Stress-induced Twinning Research Articles

Relaxation mechanisms of near – α titanium alloy during stress relaxation with synchronous aging

Stress relaxation with synchronous aging (SRSA) could not only reduce springback but as well improve the performance of titanium alloy components. To reveal the relaxation mechanisms during SRSA, solution treatments of Ti-6.5Al–2Zr–1Mo–1V alloy under different temperatures were carried out to obtain diverse initial microstructures. The evolutions of microstructure including phase transformation, dislocation movement and stress-induced twinning were meticulously characterized. The progress of activation energy was analyzed quantitatively by the Kohlrausch-Williams-Watts equation. Results show that SRSA under all different conditions exhibits a rapid stress decline initially followed by a quasi-static stage, due to a shift in the driving force from stress-driven to thermally activated driven. The activation energy changes from a quasi-static state to a rapid rise with relaxation time. After two-phase solution treatment, the primary α (αp) dominant mechanism is the dislocation movement, including dislocation tangles and then recovery. The relaxation mechanism of the supersaturated metastable β phase is phase transformation (precipitation and growth of secondary α (αs)) and stress-induced twinning, and the αs realizes stress release through the globularization in the later stage. The full martensite microstructure was obtained by solution treatment in a single-phase region, and the main relaxation mechanism involves stacking faults, stress-induced twinning and then globularization. The enhancement in strength following SRSA is primarily attributed to the formation of αs, dislocation strengthening, and stress-induced twinning.

A novel integrated hot forming with in-situ stress relaxation-aging for titanium alloy thin-walled components

The simultaneous achievement of high strength and precision in the fabrication of titanium alloy thin-walled components is a long-standing issue. This work proposes a novel integrated forming process, named Hot Forming with In-situ Stress Relaxation-Aging (short for HF-ISRA) to solve the special issue. In contrast to usual isothermal forming, the forming temperature in the proposed process is raised to the solution treatment temperature. Dies at a lower temperature realize in-situ stress relaxation-aging after hot forming. The role of the dies, in addition to forming, is also achieved post-forming heat treatment. This novel process comprises three main steps: solution heat treatment, rapid forming at solution temperature, and in-situ stress relaxation-aging. An experimental prototype of HF-ISRA was developed for V-bending test, in which a sheet blank was rapidly heated using electric current and the forming die was heated using heating rods. The process window of the proposed HF-ISRA was established based on the V-bending and uniaxial tensile tests of the TA15 titanium alloy. The results showed that compared with cold forming, the springback angle obtained using the optimized HF-ISRA decreased by 97.8 %, from 9°06′ to only 12′. The tensile strength at room temperature and 500 °C was improved by 4.4 % and 10.9 % compared with the as-received material, respectively. The relaxation mechanisms of αs were the precipitation, growth, and then globularization. The relaxation mechanism of αp was dislocation movements. The strength improvement in HF-ISRA was due to the formation of αs and dislocations strengthening. The stress-induced twinning in αs was also a contributor. The proposed novel process provides a new route for the fabrication of titanium alloy thin-walled components with high precision and strength.

Role of grain size in the evolution of stress-induced martensite and the tensile deformation behavior of Ti-7333 alloy

The evolution of martensite and the tensile deformation behavior of Ti-7333 alloy with different β grain size was studied. It is found that the β grain size (45–260 μm) significantly affects the triggering stress of stress-induced α″ martensite (SIMα″), and then affects the distribution and morphology of α″ martensite. At the initial stage of deformation, the larger β grain can inhibit the stress-induced secondary α″ martensitic transformation and the formation of α″ twinning. While under the same strain, the stress-induced martensite and martensite twinning can be activated simultaneously in the alloy with the smaller β grain, including {111}α″ type Ⅰ twinning, {130}<310>α″ compound twinning, <211>α″ type Ⅱ twinning, {021}<512>α″ type Ⅱ twinning, which can effectively accommodate localized strains and improve the work hardening rate of Ti-7333 alloy. When the β grain size varies in the range of 45–84 μm, there is a higher work hardening plateau in the deformation stage II of Ti-7333 alloy. The yield strength and β grain size are in accordance with the Hall-Petch relation. As the grain size increases to 128 even to 260 μm, the work hardening rate continues to decrease and the yield strength inversely increases. It is demonstrated that the triggering stress for SIMα″ exhibits U-shaped fluctuations as β grain size increases, and the inflection point of Ti-7333 alloy is between 84 μm and 260 μm.

