Stress-induced Reorientation Research Articles

Stress-induced martensite reorientation and its related performances in trained β-Ti based shape memory alloy

In the present study, the influence of training on the microstructural evolution, martensitic transformation behavior and mechanical/functional properties of Ti–V–Al–Zr shape memory alloy was investigated systematically. With the increasing of training cycling number, the degree of martensite reorientation became larger and larger. Moreover, higher density of dislocations appeared in the interior of martensite variants, upon the excessive training cycling number was applied. The international stress can be brought into the matrix, which not only promoted to the ω→αˊˊ transition, but also made the martensite phase stabilizer and even suppressed the αˊˊ→β martensitic transformation during training process. The superior mechanical properties including maximum fracture stress of 906 MPa and the higher fracture strain of 33.6% can be obtained in Ti–V–Al–Zr shape memory alloy subjected with the training cycles number of 5th, which can be attributed to the matrix strengthening stemmed from work hardening and suppression of ω precipitation. Meanwhile, the lower critical stress for the martensite variant reorientation and enhanced matrix strength synergistically contributed to the superior strain recovery characteristics with total completely recoverable strain of 6% in Ti–V–Al–Zr shape memory alloy by controlling training cycles number of 5th. Meanwhile, the higher micro-hardness can be obtained by training.

Unveiling the formation of nanotwin-mediated metastable ζ-hydrides in fatigued Zr-1Nb-0.01Cu cladding tube

The impact of internal pressure fatigue treatment on the α-Zr matrix and hydride precipitates in Zr-1Nb-0.01Cu cladding tube has been systematically investigated using synchrotron-based high-energy X-ray diffraction, electron backscatter diffraction and transmission electron microscopy techniques. The evolution in d-spacings of the basal plane (0002), the pyramidal plane {101¯1} and the prismatic plane {101¯0} at azimuths from 0° to 360° indicates that the internal pressure fatigue treatment has imparted a degree of plastic deformation upon the α-Zr phase, which is further confirmed by the presence of {101¯2} deformation twins within the α-Zr phase. Concurrently, the prior internal pressure fatigue treatment at 400 °C significantly increased nucleation sites for zirconium hydrides during the subsequent furnace cooling process. High-density dislocations can be seen in both the δ-hydride precipitate ({111}<110> type) and the α-Zr matrix ({101¯0}<a> type) near the hydride-matrix interface. Additionally, δ-hydride {111}<112¯> nanotwins were observed within the micro-sized δ-hydrides in the fatigued cladding tube, resulting from the nucleation and growth of intra-granular δ-hydrides at different regions of the same parent α-Zr grain. Furthermore, the HCP-structured ζ-hydride phase was detected at the δ-hydride nanotwin forefront along the growth direction and at the curved δ-hydride nanotwin boundaries, providing direct experimental evidence to the notion that the metastable ζ-hydride serves as the precursor to the stable δ-hydride phase during the precipitation process. The current experimental findings contribute to the fundamental understanding of microscopic mechanisms underlying the delayed hydride cracking and the stress-induced hydride reorientation in Zr alloys.

Coupled magneto-mechanical response of laminate composites comprising a layer of Ni-Mn-Ga microparticles

Ni-Mn-Ga ferromagnetic shape memory alloys (FSMAs), exhibiting a giant magnetic field induced strain (MFIS), high work output and fast response, are highly promising materials for emerging technologies to be used in automotive, aerospace, and robotics industries, among others. Their magnetic-to-mechanical energy conversion ability is characterized by the coupled magneto-mechanical response they show when magnetic field and mechanical stress are simultaneously applied. In the present work we elaborated a series of laminated composites comprising a layered ensemble of single crystalline Ni-Mn-Ga microparticles built-in between Cu foils via small layers of silicone rubber. Such a simple and robust design enabled an in-depth study of the simultaneous influence of the magnetic field and compressive opposing stress on MFIS and the generated force of the particle layer driving out-of-plane magnetomechanical response of the laminate. Self-consistent parameters characterizing a magnetic field- and stress-induced martensitic variant reorientation in the particles were disclosed by the measurements and comprehensive analysis of both the magnetization curves under opposing constant stresses (complemented by tracking residual strains with X-ray µ-CT imaging) and compressive stress-strain dependences under transversal magnetic field. In particular, an accurate value of the magnetic-to-mechanical energy conversion coefficient, equal to Cme = (2.3 ± 0.13) MPa/T2, and the value of maximum magnetostress of about 2.3 MPa generated by microparticles were determined, in a good agreement with bulk Ni-Mn-Ga single crystals (SC). In contrast to bulk SC, particles are technologically and costly more efficient. They have much higher degree of freedom in terms of composites design allowing the development of advanced miniature actuators and sensors.

