Papers by Keyword: Deformation Microstructure

Abstract: Recently, high strength at high temperature can be achieved by inducing kink bands in alloys having aligned lamellar microstructure. However, the kink-bands formation has been confirmed only in alloys with lamellar microstructures, where slip plane is limited to the plane parallel to the lamellar interface, and not confirmed in alloys with rod-like or Chinese script microstructures. In this study, we clarified the contribution of rod-like Si phases in Al-Si alloy on the mechanical properties and focused on the feasibility of introduction of kink bands in the alloys without lamellar structure. The results showed that in Al-Si eutectic alloys, the non-lamellar second phase, i.e., the Si phase, is aligned by directional solidification, and refined by rolling. The directionally-solidified sample showed high yield strength with long and aligned Si phase, while the rolled samples showed high ductility with refined microstructure. The rolled samples were uniformly deformed in all the samples with variety of reduction ratios, and wedge-shaped deformation bands were observed after the compression test, especially in the 5-10% rolled specimens. Crystallographic orientation analysis indicated that these deformation bands were not kink bands but were localized slip bands.

Abstract: Titanium alloys typically exhibit a limited ductility (typically 20%) and little strain-hardening. An alloy design with new concept was conducted aiming at improving both ductility and strain hardening while keeping the mechanical resistance at an excellent level. An experimental validation was illustrated with the Ti-12(wt.%)Mo alloy, exhibiting true stress - true strain values at necking, of about 1000MPa and 0.38, respectively, with a large strain hardening rate close to the theoretical limit. In order to clarify the origin of this outstanding combination of mechanical properties, detailed microstructural investigation and phase evolution analysis were conducted by means of in-situ synchrotron XRD, in-situ light microscopy, EBSD mapping and TEM microstructural analysis. In the deformed material, combined Twinning Induced Plasticity (TWIP) and Transformation Induced Plasticity (TRIP) effects are observed. Primary strain/stress induced phase transformations (β->ω and β->α’’) and primary mechanical twinning ({332}<113> and {112}<111>) are simultaneously activated in the β matrix. Secondary martensitic phase transformation and secondary mechanical twinning are then triggered in the twinned β zones. The {332}<113> twinning and the subsequent secondary mechanisms are shown to be dominant at the early stage deformation process. The evolution of the deformation microstructure results in a high strain hardening rate (~2GPa) bringing both a high tensile strength and a large uniform elongation.

Abstract: The present work examines the microstructure and texture evolution in a Ni-30wt.%Fe austenitic model alloy deformed in torsion at 1000 °C, with a particular emphasis on the orientation dependence of the substructure characteristics within the deformed original grains. Texture of these grains was principally consistent with that expected for simple shear and comprised the main A, B and C components. The deformation substructure within the main texture component grains was characterised by “organised” arrays of parallel microbands with systematically alternating misorientations, locally accompanied by micro-shear bands within the C grains. With increasing strain, the mean subgrain size gradually decreased and the mean misorientation angle concurrently increased towards the saturation. The stored deformation energy within the main texture component grains was principally consistent with the respective Taylor factor values. The microband boundaries corresponded to the expected single slip {111} plane for the A oriented grains while these boundaries for the C oriented grains represented a variety of planes even for a single grain.

Abstract: The microstructure evolution and softening processes occurring in 22Cr-19Ni-3Mo austenitic and 21Cr-10Ni-3Mo duplex stainless steels deformed in torsion at 900 and 1200 °C were studied in the present work. Austenite was observed to soften in both steels via dynamic recovery (DRV) and dynamic recrystallisation (DRX) for the low and high deformation temperatures, respectively. At 900 °C, an “organised”, self-screening austenite deformation substructure largely comprising microbands, locally accompanied by micro-shear bands, was formed. By contrast, a “random”, accommodating austenite deformation substructure composed of equiaxed subgrains formed at 1200 °C. In the single-phase steel, DRX of austenite largely occurred through strain-induced grain boundary migration accompanied by (multiple) twinning. In the duplex steel, this softening mechanism was complemented by the formation of DRX grains through subgrain growth in the austenite/ferrite interface regions and by large-scale subgrain coalescence. At 900 °C, the duplex steel displayed limited stress-assisted phase transformations between austenite and ferrite, characterised by the dissolution of the primary austenite, formation of Widmanstätten secondary austenite and gradual globularisation of the transformed regions with strain. The softening process within ferrite was classified as “extended DRV”, characterised by a continuous increase in misorientations across the sub-boundaries with strain, for both deformation temperatures.

Abstract: A phenomenologically new recovery mechanism – triple junction motion is presented. This recovery mechanism is found to be the dominant one at low and medium temperatures in highly strained aluminum, which has a very fine microstructure, composed of lamellae with the thickness of a few hundred nanometers. Triple junction motion leads to removal of thin lamellae and to a consequent increase of the thickness of neighboring lamellae. This recovery mechanism therefore increases the average lamellar boundary spacing and causes a gradual transition from a lamellar structure to a more equiaxed structure preceding recrystallization.

