Microstructure,texture and magnetic properties of strip casting Fe–6.2 wt%Si steel sheet

An Fe–6.2 wt%Si strip with equiaxed grains and mild {0 0 1}〈0 v w〉 fiber texture was produced by twin-roll strip casting process. Then the as-cast strip was treated with or without the hot rolling prior to the warm rolling and annealing. When the hot rolling was not introduced, a fine and heterogeneous warm-rolled microstructure was produced and led to a fine recrystallization microstructure and very weak {0 0 1}〈0 v w〉 fiber texture in the annealed sheets. When the hot rolling was introduced, a coarse and homogeneous warm-rolled microstructure was produced and led to a very coarse recrystallization microstructure and much stronger {0 0 1}〈0 v w〉 fiber texture in the annealed sheets. The annealed sheets with hot rolling showed a higher magnetic induction and a higher core loss than those without hot rolling.     

Microstructure evolution during semi-solid powder rolling and post-treatment of 7050 aluminum alloy strips

Semi-solid powder rolling (SSPR) combines semi-solid rolling with powder rolling to prepare high-performance metallic strips. Semi-solid powders were prepared under an inert atmosphere and subsequently rolled by a powder rolling machine. Conductive cooling between the pre-heated rollers and semi-solid powders results in a rapid solidification effect that is able to process alloys with a broad freezing range. The liquid in the semi-solid powders plays an important role in the microstructure evolution, which can improve the strength of strips. The 7050 aluminum alloy strips were obtained and used to evaluate the processing parameters and strip qualities for strips up to 100 mm wide and 1.5–2 mm thick. The process of semi-solid powder rolling was described and microstructure evolution during rolling and post-treatment was analyzed. The combination mechanism of semi-solid powders during rolling was also discussed. The results show that the best liquid fraction to prepare a strip ranges from 45 to 65%. Flowing and filling of liquid (>10%), densification by rolling and recrystallization (<10%) are the three combination mechanisms of the semi-solid powders during rolling. In addition, semi-solid powder rolled strips can be processed subsequently by hot rolling with the improved micro-hardness and relative density.     

Microstructure changes during non-conventional heat treatment of thin Ni–Ti wires by pulsed electric current studied by transmission electron microscopy

Transmission electron microscopy, electrical resistivity measurements and mechanical testing were employed to investigate the evolution of microstructure and functional superelastic properties of 0.1 mm diameter as-drawn Ni–Ti wires subjected to a non-conventional heat treatment by controlled electric pulse currents. This method enables a better control of the recovery and recrystallization processes taking place during the heat treatment and accordingly a better control on the final microstructure. Using a stepwise approach of millisecond pulse annealing, it is shown how the microstructure evolves from a severely deformed state with no functional properties to an optimal nanograined microstructure (20–50 nm) that is partially recovered through polygonization and partially recrystallized and that has the best functional properties. Such a microstructure is highly resistant against dislocation slip upon cycling, while microstructures annealed for longer times and showing mostly recrystallized grains were prone to dislocation slip, particularly as the grain size exceeds 200 nm.     

Dynamic tensile extrusion response of tantalum

The effects of the original rolling texture and impact velocity on the large-strain dynamic tensile extrusion process in a high-purity Ta have been investigated in a gas gun facility. A continuous increasing of the total elongation of the extruded Ta spheres with increasing impact velocities has been observed. The starting texture was found to influence the development of instabilities with little effect on the total elongation. Regardless of the starting textures and impact velocities, a strong <1 1 0> fiber texture was developed along the extrusion direction and similar hardness increments were recorded in the recovered Ta segments. Dislocation analysis revealed a continuous evolution of the dislocation substructures from loose planar arrays, to microbands, to elongated cells structures, and to equiaxed subgrains as a function of deformation. The evolution process of the dislocation substructures is discussed in detail in correlation with the development of the microstructure as well as the texture.     

