Intersections Of Deformation Twins Research Articles

Effects of Ni/Cu replacement on improvement of tensile and hydrogen-embrittlement properties in austenitic stainless steels

Effects of Ni/Cu replacement and subsequent aging treatment on tensile and hydrogen embrittlement (HE) properties were investigated in Fe-Cr-Ni austenitic stainless steels. With the Ni/Cu replacement, tensile strength and ductility were reduced in the as-annealed state by the increased stacking fault energy (SFE), whereas they were enhanced in the aged state due to the consumption of Cu in the matrix by the precipitation and resultantly lowed SFE. After the electrochemical H-charging, the H permeation depth decreased in the aged Cu-containing steels, indicating that Cu-rich precipitates effectively interfered the diffusion of H to increase the HE resistance. However, the excessive formation of Cu-rich precipitates destabilized the austenite phase, and induced the martensitic transformation. This α’-martensite preferably formed inside or at intersections of deformation twins, along with austenite grain boundaries, thereby providing H-induced crack propagation paths. These cracks caused the earlier brittle fracture and severe HE as a consequence. This result would suggest that the formation of Cu-rich precipitates by replacing expensive Ni could enhance both tensile properties and HE resistance, but a delicate control of SFE should be required to overcome the trade-off relationship of strength and HE resistance.

The sequential twinning-transformation induced plasticity effects in a thermomechanically processed high Mn austenitic steel

Different initial microstructures with various bimodal grain size distributions (BGSD) were produced in a high Mn austenitic steel through applying a predetermined set of thermomechanical processing cycles. The corresponding room temperature mechanical properties and the related strain hardening behaviors were assessed using tensile testing method. The results indicated that in the microstructure with high grain size bimodality, the length and amplitude of rapid hardening region was well higher than the others. This was attributed to its higher capability to α'-martensite formation. In addition, the threshold strain to initiate martensitic transformation was shifted to the lower one in the microstructure with higher bimodal grain size distribution. The latter was related to the lower arisen back stresses in the interior regions of the coarser grains. Furthermore, different transformation paths were identified as the BGSD changed. The austenite could directly transform to α'-martensite (γ→α') in the microstructure with lower BGSD; in this case the α'-martensite mainly appeared at the intersections of deformation twins. In contrast, in microstructures with higher BGSD, the nucleation occurred at the intersections of ε-martensite platelets. The co-existence of these transformation paths provided an extended transformation induced plasticity effect ending to a higher elongation to fracture in the course of deformation. In order to summarize the contribution of various strain hardening mechanisms, a deformation map was also constructed. Accordingly, the enhanced ductility/strength properties were attributed to the sequential operation of extended transformation induced plasticity and twinning induced plasticity effects.

Deformation twin and martensite in the Fe–36%Ni alloy during cryorolling

ABSTRACTIn this paper, the microstructural evolution and deformation mechanism of the cryorolled Fe–36%Ni alloy was investigated. Deformation twinning was confirmed to be activated at the early stage of cryorolling (CR) and subsequently suppressed due to microstructure refinement. The existence of and peaks revealed that the (α′-M) phase transformation was induced during the CR process. The stress-assisted martensite and strain-induced martensite were detected to be transformed at the grain boundary and intersection of deformation twins, respectively. Owing to the co-effect of the temperature raise and mechanical stabilisation, the α′-M phase transformation was suppressed during the CR process. Therefore, the amount of the α′-M was limited in the tested alloy.

Effect of Cold Working and Aging Treatment on the Mechanical Properties of Co-Ni Alloy

Co-Ni alloys in which Co, Ni, Cr and Mo are principal elements, exhibit an impressive combination of high strength, high toughness, and excellent corrosion resistance. In this study, the effects of cold-rolled combined with the heat-treatment ranged from 673 to 973 K from 1hour to 10 hours on the mechanical properties and microstructures of Co-Ni alloys were investigated systematically. The relations between the aging temperature and mechanical properties were concluded. The initial ultimate tensile strength of 790 MPa increased to 1808 MPa by cold rolled 80 pct. After aging the cold-rolled alloy (80 pct ) at temperature 773K for 4hours, the ultimate tensile strength and the hardness reached to 2220MPa and 759(HV), respectively. It is found that the material was hardened by the cold working and aging which provided the second hardening. However, TEM observations and X-ray diffractions suggested that no structural change could be found. The cold deformation introduced platelets of a few atomic layers in thickness less than 100 nm, which were identified as stacking faults. A high density of nanoplatelets and dislocations, piled up in the vicinity of twin plate strengthened materials. The aging treatment provided the second major source of strengthening after cold-working (and only after cold-working) by the formation of secondary twins. The ultimate strength resulted from that the intersection of deformation twins and secondary twins blocking the dislocation movement.