A Review of Deformation Mechanisms, Compositional Design, and Development of Titanium Alloys with Transformation-Induced Plasticity and Twinning-Induced Plasticity Effects

Metastable β-type Ti alloys that undergo stress-induced martensitic transformation and/or deformation twinning mechanisms have the potential to simultaneously enhance strength and ductility through the transformation-induced plasticity effect (TRIP) and twinning-induced plasticity (TWIP) effect. These TRIP/TWIP Ti alloys represent a new generation of strain hardenable Ti alloys, holding great promise for structural applications. Nonetheless, the relatively low yield strength is the main factor limiting the practical applications of TRIP/TWIP Ti alloys. The intricate interplay among chemical compositions, deformation mechanisms, and mechanical properties in TRIP/TWIP Ti alloys poses a challenge for the development of new TRIP/TWIP Ti alloys. This review delves into the understanding of deformation mechanisms and strain hardening behavior of TRIP/TWIP Ti alloys and summarizes the role of β phase stability, α″ martensite, α′ martensite, and ω phase on the TRIP/TWIP effects. This is followed by the introduction of compositional design strategies that empower the precise design of new TRIP/TWIP Ti alloys through multi-element alloying. Then, the recent development of TRIP/TWIP Ti alloys and the strengthening strategies to enhance their yield strength while preserving high-strain hardening capability are summarized. Finally, future prospects and suggestions for the continued design and development of high-performance TRIP/TWIP Ti alloys are highlighted.

Study on the stress relaxation with synchronous ageing of near α titanium alloy

The stress relaxation with synchronous ageing of TA15 titanium alloy was studied under a plastic pre-loading condition. The evolutions of microstructure and room temperature properties were characterized. Results show that the stress can be eliminated by 85 %, and the ultimate tensile strength can be increased by 5 % compared to the as-received sheet at room temperature. The plastic pre-loading results in dislocation entanglements in primary α (αp) and precipitation of partial secondary α (αs). The main stress relaxation mechanisms in the early stage are the precipitation and growth of αs, stress-induced twinning and the dynamic recovery in αp. The main stress relaxation mechanisms in the late stage are the globularization of αs and dislocation recovery in αp. The strength improvement after stress relaxation is mainly due to the formation of αs.

Accelerated stress relaxation with simultaneously enhanced strength of titanium alloy by phase transformation and stress-induced twinning

Stress relaxation with synchronous aging for titanium alloys was proposed to reduce the stress relaxation limit and enhance the mechanical strength simultaneously. Stress relaxation tests at 600 °C of solution treated sample and aged sample were performed to study the interplay between microstructure evolution and stress relaxation. Kohlrausch-Williams-Watts equation was employed to analyze the relaxation kinetics. Results show that the applied stress promotes the rapid precipitation and growth of the αs (secondary α) phase. Meanwhile, the applied stress also induces the presence of twinning in the αs phase, leading to the relaxation of the elastic energy in the αs. In return, both phase transformation and stress-induced twinning reduce the stress relaxation limit by 56 %. Moreover, the tensile strength of Ti-6.5Al-2Zr-1Mo-1 V alloy at room and service temperature was improved by 3.4 % and 9.2 %, respectively.