Dislocation density and sub-grain size evolution of 2CrMoNiWV during low cycle fatigue at elevated temperatures

We present the dislocation density and sub-grain size evolution for samples subjected to low cycle fatigue (LCF) loading under various conditions. Interrupted LCF tests have been performed on a cyclic softening bainitic steam turbine rotor steel, 2CrMoNiWV, at total strain amplitudes of ±0.25%, ±0.4% and ±0.7%, strain rates of 0.01 and 0.1% s^?1, and temperatures of 500 and 565 °C. Neutron diffraction experiments have been carried out on these samples, which were evaluated by means of a convolutional multiple whole profile peak shape analysis approach. With this analysis, both dislocation density and sub-grain size evolutions have been determined and compared to the results of transmission electron microscopy (TEM) and scanning TEM (STEM) spot-check evaluations. In addition, the proportions of prevailing dislocation types and the correlation factor of the microstructure have been determined. Finally, the results have been used to establish a phenomenological model describing the relationships between the observed cyclic softening and the evolution characteristics of dislocation density and sub-grain size, as a function of strain amplitude, strain rate and temperature.     

New experimental insight into the mechanisms of nanoplasticity

The evolution of microstructure and texture of a nanocrystalline Pd–10 at.% Au alloy (initial grain size 16 nm) subjected to severe plastic deformation by high-pressure torsion (HPT) at room temperature is investigated by X-ray line profile analysis and X-ray microdiffraction, respectively. In addition, changes in the microhardness are measured and the texture is modeled. During HPT the microstructure changes: the crystallite size goes over the maximum, the dislocation density goes through a minimum and the density of stacking faults decreases at/up to a shear strain of ～1, corresponding to a grain size of 20 nm. Starting with a random texture, typical brass-type shear components develop at a shear strain above ～1. The microhardness with decreasing crystallite size goes over a maximum at ～20 nm. The correlated changes in microstructure, texture and strength strongly suggest the transition from a dislocation slip to a grain boundary sliding (GBS)-dominated deformation mechanism. The unexpected brass-type texture and its deviation from the ideal position can be simulated with the Taylor model assuming dominant partial dislocation slip and a certain contribution of GBS, respectively. Taken together, the results of many techniques applied to the same material, in particular those of the texture investigations, provide a more comprehensive and consistent picture of nanoplasticity than reported before for face-centered cubic metals.     

The effect of β phase on microstructure and texture evolution during thermomechanical processing of α + β Ti alloy

Microstructure and texture evolution have been investigated in both α and β phases during the hot rolling of β-quenched Ti–6Al–4V at 800 and 950 °C, followed by annealing at 950 °C and air cooling using detailed electron backscattered diffraction mapping. The textures of primary and secondary α in the bi-modal microstructure were analysed separately, and the high-temperature β orientations were calculated by a variant based reconstruction from the inherited α_s orientations. Crystal plasticity finite element modelling has been employed to predict the rolling texture based on common α phase slip systems and compare with the measured α texture. It was found that despite the severe deformation during rolling, a large proportion of the primary α grains retain a Burgers relationship with the β phase. Consequently, the β phase in combination with a variant selection mechanism seems to control the α texture, which explains the discrepancy between predicted and measured rolling textures. The consequence of this mechanism for macrozone formation is also discussed.     

High strength and ductile ultrafine-grained Cu–Ag alloy through bimodal grain size,dislocation density and solute distribution

Ultrafine-grained materials produced by different severe plastic deformation methods show very high strengths but their tensile ductility is often very low. In the present work, we demonstrate an approach for retaining high strength while recovering ductility in a Cu–3 at.% Ag alloy through cold rolling and short-time annealing. X-ray line profile analysis of cold-rolled and annealed samples reveals the development of a heterogeneous solute atom distribution due to the dissolution of nanosized Ag particles in some regions of the matrix. In regions with higher solute (Ag) content, the high dislocation density present following rolling is stabilized, while in other volumes the dislocation density is decreased. High-resolution scanning electron microscopy confirms the presence of regions of varying Ag content in the matrix. Microstructure analysis of the rolled and annealed samples revealed bimodal grain size, dislocation density and solute Ag distributions as well as nanosized Ag precipitation. The as-rolled samples exhibit high tensile strengths of ～600–700 MPa with negligible uniform elongation (～1%). After short-time annealing the strength decreases only slightly to ～550–620 MPa with significant improvement in uniform elongation (from 1 to 10%); this is mainly attributed to the bimodal microstructure.     