Influence of Al on the temperature dependence of strain hardening behavior and glide planarity in Fe–Cr–Ni–Mn–C austenitic stainless steels

Two austenitic stainless steels of compositions Fe–17Cr–9Ni–6Mn–0.4C–(0 and 4)Al (mass-%) were tensile tested at temperatures between −196°C and 200°C. The influence of Al on the temperature dependence of the strain hardening behavior, tensile properties, and deformed microstructures was subsequently studied. For both alloys, the strain hardening at high deformation temperatures was characterized by near-linear hardening at low stresses and transition to a regime of non-linear hardening at higher stresses. Lower tensile temperatures resulted in the extension of the initial near-linear regime of hardening. Near-linear hardening until necking was established at a lower temperature for the Al-containing alloy than for the Al-free alloy (−100°C vs −40°C). This was correlated with a higher glide waviness in the Al-containing alloy. Deformation twinning was the most obvious feature of glide planarity in the deformed microstructures. Tensile elongation of both steels showed a maximum within the temperature range studied. The peak elongation temperature almost exactly coincided with the temperature at which near-linear hardening until necking occurred and was therefore lower for the Al-containing alloy. The loss of ductility at temperatures below the peak elongation temperature was attributed to the occurrence of a small fraction of deformation-induced α′ at intersections of deformation twins. In agreement with the reported stacking fault energy-raising effect of Al, results indicated an increased stability of austenite upon Al addition.

Effect of Ultrasonic Nanocrytalline Surface Modification on Hardness and Microstructural Evolution of Cu-Sn Alloy

In this study, a Cu-Sn sintered bronze, used largely for con-rod bushing and automotive transmission, was treated by ultrasonic nanocrystalline surface modification (UNSM). Then, Vickers hardness and microstructural evolution of the treated region were investigated by using scanning electron microscope (SEM), X-ray diffraction (XRD) and transmission electron microscope (TEM). The hardness of the treated surface doubled, which is attributed to the developed of nanoscale grains, deformation twins, and high density of dislocations induced by the UNSM. Microstructural modification beneath the UNSM treated surface was typically characterized with increase of the depth: (i) nanoscale grains (top surface), (ii) intersection of deformation twins (~30 μm), (iii) high density nanoscale twin/matrix lamellae (~50 μm), (iv) interception of micro band and deformation twins (~100 μm), (v) dislocation arrays (~200 μm), (vi) low density dislocations (~300 μm) and (vii) pre-existing coarse grains and annealing twins in unaffected region (400 μm ~deeper).

Nanoscale cracks at deformation twins stopped by grain boundaries in bulk and thin-film materials with nanocrystalline and ultrafine-grained structures

Nanoscale fracture processes initiated at intersections of deformation twins and grain boundaries (GBs) in nanocrystalline and ultrafine-grained materials are theoretically described. Within the suggested description scheme, nanocracks are formed at GBs at which deformation twins are stopped and thereby produce specific GB defect configurations that create high local stresses initiating nanocracks. The conditions for realization of this new fracture mode in nanocrystalline and ultrafine-grained materials with face-centred cubic and body-centred cubic crystal lattices are calculated. The free-surface effect on crack generation at deformation twins stopped by GBs in thin-film nanocrystalline specimens is theoretically described. The critical parameters for crack generation in nanocrystalline and ultrafine-grained bulk materials and thin films are revealed. It is shown that in bulk nanomaterials nanocracks are generated at twin thicknesses of several nanometres and grow very fast with increasing twin thickness. At the same time, in thin-film nanomaterials, the equilibrium nanocrack lengths significantly decrease if the distance between the twin lamella and the closest film surface does not exceed several twin thicknesses.

Twin intersection mechanisms in nanocrystalline fcc metals

Deformation twins have been reported to produce high strength and ductility. Intersections of deformation twins may affect the microstructural evolution during plastic deformation and consequently influence mechanical properties. However, the mechanisms governing twin-intersection behavior remain poorly understood. In this study, we investigated twin intersection mechanisms by observing twin transmission across the boundary of another twin using high-resolution transmission electron microscopy. Based on the experimental observations, mechanisms were proposed for twin–twin intersections and associated dislocation reactions in nanocrystalline fcc materials.

