Non-recrystallized Grains Research Articles

Origin of unusual fracture in stirred zone for friction stir welded 2198-T8 Al-Li alloy joints

Friction stir welded (FSW) joints of conventional precipitation-hardened aluminum alloys usually fracture in the lowest hardness zone (LHZ) during tension testing. However, all of the FSW joints of a 2198-T8 Al-Li alloy fractured in the stirred zone (SZ) instead of the LHZ with the welding parameters of 800rpm-200mm/min and 1600rpm-200mm/min under the condition that no welding defects existed in the SZ. The experiment results revealed that lazy S was not the dominant factor resulting in the unusual fracture. The SZ consisted of three subzones, i.e., the shoulder-affected zone, the pin-affected zone, and the transition zone between them. While the former two zones were characterized by fine and equiaxed recrystallized grains, incompletely dynamically recrystallized microstructure containing coarse elongated non-recrystallized grains was observed in the transition zone. The transition zone exhibited the lowest average Taylor factor in the SZ, resulting in a region that was crystallographically weak. Furthermore, obvious lithium segregation at grain boundaries was observed in the transition zone via time-of-flight secondary ion mass spectroscopy analysis, but not in the shoulder-affected zone or the pin-affected zone. The combined actions of both the two factors resulted in the appearance of preferential intergranular fracture in the transition zone and eventually caused the failure in the SZ. The lithium segregation at grain boundaries in the transition zone was closely associated with both the segregation in the base material and the partially dynamically recrystallized microstructure resulting from the inhomogeneous plastic deformation in the SZ.

Evolution of twinning in extruded AZ31 alloy with bimodal grain structure

Twinning in extruded AZ31 alloy with a bimodal grain structure is studied under compression along the extrusion direction. This study has combined in-situ measurements during the compression tests by Synchrotron Radiation Diffraction and Acoustic Emission techniques and the evaluation of the microstructure and texture in post-mortem compression samples deformed at different strains. The microstructure of the alloy is characterized by the coexistence of large areas of fine dynamic recrystallized grains and coarse non-recrystallized grains elongated along extrusion direction. Twinning occurs initially in large elongated grains before the macroscopic yield stress which is controlled by the twinning in equiaxed dynamically recrystallized grains.

Magnetization reversal processes in hot-extruded τ-MnAl-C

The magnetic domain structure of hot-extruded bulk τ-Mn53Al45C2 was studied by Kerr microscopy under application of a magnetic field in-situ. The microstructure consists of recrystallized, fine-grained regions and large non-recrystallized grains which contain a high density of twins. Within these large polytwinned grains, a clear pinning interaction of magnetic domain walls at twin boundaries was observed but with a rather small pinning force. The smaller, recrystallized grains show a higher resistance to magnetization reversal. The critical single domain particle size of this material was estimated at 773nm and the fine, recrystallized grains are in the range of this size. Demagnetizing the sample following saturation using a 3T field pulse revealed that individual fine grains reverse independently from their neighbours.

Effect of cold rolling on recrystallization and tensile behavior of a high-Mn steel

The microstructure evolution in an Fe–17Mn–1.5Al–0.3C steel processed by cold rolling with subsequent annealing was studied. Cold rolling was accompanied by a significant increase in dislocation density and the development of deformation twins followed by the formation of microshear bands. The recrystallization of cold rolled specimens was observed at temperatures of T≥873K for annealing times of 30min. An increase in the cold rolling strain increased the density of recrystallization nuclei leading to fine-grained microstructure and accelerated the recrystallization kinetics. The steel with partially recrystallized microstructure exhibited the yield strength and total elongation above 600MPa and 30%, respectively. The same level of strength along with total elongation above 60% was achieved by the development of a uniform fine-grained recrystallized microstructure with a grain size below 1μm in the sample subjected to cold rolling to a strain of 3 followed by an annealing at 873K for 1h. The influence of the initial cold rolling strain on the hardness after cold rolling and on the recrystallized fraction were formulated using power law relationships, whereas the yield strength after annealing was found to be predictable by consideration of dislocation density in non-recrystallized grains and size of recrystallized grains.