Phase transformation behavior and mechanical properties of NiTi shape memory alloys fabricated by directed laser deposition

Directed laser deposition shows promise as a method for fabricating complex structure NiTi shape memory alloy parts in engineering, aerospace, automobile, and biomedical fields. The dislocation density has an impact on phase transformation behavior and mechanical properties of NiTi manufactured by directed laser deposition. In this work, NiTi samples were produced by directed laser deposition, and the mechanism of martensitic phase transformation and its interactions with dislocation density were revealed through molecular dynamics. Laser power and scanning speed are the main factors affecting dislocation density, the phase compositions of the samples are B2, B19′, and Ti2Ni phase, and the samples with dislocation density below 2 × 1015 m−2 show single-step B2–B19′ martensitic transformation, stress plateau and improved elongation, the samples with dislocation density above the value exhibit no significant transformation peak and no stress plateau. The movement of (100) twin planes and the growth of advantageous variants are observed in the simulation which can explain stress-induced martensitic reorientation and the stress plateau phenomenon. The bending test suggested that the sample under the optimal parameters has an 85 % recovery ratio within 8 % pre-strain.

Hydride precipitation dependence on α-parent orientation and plastic deformation in zirconium alloys

Hydrides are at the center source of mechanical degradation affecting Zr-based nuclear fuel cladding integrity during in-reactor service and even spent fuel storage. This study presents investigations on hydride precipitation in response to thermo-mechanical treatment in a hydrogenated Zircaloy-4 plate. Correlated EBSD and ECCI characterizations demonstrate stress-induced variations in α-parent orientation and stacking structure between the in-plane and reoriented hydrides. Based on the hydride strains quantified by HR-EBSD, external tensile stress is found to favor the α-grains submitted to significant {0001}<10-10> shear stress as preferential nucleation sites for reorientated hydrides. Regardless of elastic or plastic stress, the interaction between external stress and hydride strains, particularly the shear component, is expected to govern hydride nucleation probability as a function of α-grain orientation and eventually induce hydride reorientation. Influence of plastic deformation is mainly manifested in hydride distribution, where the {10-10}<11-20> slipping promotes reoriented hydride interconnection up to hundreds of microns while the {10-11}<10-1-2> twining results in a fragmented distribution, as visualized by X-ray tomography. The presented results provide solid experimental evidences for the auto-catalytic nucleation and martensitic transformation of hydride precipitation, as well as new insights into stress-induced hydride reorientation in the light of stress interaction with hydride strain.

Tetragonal dipole dominated Zener relaxation in BCC-structured Fe-17at.%Ga single crystals

The stress-induced reorientation of the low-symmetry defects due to substitution atom pairs can give rise to an internal friction peak (Zener relaxation) and the first-nearest-neighbor substitutional atom pairs are plainly considered as the dominant contribution. Here, the Zener relaxation in the body-centered cubic (BCC) FeGa single crystals with different orientations was investigated, which against intuition originates primarily from the stress-induced reorientation of the second-nearest-neighbor Ga-Ga atom pairs. This Ga-Ga atom pair configuration is prevalent at high temperatures as directly observed by high-temperature in situ electron microscopy experiments. This phenomenon establishes non-ambiguously a direct correlation between Zener relaxation strengths and magnetostrictive properties. In addition, it was found that the diffusion activation energy of Ga atoms in FeGa single crystals is significantly lower than that in other iron-based alloys, suggesting that this value is closer to the actual diffusion activation energy of Ga atoms in FeGa alloys. These findings offer valuable insights into the study of short-range ordering in BCC alloys and the exploration of their functional properties.