Ductile-to-brittle transition behavior of high-interstitial Fe-Cr-Mn alloys

The ductile-to-brittle transition behavior of high-interstitial Fe-Cr-Mn alloys with different N and C contents is discussed in terms of the deformation microstructure and the mode of brittle fracture. The combined addition of N + C improved the low-temperature toughness by decreasing the ductile-to-brittle transition temperature, compared to the addition of N alone, by effectively increasing the free-electron concentration and enhancing the metallic component of interatomic bonding. Transmission electron microscopy observations on deformed regions beneath the fracture surface of Charpy impact specimens tested at low temperatures indicated that α′-martensite was formed at the intersections of deformation twins in the N + C alloy, unlike in the N alloy. Thus, it is suggested that the formation of α′-martensite exerts a beneficial influence on low-temperature toughness because it suppresses the initiation of brittle crack by reducing the internal stresses of intersecting twins. On the other hand, the ductile-to-brittle transition temperature of the N + C alloys increases with increasing C content, which could be explained by the occurrence of intergranular fracture resulting from the excessive content of C above a certain level.

Twin intersection in tensile deformed γ-TiAl intermetallic compounds

Intersections of deformation twins in γ-TiAl grains were examined in Ti-47at.%Al alloy specimens tensile-tested at 873 K. Transmission electron microscopy (TEM) revealed 1 / 2 < 1 1 ¯ 0 ] { 111 } dislocations and 1 / 6 < 11 2 ¯ ] { 111 } twins in deformed γ-TiAl grains. Some γ-TiAl grains contained twins on two different {111} planes, which intersect along <110] directions (type-1). No twin intersection along <101] (type-2) was observed, which was often reported in compressed specimens. Results are discussed with the Schmid factor distributions of twinning systems and slip systems. It should be noted that twinning shear in L1 0 structure is limited only in one 1/6< 11 2 ¯ ] movement on a {111}. Considering Schmid factors of possible four twin systems, both type-1 and type-2 twin intersections are expected. Tensile load direction which can activate both twinnings of type-2 intersection can also activate ordinary 1 / 2 < 1 1 ¯ 0 ] { 111 } dislocations. The absence of type-2 intersection suggests that ordinary slip systems were favored in these cases.

Crystallography of nanometre-sized α′-martensite formed at intersections of mechanical γ-twins in an austenitic stainless steel

Nanometre-sized !′-martensites (bcc, denoted by α), a few nanometres in cross-sectional diameter, formed at intersections of deformation twins, have been found in a heavily deformed 316-type austenitic stainless steel (fcc, denoted by γ) and examined by high-resolution transmission electron microscopy (HRTEM). An orientation relationship was found to be close to the Kurdjumov-Sachs relationship, that is (111)γ//(101)α, [10amp;1macr;]γ//[11amp;1macr;]α. HRTEM observations were conducted with a beam direction parallel to [10amp;1macr;]γ//[11amp;1macr;]α. The observed nanometre-sized !'-martensites exhibited a (2amp;5macr;2)γ habit plane and the (10amp;1macr;)γ//(11amp;1macr;)α cross-section of the nanometre-sized !'-martensites were elongated along [545]γ. It has been revealed that the nanometre-sized !'-martensites of a few nanometres diameter formed at the intersections of deformation twins have a typical habit plane of (2amp;5macr;2)γ, which is identical with that reported for a fully grown plate such as !'-martensite. The origin of the observed habit plane is discussed on the basis of the ⟨112⟩{111}γ shear displacements assisting the γ → α′ transformation.

Microstructural instability in a crept fully lamellar TiAl alloy

A fully-lamellar TiAl alloy creep deformed at 760°C with a stress of 138 MPa and a strain of ∼1.5% was employed in the examination of microstructural instability. Fragmentation and spheroidization of α 2 lamellae and formation of recrystallized γ grains ( γ G ) inside the lamellar colony were observed. It was proposed that the fragmentation and spheroidization of α 2 lamellae during creep could be caused by either a subgrain boundary groove mechanism or an intersection of deformation twins with α 2 phase lamellae. Possible mechanisms were proposed. The formation of coarsened γ G grains was frequently observed. The γ G grains had crystallographic relationships with the lamellar structure as < 1 ̄ 10> γ G // < 1 ̄ 10 γ>// [11 2 ̄ 0] α 2 and {111} γ G ⊥{111} γ//(0001) α 2 . Simple thermodynamical analysis was done on the formation of γ G grains. Deformation and dynamically recrystallized structures in the γ G grains were also observed.