Deformation mechanisms in an austenitic single-phase duplex microstructured steel with nanotwinned grains

A novel type of duplex microstructure is generated in a single-phase austenitic steel (AISI 316L; X2CrNiMo19-12), consisting of plastically compliant recrystallized austenitic grains as the matrix containing coarse non-recrystallized grains with a nanotwinned austenitic (nt-γ) structure as strengthening inclusions. This novel type of single-phase yet duplex microstructured steel exhibits an excellent combination of strength and ductility. We study the plastic co-deformation mechanisms between the nanotwinned and the recrystallized grains under tension using electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). At tensile strains below 5%, the nt-γ grains nearly deform homogeneously in conjunction with the surrounding statically recrystallized (SRX) grains without generating notable strain localization near their interfaces. The anisotropic plastic deformation of the nt-γ grains with predominant shear parallel to the twin boundaries results in a higher dislocation density in the neighboring SRX grains. As the strain exceeds 12%, localized deformation occurs within the nt-γ grains in the form of shear banding. A strain gradient is developed in the surrounding SRX grains as a function of distance from the nt-γ/SRX interface. Deformation twinning is observed in the SRX grains near the nt-γ grains, while away from nt-γ grains dislocation slip dominates the deformation. The strengthening effect of the strong and ductile nt-γ grains may offer a novel approach to strengthen austenitic steels and related alloys by generating a nanotwinned/recrystallized duplex microstructure.

Recrystallization and microstructural evolution during hot extrusion of Mg97Y2Zn1 alloy

This study revealed that the extrusion temperature has a great influence on microstructure and mechanical properties of the Mg97Y2Zn1 alloy. The average grain sizes increased from 3 μm to 8 μm with increasing extrusion temperatures from 623K to 773 K. Both dynamic recrystallization (DRX) and static recrystallization (SRX), which occur during and after deformation, respectively, were observed. The alloy, which extruded at a relatively high temperature, exhibited lower strength because the strain strengthening was balanced by the softening that originated from DRX. Three types of morphologies, namely, big recrystallized grains, fine recrystallized grains, and non-recrystallized grains, were observed in the extruded microstructures obtained at 623 K. The dislocation density was quite high in the fully recrystallized grain. The extruded microstructures obtained at 773 K were composed of large grains with more uniform size. Their degree of recrystallization was higher and the dislocation density also declined. All dislocation in the grain were distinguished as 〈c+a〉 dislocations. Submicron scale precipitates were distributed along the newly formed recrystallized grain boundaries and had a remarkable pinning effect on the recrystallized grain growth after extrusion at 773 K. The precipitates can be divided into two main types: mixed type and single type.

A New Metallographic Technique for Revealing Grain Boundaries in Aluminum Alloys

A new metallographic technique has been developed for revealing the grain structure of common 5000 and 2000-series aluminum alloys. Grain structures of these aluminum alloys are normally difficult to reveal with common etchants. The preferred metallographic etchant for structural aluminum alloys is generally based on HCl, HNO3, and HF, usually referred to as Keller’s reagent. At times, however, this etchant does not give completely satisfactory results. This is true when the structure is inhomogeneous, having patches of recrystallized and non-recrystallized grains, or clusters of intermetallic particles in material containing few particles. Samples with a poor final polish and a smeared surface also do not respond well to Keller’s etchant. The new technique described in this article consists of two steps: 1. a pre etchant with a solution of 1 g NaCl in 50 ml H3PO4 (40%) and 2. etch with Weck’s reagent. The microstructures obtained with this new method are compared with Keller’s and Weck’s reagents.

Flow softening behavior during dynamic recrystallization in Mg–3Al–1Zn magnesium alloy

The flow softening behavior of Mg–3Al–1Zn magnesium alloy in the temperature range 325–500 °C is quantified using plane strain compression. The flow softening ratio, defined as the steady-state stress at 0.6 strain to the peak stress, shows a linear relationship with the Zener–Hollomon (Z) parameter and varies from 45 to 70%. Higher levels of flow softening at high Z are associated with fully recrystallized structures, whereas lower flow softening levels at low Z exhibit a mix of large recrystallized and fine non-recrystallized grains.

Effects of Niobium on Mechanical Twinning and Tensile Properties of a High Mn Twinning-Induced Plasticity Steel

The effects of Nb on mechanical twinning and tensile deformation behavior were investigated in hot-rolled Fe­18Mn­0.6C­1.5Al twinning-induced plasticity (TWIP) steels without and with 0.1Nb. In comparison to the Nb-free TWIP steel with fully-recrystallized and equiaxed grains, the Nb-added TWIP steel showed non-recrystallized and elongated grains with well-dispersed NbC particles and a high density of dislocations. The increased yield and tensile strengths in the Nb-added TWIP steel were mainly caused by the hardening of dislocations in non-recrystallized grains rather than the precipitation hardening of NbC particles or the solid solution hardening of Nb atoms. Meanwhile, the uniform elongation and strain hardening rate of the Nb-added TWIP steel were decreased due to inactive mechanical twinning, which is probably because the dislocations interfere with the movement and dissociation of dislocations necessary for mechanical twinning. [doi:10.2320/matertrans.M2012273]