Characterization of local deformation around hydrides in Zircaloy-4 using conventional and high angular resolution electron backscatter diffraction

Zircaloy-4 is used as a fuel cladding material for water reactors, as it has good mechanical properties, corrosion resistance, and a low thermal neutron absorption cross section. However, the mechanical performance of Zircaloy-4 can be reduced during service due to hydrogen uptake and hydride formation. These hydrides are brittle, and often reduce the strength and toughness of materials as well as increase susceptibility to delayed hydride cracking (DHC). In this work, large grain Zircaloy-4 with hydrides was prepared and then cross sectioned using cryo-ion beam polishing, using plasma focused ion beam (pFIB) and broad ion beam (BIB) approaches to enable the preparation of a very high quality flat surface with no preferential etching of either the hydride or zirconium metal (typically metallographic polishing preferentially removes hydrides). Conventional and high angular resolution electron backscatter diffraction (EBSD) analysis were then used to explore morphology, deformation fields, and orientation relationships between the zirconium matrix and hydrides. Four maps were collected for analysis which included hydrides near grain boundaries: (a) where the hydride smoothly decorates across two of the connecting boundaries near a triple junction; (b) where the hydride smoothly decorates the boundary; (c) a mixture of smooth decoration of the interface and protrusion into the grains; (d) fine scale hydride that protrudes into one grain. This work highlights that incompatibility of the hydride within the zirconium matrix is strongly linked to the orientation relationship of the hydride and matrix, and the grain boundary character. These results may enable enhanced understanding of the role of hydrides in fracture as well as stress-induced hydride reorientation and DHC susceptibility.

Exceptionally small Young modulus in 10M martensite of Ni-Mn-Ga exhibiting magnetic shape memory effect

A Ni50Mn28Ga22 single crystal exhibiting 10M martensitic structure at room temperature was studied by X-ray diffraction measured in situ under uniaxial tensile stress up to 20 MPa and in different initial orientations of the sample. When the c axis lies along the direction of the applied stress, the sample is elongated primarily by the stress-induced reorientation. In comparison, with a or b axis lying along the stress direction, the overall elongation happens primarily due to the change of the lattice parameters and also the modulation amplitude of 10M martensite rapidly decreases with the applied tension. The initial tensile Young modulus calculated from the refined lattice parameters is very low and exhibits gradual stiffening with the increasing tensile stress. Close to zero stress the modulus has an extremely low value of about 0.3 GPa.

Root twisting drives halotropism via stress-induced microtubule reorientation.

Plants have evolved signaling mechanisms that guide growth away from adverse environments that can cause yield losses. Root halotropism is a sodium-specific negative tropism that is crucial for surviving and thriving under high salinity. Although root halotropism was discovered some years ago, the underlying molecular and cellular mechanisms remain unknown. Here, we show that abscisic acid (ABA)-mediated root twisting determines halotropism in Arabidopsis. An ABA-activated SnRK2 protein kinase (SnRK2.6) phosphorylates the microtubule-associated protein SP2L at Ser406, which induces a change in the anisotropic cell expansion at the root transition zone and is required for root twisting during halotropism. Salt stress triggers SP2L-mediated cortical microtubule reorientation, which guides cellulose microfibril patterns. Our findings thus outline the molecular mechanism of root halotropism and indicate that anisotropic cell expansion through microtubule reorientation and microfibril deposition has a central role in mediating tropic responses.