Fragmentation of α2 Plates in a Fully Lamellar TiAl During Creep

ABSTRACTThe fragmentation and spheroidization of α2 laths in a fully-lamellar TiAl alloy during creep were examined. Three possible mechanisms, Rayleigh's perturbation model, subgrain boundary groove mechanism and intersection of deformation twins with α2 lamellae were presented and discussed. During creep deformation, the pile-up of interfacial dislocations leads to a change of planar interface, which, in turn, causes a difference in local chemical potential, and further results in the spheroidization of α2 lamellae. On the other hand, the deformation of the α2 phase is expected to be induced by the high local stress concentration introduced by the pile up of interfacial dislocations. The dynamic recovery process may lead to the formation of subgrain boundaries in the α2 lamellae, which results in the spheroidization and termination of α2 lamellae with the aid of diffusion during creep.

The Effect of Fracture Mechanism on Low-Cycle Fatigue Life of Pure Iron at Low Temperature.

Low-cycle fatigue tests were performed at -140°C using commercial-grade pure iron having an average ferrite grain size of 400 μm. The effect of fracture mechanism on fatigue life properties was investigated, especially on the dispersion of fatigue lives in the high plastic strain range (Δεp) where fracture occurs in an internal fracture mode. Fracture surfaces were observed using a scanning electron microscope. The dispersion of fatigue lives is due mainly to the difference of internal fracting mode. Specimens fractured by intersection of a deformation twin and a grain boundary (T-B type) show longer fatigue lives than those fractured by intersection of deformation twins (T-T type) or a deformation twin and an inclusion (T-I type). The effect of grain size at the internal fracture origination site was also discussed.

Interaction of deformation twins with α 2 plates in γ-TiAl base alloy deformed at room temperature

Intersections of deformation twins with α plates in a two phase γ TiAl alloy deformed at room temperature have been investigated in detail by transmission electron microscopy (TEM). It has been found that two types of configurations, depending on the orientation of the intersection line, should be distinguished. For a type-I intersection with the intersection line parallel to 〈110] y, the deformation twins are blocked by the α 2 plate. At these intersections ordinary glide dislocations were emitted back into the γ matrix on cube slip planes, together with the production of twinning in γ parallel to the γ/ α 2 interface. No shear transfer across the γ/ α 2 interface was observed in this case. For a type-II intersection with the intersection line parallel to 〈011] γ, the incident twinning shear is usually accommodated by a-type slip on {11¯00} planes in the α 2 plate. In one particular case, {22¯01} twinning was observed in the α 2 plate at the intersection. The observations were analysed by taking into account all possible dissociations of a twinning partial (or a number of them) into two glissile dislocations, or into one glissile dislocation (glissile in matrix or plate) and a residual interface dislocation. This was done by considering a number of conditions for dissociation of twinning partials at the intersection and subsequent relaxation by slip. These conditions are similar as reported earlier for twin—twin intersections in the γ phase, and for slip transfer at grain boundaries in metals and alloys. The analysis showed that for a particular dissociation not all conditions could be fulfilled at the same time. The combination of the analysis and the observations showed the following order of priority of conditions: (i) a common line of intersection with the γ/ α 2 interface of the incoming twinning plane and the relaxation slip planes; (ii) the slip directions of the glissile product dislocations are consistent with the applied load, which was checked by taking into account the sign of the Schmid factor S; (iii) a zero or small residual interface dislocation, and (iv) a high value of ‖ S‖ (sometimes S was low).

Fracture Mode Transition in Low-Temperature Low-Cycle Fatigue of Commercial Pure Iron.