Preparation, microstructure and properties of Al-Zn-Mg-Sc alloy tubes

The Al-6.0Zn-2.0Mg-0.2Sc-0.10Zr hollow tube ingots, prepared by semi-continuous casting technology, were subjected to homogenization treatment, hot extrusion, intermediate annealing, tension, solution and aging treatment. The microstructures and properties of as-cast Al-Zn-Mg-Sc alloy at different homogenization treatment conditions were studied using hardness measurement, X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analysis. The results showed that the phases in the as-cast Al-Zn-Mg-Sc alloy were α(Al) and T(Mg 32(Al,Zn) 49). The metastable supersaturation solid solution decomposed firstly. Then the equilibrium T phases and dispersed Al 3(Sc,Zr) particles precipitated and dissolved into the solution matrixes at high homogenization temperature. The appropriate homogenization processing was determined to be 470 °C for 24 h. The microstructures after extrusion were non-recrystallized grains because of dispersed Al 3(Sc,Zr) particles. After intermediate annealing most of the deformed microstructures were eliminated and softened. During solution treatment the equilibrium phases dissolved into the matrix, leading to the formation of supersaturation solid solution. The η' phases precipitated from the solid solution during aging process. The η' phases in the grain and the equilibrium phases on the grain boundaries were coarsening and the precipitate free zone (PFZ) was wider with increasing of the aging temperature and time. The suitable heat treatment for Al-Zn-Mg-Sc-Zr aluminum tubes was solution treated at 470 °C for 1 h, followed by aging at 120 °C for 24 h. At this condition, the ultimate tensile strength, yield strength and elongation could reach 520 MPa, 510 MPa and 11%, respectively. The high strength was mainly due to the fine grain strengthening caused by the addition of Sc and Zr, the subgrain strengthening and precipitation strengthening of Al 3(Sc,Zr) particles and η' phase.

Mechanism of Grain Growth During Annealing of Si-Al Electrical Steel Strips Deformed in Tension

AbstractThe mechanism of recrystallization as a result of annealing during 600–7200 seconds at 700 °C of a Si-Al, low C electrical steel strip is investigated in samples deformed in tension. The evolution of grain size during annealing is evaluated by optical microscopy and electron backscatter diffraction in the scanning electron microscope. It is found that grain growth starts after an incubation time of 600 s with no apparent evidence of primary recrystallization. After that, the grain size-time relationship exhibits two different stages. Initially, the grain size increases linearly with time up to about 3600 s. During this time, some selected grains grow until they consume the deformed microstructure. In the second stage, the rate of growth decreases significantly and a final grain size of about 150 m is reached after 7200 seconds of annealing. Grain orientation spread maps obtained from EBSD data of deformed and partially recrystallized samples during the stage of linear growth reveals that the growing grains exhibit lower misorientation and therefore smaller stored energy than the non-recrystallized matrix grains. Analysis of image quality maps reveal that the IQ values for {100}<uvw>orientations are higher than those observed for {111}<uvw>orientations thus suggesting that the {100}<uvw>orientations grow at the expense of {111}<uvw>orientations by a mechanism of strain-induced boundary migration.

Effects of Friction Stir Welding Parameters on Microstructures and Mechanical Properties of Brass Joints

5 mm thick brass plates were successfully friction stir welded at the tool rotation rates from 400 to 1000 rpm for a constant traverse speed of 100 mm/min. The nugget zone (NZ) consisted of the incompletely and completely-recrystallized regions. With increasing rotation rates, the fraction of the non-recrystallized grains decreased and the size of the recrystallized grains increased. The hardness values in the NZs were higher than those in the parent material (PM). Increasing the rotation rate did not exert a noticeable effect on the tensile and yield strengths of the welds, but increased the elongation. The tensile and yield strengths of the welds reached up to ∼99 and 80% of the PM, respectively. The fracture occurred in the heat affected zone that had the lowest hardness.

Relationship between deposition and recrystallization textures of copper and chromium electrodeposits

The 〈100〉, 〈111〉 and 〈110〉 textures of copper electrodeposits obtained from copper sulfate baths changed to the 〈100〉, 〈100〉 and 〈 $$\sqrt {\text{3}} 10$$ 〉 textures, respectively, after recrystallization. The textures of chromium electrodeposits obtained from the standard Sargent bath remained unchanged after recrystallization. The results are in agreement with the prediction of the strain energy release maximization model, in which the recrystallized grains orient themselves so that their minimum elastic modulus direction can be parallel to the absolute maximum internal stress direction due to dislocations in the non-recrystallized grains.