Threshold stress of hydride reorientation in zirconium alloy nuclear fuel cladding tubes: A theoretical determination

One of the most critical issues affecting the integrity of nuclear fuel cladding tubes is hoop stress-induced hydride reorientation from the circumferential to the radial direction. The theoretical prediction of the threshold stress for the hydride reorientation is a long-standing but unrealized objective of material-design engineers in the nuclear industry. In this paper, we propose a criterion to determine the threshold stress based on the thermodynamic model. The results obtained this way agree very well with experimental observations. Multiple factors, such as crystallographic texture, grain-boundary structure and distribution, grain morphology and size, the hydride-matrix misfit strain, hydrogen content, the chemical composition, the temperature and the mechanical strength and anisotropy of the zirconium matrix, simultaneously control the difficulty level of hydride reorientation. Strengthening only the radial basal pole texture of the zirconium alloy tube is not enough to effectively enhance the threshold stress of hydride reorientation; other factors, especially the grain-boundary structure and distribution, also need to be considered. The model presented in this study elucidates the mechanism of stress reorientation of hydrides and offers new insight on how to further increase the resistance to hoop stress-induced hydride reorientation in nuclear fuel cladding tubes

Stress-induced reorientation of hydrides in Zr-1Nb-0.01Cu cladding tube studied by synchrotron X-ray diffraction and EBSD

High-energy X-ray diffraction and electron backscatter diffraction techniques were employed to investigate the hydride precipitation in Zr-1Nb-0.01Cu cladding tube under different internal pressures up to 30 MPa. Hydride reorientation occurs when the internal pressure exceeds 10 MPa. Higher internal pressure applied to the cladding tube induced plastic deformation, which significantly increases the nucleation sites of the δ-hydrides. The different evolutions of the (111) d-spacing with the azimuth angles indicated that the δ-hydrides precipitated from the cladding tubes under different internal pressures possessed different residual stress fields. Detail analysis of stereo-graphic projections confirmed that both the circumferential and radial δ-hydrides precipitated under different internal pressures followed the {0001}α-Zr // {111}δ-hydride and <112¯0>α-Zr // <110>δ-hydride orientation relationship with their parent α-Zr matrix. The δ-hydride {111}<112¯> twins may form when the Shockley partial dislocations glide on the alternate close-packed (0001) basal planes and the alternating distribution of δ-hydride twins may result in a deflection of hydride packet. The δ-hydride precipitates along the radial direction of the cladding tube under internal pressure is more adaptable to the volume expansion induced by phase transformation.

Mechanical spectroscopy of atomic ordering in Fe-(16−21)Ga-RE alloys

Anelasticity of Fe-(16−21)at% Ga-RE (rare earth RE = La, Tb, Dy, Er, Yb elements) alloys at room and elevated temperatures is studied in a sub-resonance mechanical spectroscopy. Two thermally-activated and two transient effects are recorded in most of the studied alloys. To explain these phenomena, structure and phase transitions in several binary and ternary Fe-Ga alloys are investigated. The D03 ordering of rapidly cooled alloys with the A2 structure at heating at around 300 °C and the disordering at heating and the D03 ordering at cooling around 500 °C for annealed samples are proved using three in situ techniques: neutron diffraction, vibrating sample magnetometry, and internal friction and supported by positron annihilation experiments. Thermally activated transitory effects are tentatively explained by stress-induced reorientation of Ga-Ga, vacancy-vacancy pairs, and carbon atoms jumps.

Correlation of magnetic field and stress-induced magnetic domain reorientation with Barkhausen Noise

This paper investigates magnetic field and stress-induced magnetic domain reorientation and its correlation with Magnetic Barkhausen Noise (MBN). The magnetic domain dynamics and MBN of grain-oriented transverse and longitudinal electrical steels are studied by using time-resolved magneto-optical imaging. Time and frequency-domain characteristics of MBN signal are investigated for evaluation of stress state and magnetocrystalline anisotropy. Transitions between magnetic domain states with differing tensile stress and magnetic anisotropy directly influence the MBN signal. Its shown that 180° domain wall (DW) reorganization and DW-type conversion generate peaks in the MBN measurement for longitudinal and transverse electrical steel, respectively. The magnetic domain imaging reveals complicated domain arrangement processes in transverse electrical steel. The results provide a physical insight into bulk micromagnetic phenomena.

