Twin Intersections Research Articles

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.

Microstructures and mechanical properties of β forging Ti17 alloy under combined laser shock processing and shot peening

The aim of this paper was to utilize combined laser shock processing (LSP) and shot peening (SP) to study the grain refinement mechanism and mechanical properties modification of β forging Ti17 alloy. Firstly, LSP experiments were conducted on the surface of Ti17 alloy by a YAG laser system and square spots of 8% overlapping rate. LSP process parameters were pulse width of 15ns, pulse energy of 30J and spot size of 4mm×4mm. Secondly, SP experiments were carried on the surface of LSPed Ti17 alloy. ASH230 steel shots were used to SP with a diameter of 0.3mm and an intensity of 0.3mmA (A-type Almen stripe). Lastly, the microstructures and mechanical properties in the surface layer of Ti17 alloy were investigated by surface topography and roughness, micro-hardness, X-ray diffraction (XRD) analysis, residual stress, scanning electron microscopy (SEM) and transmission electron microscope (TEM). Results showed that surface topography and surface roughness amplitude were increased by combined LSP and SP. The amplitude and depth of the micro-hardness in the surface layer were also significantly improved. No new phase was formed after combined LSP and SP. High amplitude compressive residual stress with −613.5MPa was induced in the surface with combined LSP and SP. The smaller phase sizes in β phase and more obvious crystal defects (deformed twining and dislocations in α phase and dislocations in β phase) were generated by combined LSP and SP. Grain refinement mechanisms were attributed to high density dislocations in α and β phases and multidirectional twin intersections in α phase.

Effect of Shot Peening Coverage on Surface Nanostructuring of 316L Stainless Steel and its Influence on Low Temperature Plasma-Nitriding

Abstract Air-blast shot peening (ABSP) is a cost effective and industrially viable technique to produce nanostructured surface layer on metallic materials. In the present study, 316L stainless steel samples were subjected to shot peening at different peening coverage, from conventional to severe peening. Nanocrystalline structure was observed on the sample surface after peening and mechanical twins; intersection of multidirectional twins producing rhombic blocks were observed in the subsurface layer. Peening process led to the formation of strain induced martensite (α‘), and its fraction was found to increase with the coverage. Depth of nanostructured layer and surface microhardness also increased with the increase in coverage, whereas surface roughness followed an opposite trend. Both peened and un-peened samples were subjected to plasma nitriding at 400°C for 4 h. Uniform and appreciably high case depth of about 45 μm was observed in severely pre-peened (1000 % coverage) sample after nitriding treatment. No precipitation of CrN was observed. This highlights the marked influence of severe shot peening as a pre-treatment for low temperature plasma nitriding of austenitic stainless steels.

Deformation response of AgCu interfaces investigated by in situ and ex situ TEM straining and MD simulations

The mechanisms of strain transfer across Ag/Cu interfaces were determined by a combination of in situ and ex situ TEM straining experiments and molecular dynamics simulations. Minimizing the magnitude of the Burgers vector of the residual dislocation generated in the interface was the dominant factor for determining the outcome of dislocation and deformation twin interactions with both non-coherent twin and cube-on-cube interfaces. This included the unexpected finding, due to the loading condition, of deformation twin activation in the Cu layer due to the intersection of deformation twins in Ag with the interface. Deformation twin nucleation in Ag from the non-coherent twin interfaces was also explained by a Burgers vector minimization argument.

Static recrystallization behavior of hot-rolled Mg-Zn-Ce magnesium alloy sheet

Static recrystallization behavior of Mg-1.5Zn-0.2Ce (wt%) alloy rolled at 480 °C was investigated at different annealing stages using quasi-in-situ electron backscattered diffraction (EBSD) analyses, which were carried out repeatedly in the same region to trace microstructural evolution. In the as-rolled condition, double twins exhibit the highest fraction compared with tensile and compression twins. Double twins mostly exist in deformed grains with basal orientations, and statically recrystallized grains mainly nucleate at the intersections of double twins and pre-existing grain boundaries. Statically recrystallized grains tend to exhibit TD-tilted orientations and grow along double twins to consume adjacent basal-oriented grains. This is the main reason for inducing the change from a RD-split texture to a well-weakened TD-split texture during annealing. The residual unrecrystallized grains with TD-tilted orientations and the growth advantage of TD-tilted grains also enhance the formation of TD-split texture.

