 epilayer are rotated by 180° about the [111] direction. In metal-semiconductor-metal Schottky photodiodes fabricated with these all-epitaxial Ge-on-insulator (GeOI) samples, significant suppression of dark current is observed due to the presence of the Gd2O3 epilayer. These results are promising for applications of these GeOI structures as virtual substrates or for realization of high-speed group-IV photonic components.

An empirical non-proportional cyclic plasticity approach under multiaxial low-cycle fatigue loading

The non-proportional hardening is revealed to be a main reason to influence the fatigue life of materials which are sustained to multiaxial low-cycle fatigue loading. Microstructurally speaking, increase of stacking faults to the grain size, formation of micro-twins in deformation bands and/or nucleation of martensite particles under non-proportional loading can introduce the strain hardening effect. Nevertheless to quantify the correlation between the microstructural phenomena and the macrostructural characteristic of material is not yet a trivial task. After quantitative analysis of the correlation between microstructural development and periodic deformation behaviour, an empirical non-linear hardening approach based on Benallal's NP factor estimation, which reflects the relationship between microstructural condition and internal stress responses is presented in this paper. Our approach is verified to be effective by experimental observations under different loading paths.

Creep deformation mechanisms and CPFE modelling of a nickel-base superalloy

Different cooling paths from a supersolvus temperature have been applied to FGH96, a polycrystalline nickel base superalloy for turbine disc applications, in order to simulate the different microstructures that exist through the thickness of a disc following an industrial heat treatment. The microstructures have been evaluated in terms of γ’ size distribution, morphology and volume fraction for the different heat treatments using SEM and post digital image software. The results illustrate that γ’ size decreases with cooling rate and interrupted cooling leads to a higher tertiary γ’ but lower secondary γ’ volume fraction. The heat treated samples were creep tested under 690 MPa at 700 °C. It was found that the creep response it inversely proportional to secondary γ’ size. Tertiary γ’ volume fraction was also found having a great impact on the material's creep properties. The TEM micrographs show that matrix dislocations could be the precursor of microtwin formation and creep deformation modes are closely related to tertiary γ’ precipitates. Tertiary γ’ size determines whether a/2<110> dislocations shear or dissociate before entering tertiary γ’ and the tertiary γ’ volume fraction determines how matrix dislocation dissociates. A CPFE model has been developed based on the experimental results. The simulation results for both the single element and the poly-crystalline model indicated that the modelled secondary stage creep rate is in very good agreement with the experimental results. The modelling enables the consequences of this to be investigated for representative different spatial microstructural disc conditions.

A thermodynamically consistent constitutive model for diffusion-assisted plasticity in Ni-based superalloys

An elasto-viscoplastic thermodynamically consistent constitutive model for diffusion-assisted phase transformations is presented here. The model accounts for the different deformation mechanisms, their time dependence, the crystal rotations produced by microtwin propagation and the chemistry-plasticity coupling occurring at high temperature. It is applied to the study of the chemically assisted microtwinning observed in Ni-based superalloys in the temperature range of 600–800 °C. The model parameters are calibrated against multi-directional mechanical data from tensile creep tests of single crystal superalloy MD2. The constitutive model is then implemented into a crystal plasticity finite element code to study the activation of the different deformation mechanisms within single crystal and polycrystalline aggregates. Doing so, a relation between the rotations of the crystal and the creep life of the different crystal orientations is established. The results eventually reveal the critical role of the strong anisotropy of microtwin formation on the asymmetric behavior of the alloy and its relevant role on the mechanical performance.

Diffusion processes during creep at intermediate temperatures in a Ni-based superalloy

The local compositional changes associated with stacking fault and microtwin formation during creep at intermediate temperatures in a commercial Ni-base disk superalloy are explored. In order to investigate microtwin formation, an [001] single crystal of ME3 was tested in compression at 760 °C under a stress of 414 MPa – a stress-temperature regime found to promote microtwinning. Atomic resolution scanning transmission electron microscopy combined with state-of-the-art energy dispersive X-ray (EDX) spectroscopy analysis reveals the presence of Co and Cr rich Cottrell atmospheres around leading dislocations responsible for the creation of SISFs, SESFs, and microtwins. This analysis also highlights the role that tertiary γ particles inside γ′ precipitates have on γ′ shearing deformation mechanisms. Through the use of CALPHAD calculations, combined with new experimental observations, new insights into the rate-controlling processes during creep deformation are discussed.

Investigation of fracture mechanisms and substructural changes under sequential fatigue cycling involving LCF and HCF loads in a type 316LN stainless steel at 923 K

Cumulative fatigue damage under sequential low cycle fatigue (LCF) and high cycle fatigue (HCF) loading was investigated at 923K by conducting HCF tests on specimens subjected to prior LCF cycling at various strain amplitudes. Under such loading conditions, a critical damage marking the strongest LCF-HCF interaction was found to occur at specific pre-exposures of LCF depending inversely on the strain amplitude employed. Fracture surface examination of specimens failed in HCF with prior exposure in LCF showed formation of isolated striation pockets as the main manifestation of LCF-HCF interaction. Detailed Transmission Electron Microscopy (TEM) analysis revealed extensive formation of microtwins as prominent microstructural manifestations of LCF-HCF interaction. All these features appear within the window of effective LCF-HCF interaction and hence are a strong function of the applied strain amplitude as well as the degree of prior LCF exposure. It was found out that even though formation of major crack (critical damage) is a pre-requisite for microtwins to appear, there are several microscopic factors associated to the formation of such twins, which were discussed in detail.