Abstract: The changes of roll casting microstructure under different casting conditions on experimental casting machine was studied, which showed that solidification microstructure is different with different casting speed and thickness. When the casting speed is 1.3m/min,the thickness of strip is 6mm, coarse columnar grain dominates the solidification structure; when it continuously increases to 4m/min, the thickness of strip is 3mm, aluminum strip shows as hybrid organization with columnar grains in two sides and small equiaxed grains in the center; when the casting speed is over 12m/min,the thickness of strip is 1.75mm, columnar grains all convert into equiaxed grains in solidification structure. The deformation microstructures are different with the different casting speed and thickness of strip. When the casting speed is 0.9m/min, the thickness of strip is 6mm, typical processing flow lines come into being along longitudinal direction and transverse direction shows as squashed grains without re-crystallization. When the casting speed is 5.4m/min, the thickness of strip is 2.5mm, microstructure of aluminum strip becomes chevron organization from columnar crystal, there is a condensate in the center of aluminum strip, and the deformation of outer metal is more serious than inner, incomplete dynamic recrystallized grains come into being in the outer metal of the aluminum strip.

Abstract: Two-phase single-crystal intermetallic alloys composed of Ni₃Al (L1₂) and Ni₃V (D0₂₂) with some orientations were compressed at various temperatures, and their deformation microstructures were observed by transmission electron microscopy (TEM). The deformation at room temperature was governed by the glide motion of dislocations in the primary Ni₃Al precipitates and the activation of the microtwins in the Ni₃V variant structures in the channel regions. The interfaces between the primary Ni₃Al precipitates and the Ni₃V variant structures are suggested to work as the barriers to the dislocation motion. While, at temperature above the peak temperature (873 K), the deformation microstructures of the two-phase intermetallic alloy exhibited the ribbon-like deformation microstructures penetrating the constituent phases i.e. through the interfaces between primary Ni₃Al precipitates and the Ni₃V variant structures in the channel regions. It was also suggested that the superior strength in the two-phase intermetallic alloys is due to the high flow strength of the Ni₃V phases and to the interfacial hardening receiving when the dislocations activated in the primary Ni₃Al precipitates propagate to the channel regions.

Abstract: In the present study, the micro-sized cantilever-beam type specimens containing only one block of lath martensite were fabricated, and change in deformation microstructure inside a block with strain was observed directly by scanning electron microscopy and transmission electron microscopy. A number of slip bands appeared in the fixed end of the specimen by deformation. The propagations of slip bands, however, terminate due to the gradient of strain inside the specimen. The direction of slip bands changed during the propagation by the low angle boundaries inside the block. The shear localized region becomes narrow with an increase in strain. Furthermore, the width of laths increases greatly at the large strain region. The increase in width of laths is attributed to the disappearance of some initial lath boundaries by deformation.

Abstract: Inhomogeneous deformation in a magnesium alloy with long-period stacking order (LPSO) phase has been investigated using high-precision markers drawn by electron beam lithography. Mg alloys containing Zn and rare earth elements such as Y have a characteristic microstructure including the LPSO phase and the usual hcp matrix phase. The mechanical performance of this alloy is remarkably enhanced by warm-extrusion. The microstructure developed by such extrusion consists of elongated grains with fine-lamellae of LPSO phase and fine-grained matrix of hcp phase. In order to clarify the details of inhomogeneous deformation which should relate with the superior mechanical properties in this alloy, high-precision marking method using electron beam lithography has been employed. By using the method, local displacement due to tensile deformation in the Mg alloy was directly measured.

Abstract: The low-cycle fatigue (LCF) properties of a nickel-base precipitation-strengthened superalloy (GH4145/SQ), obtained at a temperature of 538 o C, were reported and discussed in this paper. The properties investigated include cyclic stress response, fatigue life, deformation microstructure and final fracture features as a function of applied strain amplitude. It was shown that the alloy exhibited a pronounced initial hardening followed by continuous softening to failure at high plastic strain amplitudes ( > 0.2% ap ε ), while at low plastic strain amplitudes ( < 0.2% ap ε ) the initial hardening was followed by a well-defined saturation stage. Bilinear behavior with a change of slope at a plastic strain amplitude of about 0.2% was observed in the cyclic stress-strain (CSS) and Coffin-Manson (C-M) plots. TEM observations revealed that slip band density increased with increasing total strain amplitude and precipitate degradation resulting from dislocation-precipitate interactions took place with continuous cyclic straining. The change in the microstructure during cycling is thus responsible for the fatigue hardening / softening behavior of the alloy. SEM examinations indicated that at low plastic strain amplitudes ( < 0.2% ap ε ) crack propagation was basically transgranular, while at high plastic strain amplitudes ( > 0.2% ap ε ) crack propagation exhibited intergranular features, as a whole. The variation in both the number of operating slip systems and the fracture modes with the strain amplitude employed was used to explain the observed two-stage LCF behavior of the present investigated superalloy.