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.

Effect of Eu on the silicon phase in Al-40Zn-5Si alloys

A series of Al-40Zn-5Si alloys with different additions of Eu were prepared by permanent mold casting. The nucleation phenomenon incorporating the modification of silicon phase was investigated by using the synchrotron radiation imaging technology, scanning electron microscopy, electron probe microanalysis and high resolution high-angle annular dark-field scanning transmission electron microscopy. It was found that the primary phase was α-Al dendrites and the pre-eutectic Si particles were precipitated continuously at the front of the solid/liquid interface in Al-40Zn-5Si alloys. The nodular pre-eutectic Si particles were observed when the Eu addition was 0.3%. Meanwhile, a flake-to-fibrous transition of eutectic Si was also observed. The needle-like eutectic Si crystals were seen to shoot out from the vicinity of primary α-Al dendrites into melt with high velocities leading the irregular eutectic interface in the unmodified Al-40Zn-5Si alloy. However, the eutectic grains with smooth interfaces nucleated mainly near primary or secondary α-Al branches in 0.3% Eu-modified Al-40Zn-5Si alloy. The eutectic grains began to nucleate on the nodular pre-eutectic Si particles when the α-Al surface reached so close to them. The formation of nodular pre-eutectic Si particles was attributed to the adsorption of Eu atoms along the <112> growth direction of Si and at the intersection of Si twins. This is fully consistent with the well-known poisoning of twin plane re-entrant edge (TPRE) and the impurity induced twinning (IIT) modification mechanisms proposed for interpreting the modification of eutectic Si.

Growth mechanism of extension twin variants during annealing of pure magnesium: An ‘ex situ’ electron backscattered diffraction investigation

Pure magnesium was subjected to plastic deformation through CSM (continuous stiffness measurement) indentation followed by annealing at 200°C for 30min. Nucleation of no new grains was observed neither at the twin–twin intersections nor at the multiple twin variants of a grain after annealing. Significant growth of off-basal twin orientation compared to basal twin orientation was observed in the sample after annealing and is attributed to the partial coherent nature of twin boundary in the later case. Further, growth of twins was independent of the strain distribution between parent and twinned grains.

In situ electron backscatter diffraction study of twinning in commercially pure titanium during tension-compression deformation and annealing

Tension-compression forward and reverse loading followed by annealing at 773K for 1h was carried out on commercially pure titanium using an in situ heating stage with tensile testing facility in scanning electron microscope fitted with electron backscatter diffraction setup. Twins of four different variants were observed during tension in load control mode in a relatively large grain within the area of inspection. Morphology of the twins changed during subsequent deformation stages and un-indexed points within the twins decreased or increased which may be due to changes in dislocation density during unloading and compression loading stages. Reloading in tension caused barrelling of the twins. Some of the twins disappeared and new twins formed during annealing. Although all the variants show similar Schmid factor for extension twinning, there is variation in lateral thickening of different twin variants within the same grain which can be explained from variation in elastic modulus. It is also observed that twin intersections can play an important role in nucleation and growth of new twins.

Microstructure and tensile behaviour of pure titanium produced after high-energy shot peening

In this study, the microstructure and tensile behaviour of pure titanium processed by means of high-energy shot peening have been studied. The results show that a nanocrystalline surface layer is prepared on the surface of pure titanium. During severe deformation, the dislocations, twins and slip bands with single orientation are formed first, then the intersections of twins and slip bands are observed, and then the subgrains appear near or in the slip bands and finally the randomly oriented nano-grains are formed due to the further breakdown. After high-energy shot peening, the ultimate tensile strength and the yield strength increase by 27 and 40%, while the elongation decreases by 64% after treatment, due to the increasing dislocation density, micro-strain and the grain refinement.