Strain controlled low-cycle fatigue tests were performed under push-pull loading conditions at -140°C, -90°C and room temperature (RT) using commercial pure iron having ferrite grain size of 400μm. To clarify the fracture mechanisms, observation of the fracture surfaces was also carried out with special focus on the role of deformation twins in the generation of microcracks which lead to final fracture of the specimen. The transition of the fracture modes occurs from surface to internal fracture modes with an increase in the plastic strain range Δep at testing temperatures of -90°C and -140°C. In a large Δep regime crack initiation occurs inside the material, and this internal crack leads to final fracture. Deformation twins strongly affect the generation of internal cracks at -140°C and -90°C. Three internal crack initiation sites were observed: (i) the intersection of deformation twin and grain boundary, (ii) the intersection of several deformation twins and (iii) the intersection of deformation twin and inclusion. The applicability of the Manson-Coffin low-temperature low-cycle fatigue of commercial pure iron is also discussed in relation to the fracture mode transition behavior.

Brittle fracture in austenitic steel

The fracture mode of austenitic steel may feature a ductile to brittle transition (DBT), depending on alloy composition and temperature. The DBT variation can be explained in terms of the actual deformation structure evolving during cold work and the correlated internal stresses. The crucial features of microstructure causing brittle fracture are found to be the intersections of deformation twins and the total density of free dislocations. To avoid brittle fracture, the stresses of intersecting twins must be screened by dislocations. Manganese and nitrogen promote brittle fracture since they lower the stacking fault energy and thus shift the onset of twinning to low strain levels where the total dislocation density is low. Nickel additions oppose this trend. The results of the microstructural fracture model agree well with experimental results and the model is confirmed by continuum-mechanical considerations.

Intersections of deformation twins in TiAl II. Models and analyses

Abstract In TiAl with the L10-ordered structure there are two basic crystallographic configurations in which twin bands from two systems intersect each other, both of which have been investigated in this work. Transmission electron microscope observations described in part I have shown the structural changes associated with twin intersections in these two configurations. The aim of the present paper is to examine mechanisms in which the shear imposed by the incident twin on the barrier twin is accommodated. It is shown that in L10 many crystallographic relations exist in which the displacement continuity at the matrix/twin interface is maintained in shear transmissions. Most of them, however, are found to be inconsistent with the experimental observations. The present paper aims to investigate factors that have limited the occurrence of certain reaction schemes. It is found that the energetics measured in terms of the balance in the self energies of the dislocations involved is not sufficient to acount fo...

The Origins of Tritium and Helium Effects on the Tensile Properties of Metals

In this paper, the effects of internal tritium and helium on the tensile properties of two austenitic stainless steels and an iron-based superalloys have been studied. The materials tested were, forged 21 Cr-6Ni-9Mn and 304L (tested in the annealed condition and tow forged conditions), and a modified A-286 alloy. The accumulation of {sup 3}He from the radioactive decay of tritium caused an increase in the yield strength and a continuous decrease in the ductility in almost all materials tested. Increased {sup 3}He concentrations also caused a change in fracture mode form ductile rupture to predominantly intergranular fracture. The property changes resulted from {sup 3}He bubble-induced strengthening, which produced a change in deformation mode form long-range dislocation activity to deformation twinning. In the deformation-twinning mode, the {sup 3}He-accelerated fracture initiated at the intersections of deformation twins with grain boundaries. High-strength forged 304L was most resistant to {sup 3}He effects owing to the redistribution of {sup 3}He on dislocations.

Accelerated fracture due to tritium and helium in 21-6-9 stainless steel

The effects of internal tritium and helium on the room-temperature tensile properties of a nitrogen-strengthened stainless steel, forged 21Cr-6Ni-9Mn (NITRONIC 40), were investigated by thermally charging tritium into tensile specimens and aging for selected times. The precipitation of helium as bubbles on dislocations greatly increased the yield strength, and as a consequence of dislocation pinning, the deformation mode changed from long-range dislocation motion to deformation twinning. The tensile specimens exhibited a 90 pct decrease in tensile ductility at 1438 appm 3 He, accompanied by a severe change in fracture mode from ductile rupture to a dominantly intergranular fracture mode. Grain-boundary facets showed multiple striations where deformation twins had intersected the boundaries. Twinning began immediately upon yielding, and at small strains, the microstructure evolved into a fully hardened state, normally observed at 40 pct or greater strain in unexposed or hydrogen-charged 21-6-9. Fracture occurs at low strains in tritium-charged and aged 21-6-9, in part, because helium bubble precipitation causes the deformed microstructure to evolve to a heavily deformation-twinned state at a lower strain. Helium bubble precipitation on the grain boundaries may have caused a further loss in ductility. Fracture appeared to nucleate at the intersection of deformation twins with the grain boundary.