Design of metastable β-Ti alloys with enhanced mechanical properties by coupling αS precipitation strengthening and TRIP effect

A strain-transformable microstructure was successfully designed in a metastable β Ti–7Mo–3Nb–3Cr–3Al alloy with enhanced mechanical properties, by introducing ∼28% α precipitates coupled with the TRIP effect, overcoming the traditional trade-off dilemma between strength and ductility in most metastable β-Ti alloys. The as-designed lamellar microstructure was predominantly deformed by stress-induced martensitic (SIM α") phase transformations, martensitic twinning and dislocation slip of the parent β grains and α laths. The β→α" transformation followed the [113]β//[112]α"//[-310]α"//[1-21]α" orientation relationship, with the {133}β habit plane predicted by the Phenomenological Theory of Martensite Crystallography (PTMC). A novel <211>α" type II martensitic twinning mode was also found, in addition to the {111}α" type I mode at SIM α"/α impinging region. The results show that, not only the lamellar α precipitates play a major role in precipitation strengthening, but they can also effectively block SIM α" propagation at the initial stages of deformation. However, SIM α" transmission across the α laths was also observed for large strains. Moreover, <c+a> pyramidal slip and shear of the α laths also contributed to the accommodation of internal stresses. Therefore, the origin of the enhanced tensile mechanical properties can be attributed to the combined effects of α precipitation strengthening coupled with the TRIP softening effect and the extra interaction stresses introduced by the α laths and other deformation products, validating the design concept. The current investigation may provide a novel strategy for designing new high-performance metastable β-Ti alloys.

Crystal plasticity finite element simulation of NiTi shape memory alloy under canning compression based on constitutive model containing dislocation density

Crystal plasticity finite element (CPFE) simulation is employed for revealing plastic deformation mechanism of NiTi shape memory alloy (SMA) under canning compression at 400 °C, where only B2 austenite phase exists and it presents plastic deformation by dislocation slip without stress-induced martensite phase transformation and deformation twinning. Statistically stored dislocation (SSD) and geometrically necessary dislocation (GND) densities are incorporated into crystal plasticity constitutive model for strain gradient. Texture evolution, stress and strain fields, SSD and GND densities are obtained on the basis of CPFE simulation. With progression of plastic deformation, γ-fiber (<111>) texture is gradually strengthened, which is further validated by the experimental data. Heterogeneous plastic deformation of NiTi polycrystalline cylinder model subjected to canning compression is illustrated by distribution of stress and strain based on all the slip systems, where stress concentration mainly emerges near the grain boundaries and large strain appears in the core location of NiTi polycrystalline cylinder. SSD density and GND density exhibit a heterogeneous distribution in the similar manner. Both SSDs and GNDs aggregate near the grain boundaries. SSD density increases with increasing plastic strain, whereas GND density decreases with increasing plastic strain. In addition, the total dislocation density increases with the increase of plastic strain. The developed CPFE model is an effective tool for simulating plastic deformation of NiTi SMA in the appropriate temperature range where dislocation slip is responsible for plastic deformation and work hardening is dominant during plastic deformation.

Effect of stable stacking fault energy and crystal orientation on fracture behaviour of thin metallic single crystals

ABSTRACT Understanding the evolution of dislocations and twinning at the crack front is critical for designing micro-mechanical systems with improved performance. In this work, the dislocation evolution at the crack front in thin pre-cracked FCC single crystals is correlated with the associated fracture toughness, which is shown to be dependent on material specific properties such as stable stacking fault energy and crystal orientation using atomistic simulations. For materials with high value, sessile dislocations form at the crack front causing increased localised plastic flow stress that leads to low fracture toughness. Whereas the fracture process in materials with low value is governed by the motion of glissile dislocations and stress-induced twinning leads to high fracture toughness. For this case, twinning occurs at high stress levels followed by un-twinning due to stress relaxation at crack front by twinning. The crystal orientation influences the type of dislocations emitted (screw/edge) from the crack front which governs the mode of crack propagation. The Mode-III crack propagation by the emission of screw type dislocations causes significant decrease in the fracture toughness compared to Mode I crack propagation which is caused by simultaneous emission of edge type dislocations on the two symmetrically inclined slip planes at the crack front. For certain pre-cracked crystal orientations, twinning is seen during the early stages of plastic deformation in materials with high value. However, un-twinning is not observed in crystal orientation-based twinning at the crack front.