Hot rolling of gamma titanium aluminide foil

Metal flow and microstructure evolution during the thermomechanical processing of thin-gage foil of a near-gamma titanium aluminide alloy, Ti–45.5Al–2Cr–2Nb, with an equiaxed-gamma microstructure was investigated experimentally and theoretically. Foils of thickness of 200–250 μm were fabricated via hot rolling of sheet in a can of proprietary design. The variation in gage of the rolled foils was ±15 μm except in very sporadic (local) areas, with variations of approximately 60 μm relative to the mean. Metallography revealed that the larger thickness variations were associated with large remnant colonies lying in a hard orientation for deformation. To rationalize these observations, a self-consistent model was used to estimate the strain partitioning between the softer (equiaxed-gamma) matrix and the remnant colonies. Furthermore, the efficacy of pre- or post-rolling heat treatment in eliminating remnant colonies was demonstrated and quantified using a static-spheroidization model. The elimination of remnant colonies via spheroidization prior to foil rolling gave rise to improved gage control.     

Dislocation structure evolution induced by irradiation and plastic deformation in the Zr–2.5Nb nuclear structural material determined by neutron diffraction line profile analysis

Zr–2.5Nb samples removed after 7 years of service from a nuclear power reactor were investigated by traditional mechanical testing and whole pattern neutron diffraction line profile analysis of the irradiated and deformed materials. A significant increase in yield strength and subsequent strain softening are observed in the as-irradiated material. The line profile analysis allows the change in mechanical properties to be directly related to evolution of the microstructure. A fourfold increase in overall dislocation density accomplished entirely by an increase in the 〈a〉 Burgers vectors dislocations, and profound change in the dislocation network arrangement, are found to be created by the fast neutron irradiation. Comparison to the microstructural evolution during plastic deformation of the unirradiated sample shows a similar increase in dislocation density, but the increase is equally distributed amongst 〈a〉 and 〈c + a〉-type dislocations. Finally, plastic deformation of the previously irradiated material again increases the dislocation density significantly but, in contrast, does so through a 10-fold increase in the 〈c + a〉 dislocation density relative to the as-irradiated material, while the 〈a〉 Burgers vector density does not change. The different evolution of the 〈a〉 and 〈c + a〉 Burgers vector ratios in the unirradiated and irradiated Zr–2.5Nb during plastic deformation can perhaps be explained by the strain localization effect previously reported in irradiated Zircaloy subjected to deformation.     

Effect of Mn addition on one-dimensional migration of dislocation loops in body-centered cubic Fe

Elucidation of the one-dimensional (1-D) motion of dislocation loops is important for describing the microstructural development of materials under irradiation. In this study, the effect of Mn on radiation-induced microstructure evolution in body-centered cubic Fe was experimentally investigated by focusing on the migration of dislocation loops. Pure Fe and Fe–1.4Mn alloy were irradiated with Fe³⁺ ions to introduce dislocation loops. In pure Fe, inhomogeneous distribution of loops in the vicinity of the residual dislocation was observed. However, in Fe–1.4Mn, isolated dislocation loops were homogeneously distributed in a high number density. In situ transmission electron microscopy during annealing revealed that 1-D motion of dislocation loops occurred in pure Fe at 623 K, while 1-D motion of dislocation loops occurred minimally in Fe–1.4Mn annealed at temperatures below 773 K. These results indicate that 1-D motion of dislocation loops play a key role in producing the differences in the microstructures between pure Fe and Fe–1.4Mn. In pure Fe, dislocation loops were mobile and trapped in the strain field of a dislocation, leading to the formation of loop decoration of dislocations. However, in Fe–1.4Mn, dislocation loops were less mobile and dislocation loops were homogeneously formed in high density in the matrix. The migration of dislocation loops by Mn solute is strongly suggested as one of the key mechanisms of microstructure development in irradiated Fe–Mn alloy.     

Microstructural evolution and its influence on creep and stress relaxation in nanocrystalline Ni

Indentation creep and stress relaxation tests were performed on rolled and annealed nanocrystalline (NC) Ni to study the influence of microstructure evolution on plastic deformation behavior. Dislocation density (ρ) increases with increasing rolling strain, reaching a maximum at 20% strain, followed by a decrease at larger strain. The ρ of Ni decreases significantly with increasing annealing temperature. Softening behavior is observed in NC Ni with grain size <40 nm, i.e., an inverse-like Hall–Petch effect. For rolling NC Ni, both creep strain rate and rate sensitivity first increase and then decrease, while those of annealed Ni continuously decrease. With increasing grain size, creep activation volume unusually decreases first, then starts to rise, which is different from that of coarse-grained metal. A model involving dislocation annihilation and emission at grain boundaries under indenters is used to explain the anomalous behavior of rolled and annealed Ni, respectively.     