Theoretical study on the [formula omitted] deformation twinning and cracking in coarse-grained magnesium alloys

This paper theoretically and systematically investigates: (1) the effect of local transformed strains within deformation twinning on twin intersection; (2) the fracture mode based on type I co-zone tensile twin intersection in coarse-grained magnesium alloys, as well as the impacts of twin intersection and grain diameter on interfacial crack nucleation along twin boundaries; and (3) the influence of the local stresses arising from the encountered twin bands on crack growth. A novel dislocation-based strain nucleus model and a Green's function method, which are applicable to any material with local transformations in which elastic properties are reasonably approximated as isotropic, are specifically employed to model the concentrated transformed strain and calculate the local stress field resulting from deformation twinning and the stress intensity factors at crack tips in the magnesium alloys, respectively. In addition, an electron backscatter diffraction (EBSD) measurement is provided for crack nucleation originating from Type I co-zone tensile twin intersection. The theoretical modeling indicates: (i) the local strains within barrier twins strongly dictate the growth of incident twins and enhance the twin propagation stress; (ii) larger grains favor brittle fracture. More specifically, the dislocation reactions and pile-ups at the junctions between tensile twins can result in interfacial crack nucleation and growth along the twin boundaries, which is a brittle fracture mode based on lower twinning stress and stress concentration in the coarse-grained magnesium alloys; and (iii) the direction of crack propagation is easily changed by high-density twin bands and twin intersections owing to the local strains.

Microstructure evolution and strengthening mechanisms of pure titanium with nano-structured surface obtained by high energy shot peening

In this study, the microstructure evolution and strengthening mechanisms of pure titanium processed by high energy shot peening (HESP) have been studied. The results show that the deformation layer is formed on the surface and the microstructure exhibits with the equiaxed 20–40 μm grains in the matrix after HESP. A nanocrystal surface layer is produced by means of HESP on pure titanium. The formation of nano-grains on the surface can be separated into four steps: (1) the formation of the dislocations tangles; (2) the occurrence of the intersection of twins; (3) the appearance of slip band and subgrains; (4) the formation of uniformly distributed nanometer-scale grains. With increasing the holding time, the strength increases and the elongation decreases due to the work hardening effect and the formation of the nanocrystals on the surface.

Dislocation evolution and properties enhancement of GH2036 by laser shock processing: Dislocation dynamics simulation and experiment

This paper systematically investigated the effect of laser shock processing (LSP) on dislocation evolution and microstructure configuration of GH2036 alloy. Surface topography and roughness were tested by Axio CSM 700 microscope. The dislocation configurations were characterized by transmission electron microscope (TEM) and simulated by multi-scale discrete dislocation dynamics (DD) method. The results have confirmed that LSP had a beneficial effect on micro-hardness, which could be increased by 16%, and the surface topography exhibited excellent stability even after thermal cycle. The dislocation density and stress–strain response have strong dependence on laser power intensity. Reasonable agreement between DD simulation and experiments is achieved. The results showed that complex random microstructures can be observed in the shocked surface. The grain refinement mechanism of LSP GH2036 involves dislocation segmentation and twin intersections.

Surface nanocrystallization of Ti–45Al–7Nb–0.3W intermetallics induced by surface mechanical grinding treatment

Surface mechanical grinding treatment was conducted on a Ti–45Al–7Nb–0.3W intermetallics. The results show that the penetration depth plays an important role in the microstructural evolution. The γ→α2 phase transformation is observed at a penetration depth of 100μm. Nanocrystallized α2 grains (20nm) are found in the treated surface at a penetration depth of 200μm. After the surface mechanical grinding treatment, the micro-hardness of the top surface increases significantly, which is 50% higher than that of the substrate. It is also found that the γ→α2 phase transformation is induced by the localized strain in the surface, while the mechanism for the nanocrystallization is proposed to be the twin–twin intersection.