Influence of Nb addition on microstructural evolution and compression mechanical properties of Ti-Zr alloys

In order to expand the application horizon of Ti-Zr based alloys, the influence of Nb content on phase composition, microstructural evolution and biomechanical properties is systematically studied. The phase and microstructural characterization of the as-cast alloys is carried out by X-ray diffraction, optical microscopy and transmission electron microscopy. The results reveal that Nb-containing Ti-Zr alloys transformed from single α phase → α + α″ + β phase → single β phase → double β phases with increasing Nb content. In the case of β-type alloys, the addition of Nb improves the bonding energy between atoms, reduces the grain size, increases the elastic modulus, improves the yield strength and renders superior work-hardening behavior. Moreover, the current study provides mechanistic insights into microstructural evolution and strengthening of Nb-containing Ti-Zr alloys with increasing Nb content. Herein, the addition of 5 at.% Nb resulted in an abnormal work hardening during compression deformation under the synergistic influence of stress-induced martensite transformation of β phase and stress-induced twinning of α phase. Moreover, the biomechanical properties are evaluated to demonstrate the potential of Nb-containing Ti-Zr alloys in biological applications.

Ductile Ti-rich high-entropy alloy controlled by stress induced martensitic transformation and mechanical twinning

Body centered cubic type of refractory high-entropy alloys (HEAs) have high strength at room and elevated temperatures, but suffer from the strength-ductility trade-off dilemma, making their application for structural materials severely impeded. In this article, we report a strategy to break this puzzle by introducing successive stress-induced martensitic transformation (SIMT) and mechanical twinning in Ti-rich refractory TiZrHfAlNb HEA via controlling the grain size. The HEA exhibits superior ductility featured by fracture and uniform elongation as high as 40% and 33%, respectively, at ambient temperature. The plasticizing and toughening mechanisms for the HEA with different grain sizes can be described as following: in the HEA with small grains, the SIMT takes place followed by the martensite variants reorientation; in that with coarse grains, mechanical twinning of α″ phase comes up with the preferential orientation relationship of [111]β//[101]α″M//[-10-1]α″T after the SIMT.

Enhanced Work-Hardening and Toughening Via Contiguous Stress-Induced Martensitic Transformation and Mechanical Twinning in Ti-Zr-Hf-Nb-Al High Entropy Alloys

Refractory high entropy alloys (HEAs) have been considered as candidate alloys for next-generation high-temperature applications, but encounter the strength-ductility trade-off dilemma. In this article, we proposed a strategy of strengthening and toughening for Ti-rich refractory HEAs by the initiation of martensitic transformation and mechanical twinning successively. Three Ti-Zr-Hf-Al-Nb HEAs with the tensile strength and elongation up to 1167 MPa and 34%, respectively, which are superb among most of the reported BCC HEAs, have been successfully prepared. By using the SEM, TEM, HE-XRD and EBSD setups, the as-cast and fully recrystallized microstructures for these HEAs were characterized. And the mechanisms for the martensitic transformation and mechanical twinning in these HEAs are deeply explored. It has been clarified that under the as-cast and fully recrystallized state, these HEAs are mainly composed of β phase and a little of α′′ martensite phase. Stress-induced martensitic transformation took place in favor of special variants with the [110]β//LD, which has the maximum transformation strain of 7.8%. At the later stage, mechanical twinning α′′ phase have formed between the α′′ laths and the β matrix with the preferential orientation relationship of [111]β//[101]α′′M//[-10-1]α′′T.

Characterization of the crystal structure, kinematics, stresses and rotations in angular granular quartz during compaction.

Three-dimensional X-ray diffraction (3DXRD), a method for quantifying the position, orientation and elastic strain of large ensembles of single crystals, has recently emerged as an important tool for studying the mechanical response of granular materials during compaction. Applications have demonstrated the utility of 3DXRD and X-ray computed tomography (XRCT) for assessing strains, particle stresses and orientations, inter-particle contacts and forces, particle fracture mechanics, and porosity evolution in situ. Although past studies employing 3DXRD and XRCT have elucidated the mechanics of spherical particle packings and angular particle packings with a small number of particles, there has been limited effort to date in studying angular particle packings with a large number of particles and in comparing the mechanics of these packings with those composed of a large number of spherical particles. Therefore, the focus of the present paper is on the mechanics of several hundred angular particles during compaction using in situ 3DXRD to study the crystal structure, kinematics, stresses and rotations of angular quartz grains. Comparisons are also made between the compaction response of angular grains and that of spherical grains, and stress-induced twinning within individual grains is discussed.