A new 3D multiphase FE model for micro cutting ferritic–pearlitic carbon steels

A new three-dimensional multiphase finite element computation model is proposed for the simulation of micro drilling two-phase ferritic–pearlitic carbon steels in order to understand the cutting, ploughing, tribological and heat transfer mechanisms at the microscale. Based on the Split-Hopkinson-Pressure-Bar technique, a constitutive material law has been developed to model the thermo-mechanical material behaviour including the effect of the microstructure. Micro drilling tests using solid carbide twist drills with different diameters (d = 50 μm to 1 mm) were performed on ferrite–pearlite two-phase steel AISI 1045 for the verification of the developed 3D FE computation model regarding chip formation, feed force, and torque.     

Study on the indentation size effect in CaF2: Dislocation structure and hardness

In this study nanoindentations have been performed on a cleaved surface of a CaF₂ single crystal and the dislocation structure has been investigated by the etch pit technique using atomic force microscopy. The deformation during indentation is first purely elastic until dislocations are created observable in a pop-in in the load displacement data, as well as in a dislocation rosette around the indentation. After pop-in a relatively high hardness is observed, which gradually decreases, until at 3 μm a nearly constant hardness is found. By using sequential polishing, etching and imaging, the dislocation structure underneath indentations with indentation depths of 300 nm and 110 nm (load: 5 mN, 1 mN) is quantified. The dislocation density and radial distribution of dislocation density depend on the indentation depth, where a smaller indentation depth leads to a higher dislocation density, which is in qualitative agreement with the observed increase in hardness.     

Multi-phase microstructure design of a low-alloy TRIP-assisted steel through a combined computational and experimental methodology

The multiphase constitution of a transformation-induced plasticity (TRIP)-assisted steel with a nominal composition of Fe–1.5Mn–1.5Si–0.3C (wt.%) was designed, utilizing a combination of computational methods and experimental validation, in order to achieve significant improvements in both strength and ductility. In this study, it was hypothesized that a microstructure with maximized ferrite and retained austenite volume fractions would optimize the strain hardening and ductility of multiphase TRIP-assisted steels. Computational thermodynamics and kinetics calculations were used to develop a predictive methodology to determine the processing parameters in order to reach maximum possible ferrite and retained austenite fractions during conventional two-stage heat treatment, i.e. intercritical annealing followed by bainitic isothermal transformation. Experiments were utilized to validate and refine the design methodology. Equal channel angular pressing was employed at a high temperature (950 °C) on the as-cast ingots as the initial processing step in order to form a homogenized microstructure with uniform grain/phase size. Using the predicted heat treatment parameters, a multiphase microstructure including ferrite, bainite, martensite and retained austenite was successfully obtained. The resulting material demonstrated a significant improvement in the true ultimate tensile strength (～1300 MPa) with good uniform elongation (～23%), as compared to conventional TRIP steels. This provided a mechanical property combination that has not been exhibited before by low-alloy first-generation high-strength steels. The developed computational framework for the selection of heat treatment parameters can also be utilized for other TRIP-assisted steels and help design new microstructures for advanced high-strength steels, minimizing the need for cumbersome experimental optimization.     

The evolution of dislocation microstructure in electron beam melted Ta-2.5W alloy during cold rolling

The evolution of dislocation microstructure in electron beam melted Ta-2.5W alloy was investigated by transmission electron microscope (TEM). Long straight dislocations and dislocation loops are formed in Ta-2.5W alloy cold-rolled by 5%. A set of long, continuous extending planar boundaries (EPBs) are formed when the reduction reaches 20%. In the early stage of development, EPBs are fragmented, diffused and curved, which are connected by non-crystallographic cells boundaries to maintain their continuity. The straight segments of EPBs are usually parallel with the trace of {110}, and incline at about 25–35° to the rolling direction (RD). Two groups of EPBs are formed in a grain when the reduction is larger than 30%. The dislocations within EPBs tend to rearrange themselves with increasing strain in a sequence, from tangled dislocations, followed by parallel long straight screw dislocations and finally into dislocation nets, which are composed by 1/2 < 111 > and [100] type dislocations. The relaxation process of dislocations and the interaction of dislocations with EPBs make EPBs appear wavy and deviate from the trace of slip planes.     