High-temperature phenomena in an advanced intermetallic nano-lamellar γ-TiAl-based alloy. Part I: Internal friction and atomic relaxation processes

Intermetallic γ-TiAl based alloys have found applications in the low-pressure turbine of aircraft engines as well as in the turbocharger unit of automotive engines. However, these light-weight alloys must still be improved, through micro-alloying and tailoring the microstructure, to increase their creep resistance and consequently their maximum working temperature. In this work, a fully nano-lamellar advanced γ-TiAl based alloy doped with small amounts of C and Si is investigated in order to gain a deeper understanding of the atomic mobility mechanisms taking place at high temperature, thus controlling the creep properties. The study was approached through internal friction measurements up to 1223 K. We demonstrate that C has a notable influence on Ti diffusion in α2 phase, leading to an increase of the activation energy for Ti diffusion, which is assessed at ΔETi(α2)=0.32 eV per at% C. An atomic model for the relaxation process is proposed capable to explain this phenomenon. An additional internal friction peak, which, up to now, remained hidden by the high temperature background, was observed in this nano-lamellar TiAl alloy and analyzed through a careful de-convolution of the internal friction spectra. This new relaxation process, with activation energy of 3.70 eV, is attributed to the short distance diffusion of Al atoms in the γ-TiAl lattice. A novel concept of stress-induced cell-lattice reorientation is proposed to explain this relaxation. Finally, a new experimental method to analyze the high temperature internal friction background, which is closely related to the creep behavior, was developed to study the fully nano-lamellar microstructure, whose high temperature background exhibits the highest activation energy ever measured in a γ-TiAl based alloy.

Loading influence on the corrosion assessment during stress-induced martensite reorientation in nickel-titanium SMA

The effect of stress-induced martensite reorientation on the corrosion behavior of a nickel-titanium (NiTi) shape memory alloy (SMA) was characterized by mechanical-electrochemical techniques. This martensite reorientation influenced the passive state of NiTi SMA. These passive to active state transitions could be sensed by the global interfacial electrochemical measurements, such as electrochemical impedance spectroscopy (EIS), open circuit potential (OCP) and linear polarization resistance (LPR) methods. The increase in the loading when exposed to a corrosive environment resulted in the breakdown of the passive layer. A later repassivation stage was reached due to the formation rate becoming more dominant than the breakdown rate during loading conditions. The integration of the mechanical loading by tensile testing, strain distribution by digital image correlation (DIC) and electrochemical methods by OCP and EIS, characterized and detected the influence of the mechanical effect in the corrosion assessment and interfacial mechanisms.

High-strength cellulose–polyacrylamide hydrogels: Mechanical behavior and structure depending on the type of cellulose

Two types of stiff and high-strength composite hydrogels possessing the structure of interpenetrating polymer networks were synthesized via free-radical polymerization of acrylamide carried out straight within the previously formed physical network of regenerated plant cellulose or bacterial cellulose (PC and BC respectively) that was swollen in the reactive solution. The mechanical behavior of synthesized hydrogels subjected to the action of compressive deformations with different amplitude values was studied. The analysis of the stress-strain curves of compression tests of the hydrogels of both types obtained in different test conditions demonstrates the substantial difference in their mechanical behavior. Both the PC- and BC-based hydrogels withstand successfully the one-shot compression with the amplitude up to 80%, but in the conditions of the multiple compression tests (cyclic compressions) during the subsequent compression acts the dramatic increase in the stiffness of the BC-based hydrogels was observed at the deformation region beyond 60%. This effect can be explained by the deep reorganization of the intermolecular structure of the material with the stress-induced reorientation of BC micro-fibrils. Submicron- and micron-scale specific features of structures of composite hydrogels of both types were studied by cryo-scanning electron microscopy to explain the peculiarities of the mechanical effects observed.