Formation of Deformation-Induced Products in a Metastable-β Titanium Alloy during High Temperature Compression

A metastable-β titanium alloy, Ti-10V-3Fe-3Al (wt. %), was subjected to thermo-mechanical processing including the compression test at 725 °C, which is below the β transus temperature (780 °C), and at strain rate of 10−3 s−1. The presence of phases was determined using transmission electron microscopy and X-ray diffraction. Although the dynamic recovery took place together with slip, both deformation-induced α” martensite and ω were detected as other operating mechanisms for the first time in metastable-β titanium alloy deformed in α + β region. The volume fraction of stress-induced α” was higher than that of the same alloy deformed at room temperature due to a higher strain applied. Stress-induced twinning was not operational, which could be related to the priority of slip mechanism at high temperature resulted from thermally-assisted nucleation and lateral migration of kink-pairs.

Stress-Induced Twinning and Phase Transformations during the Compression of a Ti-10V-3Fe-3Al Alloy

A metastable β Ti-10V-3Al-3Fe (wt pct) alloy containing different α phase fractions after thermo-mechanical processing was compressed to 0.4 strain. Detailed microstructure evaluation was carried out using high-resolution scanning transmission electron microscopy and electron back-scattering diffraction. Stress-induced β → α′′ and β → ω transformation products together with {332}〈113〉β and {112}〈111〉β twinning systems were simultaneously detected. The effects of β phase stability and strain rate on the preferential activation of these reactions were analyzed. With an increase in β phase stability, stress-induced phase transformations were restricted and {112}〈111〉β twinning was dominant. Alternatively, less stable β conditions or higher strain rates resulted in the dominance of the {332}〈113〉β twinning system and formation of secondary α′′ martensite.

Stress-induced twinning of nanocrystalline hexagonal close-packed titanium in Ti-6Al-4V

Twinning behavior of nanocrystalline hexagonal close-packed titanium in Ti-6Al-4V subjected to severe plastic deformation was investigated. High-resolution transmission electron microscopy observations first showed that the deformation twinning of nanocrystalline hexagonal close-packed titanium was induced. The occurrence of deformation twinning was attributed to the extremely high stress due to specific grain orientations and the large strain rate. The twinning mechanism of nanocrystalline hexagonal close-packed titanium in Ti-6Al-4V was the overlapping of two stacking fault ribbons. This study provided a new thinking that it might be possible to design nanocrystalline titanium and its alloys to extensively activate twinning for further improving their properties.

Effect of Alloying Elements in Hot-Rolled Metastable β-Titanium Alloys. Part II: Mechanical Properties

This paper describes the tensile properties, flow and work-hardening behavior of four metastable β-titanium alloys Ti-5Al-5Mo-5V-3Cr (A1), Ti-5Al-3.5Mo-7.2V-3Cr (A2), Ti-5Al-5Mo-8.6V-1.5Cr (A3), and Ti-5Al-3.5Mo-5V-3.94Cr (A4) in α+β hot-rolled condition. The decreasing order of average strength parameters (σ YS and σ UTS) is A4, A2, A1, and A3. The maximum strength observed in alloy A4 is due to the presence of highest wt. fraction of Cr. The elongation is the maximum and minimum in alloys A3 and A4, respectively. These alloys display moderate to high percent in-plane anisotropy (A IP) and reasonably low anisotropic index (δ) values. Both the A IP and δ values are maximum and minimum in alloys A1 and A3, respectively. The yield locus plots also exhibit the presence of anisotropy due to relatively large differences in yield strength values along tension and compression directions. The flow behavior of alloys A1, A2, and A4 follows Swift equation, while the alloy A3 displays best fit with Holloman equation. The presence of prestrain (e 0) in hot-rolled materials before tensile testing has an important bearing on the flow curves of A1, A2, and A4 alloys. The instantaneous work-hardening rate curves of the alloys A1, A2, and A3 exhibit all the three typical stages (stage I, stage II, and stage III) in RD samples, while the alloy A4 shows the presence of only stage I and stage III. The 45 deg to RD and TD samples of alloys A1, A2, and A4 display only stage I. The stages I and III as well as I and II are present in alloy A3 in 45 deg to RD and TD samples, respectively. Dislocation-controlled strain hardening occurs in all the three stages of these alloys in the absence of stress-induced martensitic transformation (α″) and twinning. Slip is the predominant deformation mechanism during tensile testing. Three types of slip lines, i.e., planar, wavy, and intersecting have been observed close to fracture surfaces of post tensile-tested specimens.