The role of dynamic recrystallization in the development of lattice preferred orientations in experimentally deformed quartz aggregates

Experiments on coaxially deformed quartz aggregates show that when dynamic recrystallization involves grain size reduction, the lattice preferred orientation (LPO) of the recrystallized grains depends on the recrystallization mechanism. At low temperatures and fast strain rates, strain-induced grain boundary migration recrystallization favors growth of grains oriented poorly for slip on the basal and prism planes, resulting in an LPO of c-axes parallel to the θ 1 direction. At higher temperatures or slower strain rates, progressive subgrain rotation produces recrystallized grains which inherit the small-circle girdle LPO of their deformed host grains and which continue to re-orient by slip. At even higher temperatures and slower strain rates, progressive subgrain rotation is accompanied by rapid grain boundary migration, which does not favor grains of any particular orientation; thus the LPO is again similar to that of non-recrystallized grains. In contrast, when dynamic recrystallization involves grain growth driven by surface energy reduction, a strong LPO of c-axes parallel to θ 1 develops during grain growth but then evolves with increasing strain toward a small-circle girdle. The correlation of LPOs with recrystallization mechanisms has important implications for the assumption of homogeneous strain in theoretical models of LPOs and for interpreting natural LPOs in recrystallized rocks.

Development of heat treatment parameters to improve fracture toughness and grain size of an embrittled maraging steel

An embrittled 18 Ni maraging steel rolled ring was examined and samples cut from the ring were subjected to a wide range of heat treatments including high temperature solutioning and thermal cycling. The effects of these treatments on toughness were evaluated by measuring impact energy and plain strain fracture toughness. The microstructural analyses were carried out using extensive optical and scanning electron microscopy, and scanning electron fractography. It has been established that the ring was embrittled due to the combined effects of deformed structure and grain boundary precipitation of TiC or Ti (CN). Heat treatment parameters have been devised to improve the fracture toughness and grain size of the materials affected by these two types of embrittlement. It has been suggested that toughness and grain size can be improved by (a) annealing at 1223 K followed by water quenching in the case where deterioration in toughness is marginal and is caused by nonrecrystallized grains or deformation texture, and (b) solutioning at 1473 K followed by water quenching, and thermal cycling twice between room temperature and 1198 K with a holding time of 30 min at peak temperature in the case where the loss in toughness is considerably large due to excessive grain boundary precipitation of second phase particles.

Effects of creep on the microstructural features of an oxide-dispersion-strengthened superalloy

The effects of high temperature creep on the microstructure and the grain boundaries of the oxide-dispersion-strengthened superalloy MA6000 are investigated by means of transmission electron microscopy. Rafting of the γ′ phase is observed in the elongated grains, whereas in the small non-recrystallized grains γ′ rafting does not occur. Dislocations form networks around γ′ precipitates only in particular areas of the coarse grains where the oxide particles are absent. In the rest of the areas, where oxide dispersion is homogeneous, dislocations are pinned to the hard oxide particles instead of accumulating at γ-γ′ interfaces. Dislocation networks around the γ′ phase are not observed in the small grains. There is no detectable influence of high temperature creep on the grain boundaries of the material. The majority of them are of the small angle type and exhibit a very disordered morphology. Large angle grain boundaries are randomly oriented and they are mainly decorated by chromium carbides.

Ferrite Formation from Deformed Austenite in a Ti Modified HSLA Steel

Abstract The effect of austenite deformation in the recrystallization, partial recrystallization and nonrecrystallization temperature range on the subsequent ferrite transformation was investigated in a commercial HSLA steel with 0.076% Ti and a reference C-Mn steel of same base chemistry. The steel samples were given reductions in the range 20–60% at 850, 950 and 1050°C by single pass rolling followed by isothermal transformation to ferrite at 690°C for up to 1800 s. It was found that the austenite to ferrite transformation rate was increased both with increase in the amount of austenite deformation and lowering the finish rolling temperature. The addition of titanium resulted in decrease in the rate of ferrite formation as well as led to the formation of polygonal ferrite grains. Non-recrystallized elongated austenite grains appear to give fine ferrite structure whilst mixed ferrite grains resulted from partially recrystallized austenite. It was observed that not only the austenite grain boundaries, but...