Phase field simulation to one-way shape memory effect of NiTi shape memory alloy single crystal

Based on the Ginzburg-Landau theory and in the framework of thermodynamics, a three-dimensional phase field model was developed to simulate the one-way shape memory effect (OWSME) of NiTi shape memory alloy (SMA) single crystal originated from the martensite transformation and reorientation. A new reorientation strain was simultaneously introduced when a transformation strain was considered in the proposed model, and the phase field equations corresponding to the martensite transformation and reorientation were derived, respectively. From the simulations to the OWSME, it is found that: the martensite variants are grown in a self-coordinating manner and combined by twinned interfaces to reduce the local elastic energy during the temperature-induced transformation from B2 phase to B19′ one; then, the stress-induced martensite reorientation is completed through the migration of twinned interfaces and the variants with optimal orientations are obtained during the subsequent loading, and a residual strain is produced after the unloading; during the further heating after the unloading, a uniform temperature-induced reverse transformation from the single-oriented martensite variant to B2 austenite phase occurs, and the residual strain produced by the stress-induced martensite reorientation is progressively recovered.

Achieving ultralow elastic modulus in TiNi alloy by controlling nanoscale martensite phase

A low elastic modulus is urgently expected for bio-implant to avoid the harmful stress-shielding effect. Here, by using electroplastic rolling deformation, an ultralow elastic modulus of E ∼29 GPa was achieved in a Ti-50.7at%Ni alloy. The low elastic modulus can be attributed to the existence of nanoscale lamellas of low-elastic modulus martensite phase with a high fraction of 16.2% and the sluggish stress-induced martensitic transformation and reorientation of martensite variants.

In situ synchrotron X-ray diffraction investigations of the physical mechanism of ultra-low strain hardening in Ti-30Zr-10Nb alloy

A warm-rolled, metastable β-type Ti-30Zr-10Nb alloy exhibited a peculiar two-stage yielding behavior under uniaxial tensile loading, showing a first plastic stage with obvious strain hardening at 0.4%–10.4% strain and a second plastic stage with ultra-low strain hardening at 10.4%–23.5% strain. In situ high-energy X-ray diffraction (HE-XRD) was used to reveal the stress-induced martensitic transformation scenarios and physical mechanism of the different strain hardening rates. It was found that the deformation-induced phase transformation dominated the onset of the first plastic stage corresponding to the selection of favorable martensitic variants, and their elastic interaction contributed to the obvious strain hardening. HE-XRD experiments further verified that the ultra-low strain hardening rate in the second plastic stage was related to an interesting superelasticity of the martensite, which was characterized by the reversible, stress-induced reorientation of the martensite variants. This reorientation of the martensite variants was primarily due to the rigid lattice rotation of ∼23° about the [110]α″ axis toward the tensile direction. Our investigations provide in-depth understanding of the mechanism of the excellent plasticity with ultra-low strain hardening in β-type titanium alloys.

Atomic Species Associated with the Portevin–Le Chatelier Effect in Superalloy 718 Studied by Mechanical Spectroscopy

In many Ni-based superalloys, dynamic strain aging (DSA) generates an inhomogeneous plastic deformation resulting in jerky flow known as the Portevin--Le Chatelier (PLC) effect. This phenomenon has a deleterious effect on the mechanical properties and, at high temperature, is related to the diffusion of substitutional solute atoms toward the core of dislocations. However, the question about the nature of the atomic species responsible for the PLC effect at high temperature still remains open. The goal of the present work is to answer this important question; to this purpose, three different 718-type and a 625 superalloy were studied through a nonconventional approach by mechanical spectroscopy. The internal friction (IF) spectra of all the studied alloys show a relaxation peak P718 (at 885 K for 0.1 Hz) in the same temperature range, 700 K to 950 K, as the observed PLC effect. The activation parameters of this relaxation peak have been measured, Ea(P718){\thinspace}={\thinspace}2.68{\thinspace}{\textpm}{\thinspace}0.05 eV, $\tau$0{\thinspace}={\thinspace}2{\textperiodcentered}10-15 {\textpm} 1 s as well as its broadening factor $\beta${\thinspace}={\thinspace}1.1. Experiments on different alloys and the dependence of the relaxation strength on the amount of Mo attribute this relaxation to the stress-induced reorientation of Mo-Mo dipoles due to the short distance diffusion of one Mo atom by exchange with a vacancy. Then, it is concluded that Mo is the atomic species responsible for the high-temperature PLC effect in 718 superalloy