Strain rate dependence of deformation-induced transformation and twinning in a metastable titanium alloy

A metastable β Ti–10V–3Fe–3Al–0.27O (wt.%) alloy was subjected to thermo-mechanical processing to induce α and ω phase formation, so that the alloy can exhibit the features responsible for both transformation induced plasticity (TRIP) and twinning induced plasticity (TWIP) behaviour during deformation. The alloy thereafter was deformed at different strain rates (10−3, 10−1, 101, 102 s−1) at ambient temperature. At slow strain rate (≤10−3 s−1), in addition to slip the alloy displayed a dominant deformation mechanism with α′′ martensite formation, where mechanical {332}〈113〉β twinning and deformation-induced ω phase were also activated. At an intermediate strain rate, 10−1 s−1, there was a competition between stress-induced phase transformations and stress-induced twinning deformation mechanisms. With increasing strain rate to 101 s−1 or higher; it was found that the dominant deformation mode was twinning. These results have been correlated with the β phase stability of the samples.

Deformation-induced martensitic transformation in a new metastable β titanium alloy

A new metastable β titanium alloy, Ti-4Al-7Mo-3V-3Cr (wt.%), was designed using d-electron method, aiming to tailor the deformation mechanism. Microstructural and X-ray diffraction analysis of deformed specimens conforming with the theoretical prediction of d-electron method, confirmed the formation of stress-induced martensite (SIM) and mechanical twinning as the deformation mechanisms. The effect of initial grain size, strain level and strain rate on the formation of SIM were investigated. The results showed that in a given grain size, the volume fraction of SIM initially increases intensively and then follows with lower rate reaching a saturation at 35% reduction. It was observed that after a decrease in the martensite laths interspace down to less than 2 μm as a result of increasing the strain, some secondary martensite laths forms within the primary ones. It was found that with an increase in grain size from 150 μm to 250 μm, the volume fraction of SIM increases while a further increase in grain size up to 500 μm leads to a decrease in SIM volume fraction. The compression test results at different strain rates ranging from 0.7 × 10−4 to 0.7 × 10−1s−1 showed SIM transformation occurs at all strain rates and although the triggering stress for SIM transformation increases continuously with an increase in strain rate, the volume fraction of SIM is independent of the strain rate.

Dependence of stress-induced omega transition and mechanical twinning on phase stability in metastable β Ti–V alloys

Tensile properties and deformation microstructures of a series of binary β Ti–16–22V alloys have been investigated. The results show that the plastic deformation mode changes from the plate-like stress-induced ω phase transformation with a special habit plane of {−5052}ω//{3−3−2}β to {332}<113> type deformation twinning with increasing the content of vanadium in the β Ti–16–22wt.% V alloys. The plate-like stress-induced ω phase has a special orientation relationship with the β phase matrix, i.e., [110]β//[−12−10]ω, (3−3−2)β//(−5052)ω and (−55−4)β//(30−31)ω. The alloys plastically deformed by stress-induced ω phase transformation exhibit relatively higher yield strength than those deformed via {332}<113> type deformation twinning. It can be concluded that the stability of β phase plays a significant role in plastic deformation mode, i.e., stress-induced ω phase transformation or {332}<113> type deformation twinning, which governs the mechanical property of the β Ti–16–22wt.% V alloys.