Microstructure evolution and grain refinement of Ti-6Al-4V alloy by laser shock processing

Microstructure evolution and grain refinement of Ti-6Al-4V alloy after laser shock processing (LSP) are systematically investigated in this paper. Laser shock waves were induced by a Q-switched Nd:YAG laser system operated with a wave-length of 1064nm and 10ns pulse width. The microstructures of LSP samples were characterized by scanning electron microscopy (SEM) and transmission electron microscope (TEM). Present results indicate that the surface hardness of samples subjected to LSP impacts has significantly improved. Multidirectional twin intersections and dislocation movements lead to grain subdivision in α phase with ultra-high plastic deformation. High-density dislocations are found in β phase. Multidirectional twin intersections and division of sub-grain boundaries play an important role in the grain refinement of Ti-6Al-4V alloy under LSP loading conditions.

Multiple Twinning and Stacking Faults in Silver Dendrites

Detailed defect structure of dendrite formation was studied in order to connect the mesoscopic with the atomistic structure. It was demonstrated that twinning and stacking fault formation play a central role in the growth of electrodeposited Ag dendrites. The broad faces of Ag dendrites and the main trunk growth direction were found to be (1̅11) and [1̅12̅], respectively. Dendrite branches also formed and grew from the main trunk parallel to the [121̅] and [2̅1̅1̅] crystallographic directions. Twins and stacking faults were found to reside on the {111} crystallographic planes, as expected for a face centered cubic (FCC) Ag crystal. Using electron back scattered diffraction (EBSD) we found two variants of in-plane 60° rotational twin domains in the (1̅11) broad dendrite surface plane. The intersections of twins and stacking faults with dendrite arm surfaces are perpendicular to the ⟨112⟩ arm growth directions. However, occasionally twins on the {111} planes parallel to the ⟨112⟩ arm growth directions were also observed. Although defect assisted dendrite growth is facilitated by twinning and stacking fault formation on {111} planes, the growth directions of the trunk and branches are not of the ⟨111⟩ type, but rather close to ⟨112⟩. The ⟨112⟩ growth directions are maintained by breaking dendrite facets into thermodynamically stable 111 and 200 steps and structural ledges of different length.

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.

Twinning behavior and lattice rotation in a Mg–Gd–Y–Zr alloy under ballistic impact

The deformation microstructures of a Mg–Gd–Y–Zr alloy under ballistic impact at a velocity of 400 m·s−1 were characterized using optical microscopy (OM), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) techniques. Due to the high strain rate in the test, twinning and double twinning were prevalent in the deformation microstructure; high energy {101¯3} compression twins and twin–twin intersections were also readily observed. Adiabatic shear bands, the precursor of cracks, propagated along grain boundaries and through twin-rich zones. Four different lattice rotation conditions were observed in the deformation microstructure. Low angle boundaries generated by dislocations glide and dislocation arrays were partially responsible for the nonuniform orientations of the deformed grains. Some high angle boundaries formed through twinning followed by lattice rotation. Lattice rotation was also observed in unusual tension twins with misorientation angles lower than 86°. Finally, the sectioned region and matrix rotated with small angles during the twin–twin intersection process. These unique twin behaviors and lattice rotations can be attributed to accommodation of severe plastic strain and maintaining compatibility between neighboring microstructures. These deformation behaviors are beneficial for materials to improve the ballistic performance.

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).

Influence of Intra-granular Ferrite on the Tensile Behavior of Intercritically Annealed 12 pct Mn TWIP+TRIP Steel

The influence of intra-granular ferrite islands, fully embedded in austenite matrix grains, on the tensile behavior of intercritically annealed Fe-12 pct Mn-0.3 pctC-3 pctAl TWIP+TRIP steel was investigated. The presence of intra-granular ferrite, which nucleated at twin boundaries during annealing, resulted in an increased yield strength by the Orowan dislocation looping mechanism and the generation of geometrically necessary dislocations at the ferrite particle/austenite matrix interface. The intra-granular ferrite also induced the formation of secondary twin variants. The resulting twin–twin intersections nucleated additional α′-martensite, thereby leading to an enhanced TRIP effect.