Evolution of deformation microstructures of cold-rolled Ta–2.5W alloy with coarse grains at low to medium strains

Ta–2.5W alloy with coarse grains was cold-rolled to reductions ranging from 5 to 40%. The evolution of the microstructure was investigated by optical microstructure, electron backscatter diffraction (EBSD). A few microbands appear when the reduction reaches 20%. The density of microbands increases with increasing reduction. When the reduction reaches 40%, grains are composed of one or two groups of microbands except the {001}<110 > orientations. Most of the inclination angle between microbands and RD in this condition is 20–35°. As the strain increases, the inclination angle between microbands and RD gets smaller. The habit plane of microbands can be {110} plane. The microbands and matrix usually share a common < 110 > or < 111 >. The mature body-centered cubic rolling texture, including α and γ fibers, is not developed until the reduction reaches 40%. Meanwhile, shear bands appear. New grains can be seen in shear bands and a model is proposed to explain this process.     

Formation of nanostructures in commercial-purity titanium via cryorolling

Microstructure evolution in commercial-purity titanium during plane-strain multipass rolling to a true thickness strain of 2.66 at 77 and 293 K was quantified. Deformation at both temperatures was accompanied by twinning. At 77 K, twinning was more extensive in terms of the fraction of twinned grains and the duration of the twinning stage. Rolling to a true thickness strain of 2.66 resulted in the formation of a microstructure with a grain/subgrain size of ～80 nm at 77 K or ～200 nm at 293 K. The contribution of various mechanisms to the strength of titanium following rolling at 77 and 293 K was analyzed quantitatively.     

Evolution of plastic deformation in heavily deformed and recrystallized tungsten of ITER specification studied by TEM

Microstructure induced by plastic deformation at 600 °C in as-fabricated and recrystallized tungsten grades of ITER specification is studied by means of transmission electron microscopy (TEM). Both grades are produced by AG Plansee and being used by fusion community as research material for plasma facing and high heat flux components. Reference microstructure was investigated by a combination of TEM, scanning electron microscopy and nano-indentation techniques. TEM was applied to investigate the necking region in the tensile samples deformed up to ~ 30% of stain. The deformation-induced microstructure was characterized and compared for the two grades in terms of the dislocation density, heterogeneity, observation of pile-ups and tangles specifically near high angle grain boundaries. The obtained evolution of the dislocation density was compared with the prediction of the previously developed thermo-mechanical model for the plastic deformation of polycrystalline tungsten and a good agreement was found without modification of the original parameter set.     

Ductilisation of tungsten (W): On the shift of the brittle-to-ductile transition (BDT) to lower temperatures through cold rolling

Here we show that cold rolling decreased the brittle-to-ductile transitions (BDT) temperature of tungsten (W). Furthermore, we show that the BDT temperature correlates with the grain size (the smaller the grain size, the lower the BDT temperature) following a Hall–Petch-like equation. This relation between the grain size and the BDT temperature is well known from ferrous materials and is generally accepted in the steel community.Our ductilisation approach is the modification of the microstructure through cold rolling. In this work, we assess three different microstructures obtained from (i) hot-rolled, (ii) cold-rolled, and (iii) hot-rolled and annealed (1 h/2000 °C, annealed in H₂) tungsten plates. From these plates, Charpy impact test samples with dimensions of 1 × 3 × 27 mm³, without notch, were cut and tested in the L-S and T-S directions. The results show the following BDT temperatures: 675 °C/948 K (L-S, “annealed”), 375 °C/648 K (L-S, “hot-rolled”) and 125 °C/398 K (L-S, “cold-rolled”). The microstructure of the plates is analysed by means of SEM (EBSD: grain size, subgrains, texture, KAM), FIB (channelling contrast) and TEM analyses (bright field imaging).The question of how cold rolling decreases the BDT temperature is discussed against the background of (i) microcracking, crack branching, and crack bridging effects; (ii) texture effects; (iii) the role of dislocations; and (iv) the impact of impurities, micropores, and sinter pores. Our results suggest that the availability of dislocation sources (dislocation boundaries, grain boundaries; in particular, IDBs and HAGBs) is the most important parameter responsible for the increase of the cleavage resistance stress, σ_F, or the decrease of the BDT temperature, respectively.