Development Of Ultrafine-grained Structure Research Articles

The development of ultrafine grain structure in an additively manufactured titanium alloy via high-temperature microscopy

Microstructures dominated by acicular α' martensitic phase, such as in the case of Ti-6Al-4V fabricated by laser powder-bed fusion (PBF-LB), are known to suffer from reduced ductility and low toughness. The decomposition of such metastable microstructures into α+β lamellar structures during PBF-LB requires either specific laser regimes that are often challenging to be attained or post-process heat treatments which might lead, instead, to undesirable coarsening of the grain structure. Here we propose a novel route for the formation of ultrafine lamellar α+β microstructures and demonstrate the associated advantages in terms of tensile strength and ductility. Our approach is based on a suitable modification of constitution of Ti-6Al-4V with additions of Fe, a known potent β stabiliser of high intrinsic diffusivity. After printing, this alloy presents a microstructure dominated by metastable β phase. We investigate the details of its decomposition using a combination of in-situ high-energy synchrotron X-ray diffraction and high temperature microscopy up to the β transus temperature. The microstructure evolution is comprised by homogeneous decomposition of the metastable β phase via ω-assisted nucleation of α phase, α grain growth sustained by early diffusion of Fe in the β phase followed by a conventional partitioning of V. The understanding of this transformation pathway enables the development of ultrafine grained α+β lamellar microstructures that exhibit outstanding tensile behaviour. The presented approach is machine-agnostic and offers a novel alloy design strategy for development of high-strength alloys in additive manufacturing.

Development of Ultrafine Grain Structure during Deformation-Thermal Treatment of Austenitic Manganese Steel

Development of Ultrafine Grain Structure during Deformation-Thermal Treatment of Austenitic Manganese Steel

Development of Ultrafine Grain Structure in an Al–Mg–Mn–Sc–Zr Alloy During High-Temperature Multidirectional Isothermal Forging

Mechanisms of grain refinement under multidirectional isothermal forging (MIF) at 325 °C (~ 0.65 Tm) and the strain rate 10− 4 s− 1 of the Al–Mg-based alloy with complex additions of transition metals were investigated. The starting alloy had an equiaxed grain structure with grain size 25 µm and a uniform distribution of coherent Al3(Sc,Zr) dispersoids of 20–50 nm. A distinguished structural feature in the early MIF stage was the formation of high strain- and misorientation gradients, followed by deformation banding. Due to the sequential changes of the loading axis, such bands were developed in various directions and fragmented the original grains. The number of bands and misorientation of their boundaries gradually rose with strain, resulting in formation of (ultra)fine grain structure with the grain size 2 µm. New grain formation was concluded to occur via continuous dynamic recrystallization and controlled by the nanosized precipitates, which preferably remained stable and coherent with the surrounding matrix.

Effect of pre-deformation thermomechanical processing on the development of ultrafine grain structure during equal channel angular extrusion

Two different microstructures, i.e., fine-grained solute depleted (FGSD) and coarse-grained solute enriched (CGSE), are prepared by thermomechanical processing. The samples are subjected to six pass equal channel angular pressing (ECAP). EBSD indicates formation of an ultrafine grained (UFG) structure within a size range of 632nm in FGSD in comparison to 727nm in CGSE which is attributed to more significant grain fragmentation in the FGSD sample with higher surface area of boundaries. In both as-annealed and -deformed conditions, hardness, yield strength (YS) and ultimate tensile strength (UTS) of the CGSE samples are higher than those of FGSD which is attributed to more significant solute enrichment. Elongation is found to reduce by the initial passes of ECAP and gradually improve by further processing. At a similar deformation history, the uniform elongation of the deformed CGSE samples is higher than those of the deformed FGSD samples which is attributed to more significant solute enrichment and work hardening causing a delayed necking and larger uniform elongation. However, the elongation to failure of FGSD samples after 2, 4 or 6 pass ECAP is larger than the CGSE sample with a similar deformation history which may be due to the high capacity of boundaries to accommodate dislocations and postpone fracture.

The Effect of Strain on the Development of Ultra-Fine Grain Structure in 7055 Aluminium Alloy Processed by Equal Channel Angular Extrusion

Grain refinement taking place in a commercial 7055 aluminum alloy under equal channel angular extrusion at a temperature of 250°C, was examined. The material was deformed up to a total strain, ε, of ~12. At ε≈1, the development of subgrain bands was found. Upon further straining the average misorientation of deformation-induced boundaries increases; low-angle grain boundaries (LAGBs) gradually convert into true high-angle grain boundaries (≥15°) (HAGBs). At ε≈4, a structure consisting of boundaries with low and high angle misorientations was observed. At ε≈12, a structure with an average grain size of ∼0.7 µm was formed. This size is roughly similar to that for subgrains developed at preceding strains. It was shown that the formation of submicrocrystalline grains occurs through continuous dynamic recrystallization both along initial boundaries and within interiors of original grains as well.

Superplastic behavior and microstructure evolution in a commercial Al-Mg-Sc alloy subjected to intense plastic straining

A commercial Al-6 pct Mg-0.3 pct Sc-0.3 pct Mn alloy subjected to equal-channel angular extrusion (ECAE) at 325 °C to a total strain of about 16 resulted in an average grain size of about 1 µm. Superplastic properties and microstructural evolution of the alloy were studied in tension at strain rates ranging from 1.4 × 10−5 to 1.4 s−1 in the temperature interval 250 °C to 500 °C. It was shown that this alloy exhibited superior superplastic properties in the wide temperature range 250 °C to 500 °C at strain rates higher than 10−2 s−1. The highest elongation to failure of 2000 pct was attained at a temperature of 450 °C and an initial strain rate of 5.6 × 10−2 s−1 with the corresponding strain rate sensitivity coefficient of 0.46. An increase in temperature from 250 °C to 500 °C resulted in a shift of the optimal strain rate for superplasticity, at which highest ductility appeared, to higher strain rates. Superior superplastic properties of the commercial Al-Mg-Sc alloy are attributed to high stability of ultrafine grain structure under static annealing and superplastic deformation at T ≤ 450 °C. Two different fracture mechanisms were revealed. At temperatures higher than 300 °C or strain rates less than 10−1 s−1, failure took place in a brittle manner almost without necking, and cavitation played a major role in the failure. In contrast, at low temperatures or high strain rates, fracture occurred in a ductile manner by localized necking. The results suggest that the development of ultrafine-grained structure in the commercial Al-Mg-Sc alloy enables superplastic deformation at high strain rates and low temperatures, making the process of superplastic forming commercially attractive for the fabrication of high-volume components.

W01-(4) 低炭素鋼の高 Z- 大ひずみ加工における超微細粒組織形成

W01-(4) 低炭素鋼の高 Z- 大ひずみ加工における超微細粒組織形成