High-temperature Severe Plastic Deformation Research Articles

Characteristics of oxide-dispersion strengthened alloys produced by high-temperature severe deformation

This study is to explore an economically attractive and technically feasible processing method for oxide-nanoparticle strengthened alloys for fusion reactor application. Despite many scientific merits of the advanced oxide-dispersion strengthened (ODS) alloys, such as the nanostructured ferritic alloy (NFA) 14YWT, the only viable production path for a high-quality NFA is the high-power mechanical alloying process. This process is often a multi-day high-speed ball milling of alloy powder with a small quantity of yttria (Y2O3) powder, followed by the milled-powder consolidation using extrusion or other methods and additional thermomechanical processing (TMP) for property control. This complex production path, including the low-temperature mechanical alloying in particular, has limited technical advancement toward the cost-effective and scale-up production of ODS alloy components. To overcome such a practical limitation, we proposed to explore alternative low-cost processing routes using traditional thermomechanical processing (TMP) method only. A series of continuous TMP cycles, which were designed to impose high-temperature severe plastic deformation (HT-SPD) conditions to the consolidated powder mixtures, were applied to achieve the effective distribution of oxide particles in nanograin structure and thus desirable mechanical properties. Since the reduced-activation ferritic-martensitic (RAFM) alloy powders (Fe-10Cr and Fe-14Cr alloys with various Y contents) are available in our inventory, we focused to utilize the new solid-state synthesis approach for controlling oxide (oxygen source) dissolution and nanoscale clustering in nanograin structure in those alloys. A combination of powder consolidation at 900 °C and continuous thermomechanical activation at 600 °C yielded two essential ODS alloy microstructure contents–nanograin structure and nanoparticle distribution–and thus demonstrated a good combination of strength and ductility.

Co-introduction of precipitate hardening and TRIP in a TWIP high-entropy alloy using friction stir alloying

Tuning deformation mechanisms is imperative to overcome the well-known strength-ductility paradigm. Twinning-induced plasticity (TWIP), transformation-induced plasticity (TRIP) and precipitate hardening have been investigated separately and have been altered to achieve exceptional strength or ductility in several alloy systems. In this study, we use a novel solid-state alloying method—friction stir alloying (FSA)—to tune the microstructure, and a composition of a TWIP high-entropy alloy by adding Ti, and thus activating site-specific deformation mechanisms that occur concomitantly in a single alloy. During the FSA process, grains of the as-cast face-centered cubic matrix were refined by high-temperature severe plastic deformation and, subsequently, a new alloy composition was obtained by dissolving Ti into the matrix. After annealing the FSA specimen at 900 °C, hard Ni–Ti rich precipitates formed to strengthen the alloy. An additional result was a Ni-depleted region in the vicinity of newly-formed precipitates. The reduction in Ni locally reduced the stacking fault energy, thus inducing TRIP-based deformation while the remaining matrix still deformed as a result of TWIP. Our current approach presents a novel microstructural architecture to design alloys, an approach that combines and optimizes local compositions such that multiple deformation mechanisms can be activated to enhance engineering properties.

Fabrication of a nanostructured high strength steel tube by friction-forging tubular additive manufacturing (FFTAM) technology

Abstract A newly developed solid-state 3D-printing route of friction-forging tubular additive manufacturing (FFTAM) is applied to produce a low-carbon steel tube with superior mechanical properties. FFTAM technology is based on high-temperature severe plastic deformation and consolidation of metal chips via the friction stir welding/processing (FSW/FSP) mechanism to achieve a fully-dense tubular shape in a layer-upon-layer manner. This achieve metallurgical bonding between the layers using a rotating mandrel to maintain radial friction followed by press-die forging under hydrostatic pressure. After FFTAM deposition, the initial ferrite-pearlite microstructure of the steel chips is transformed to a triple-phase alloy consisting of ferrite, martensite, and austenite grains. The formation of martensitic phases by rapid cooling during layer-upon-layer deposition led to significant material hardening, and this further modified by the thermo-mechanical history upon deposition of subsequent layers. Transmission electron microscopy (TEM) observations revealed the microstructural refinement of the manufactured steel tube at the nano-scale range (

Ultrasonic spot welding of dissimilar Al 6022 and Al 7075 alloys

Abstract Al 6022 and Al 7075 aluminum alloy sheets were joined via ultrasonic spot welding (USW). Due to mechanical and thermal cycles during USW, both diffusion and mechanical mixing facilitated the formation of welded interfaces. Grain refinement occurred in the top sheet due to high temperature severe plastic deformation and resultant dynamic recrystallization, while in the bottom sheet, grain coarsening was observed due to significant heat input. The alloying element, Zn, in Al 7075 diffused into Al 6022 during USW, and diffusion behavior varied with selection of top sheet. The fracture mechanism during lap shear testing, i.e. debonding or pullout fracture, varied based on welding power, time, and selection of top sheet. Stress concentration during lap shear testing of spot welded samples was simulated using finite element analysis.

Orientation structural mapping and textural characterization of a CP-Ti/HA surface nanocomposite produced by friction-stir processing

In this study, hydroxyapatite (HA) nanoparticles (~120 nm, 3 vol%) are incorporated within a subsurface layer based on commercial pure titanium using multi-step friction-stir processing (FSP) to fabricate an CP-Ti/HA surface nanocomposite. A combination of electron backscattering diffraction (EBSD) and transmission electron microscopy (TEM) analyses are used to developed a correlation between microstructural features and crystallographic texture formed by material flow and dynamic restoration phenomena in the processed titanium versus CP-Ti/HA surface nanocomposite. An equiaxed ultra-fine grained structure with a mean size <1.4 μm forms on the surface of both samples caused by N and O absorption into the structure of the titanium substrate during high-temperature severe plastic deformation via FSP. The annealed BM does not contain any preferred orientation and its texture is completely random. However, the scenario is significantly altered by FSP treatment due to martensitic phase transformation as well as the absorption of impurities on the titanium surface, the incorporation of HA inclusions, and thermo-mechanical treatment. This results in new crystallographic textures consisting of weak shear and strong prismatic P-fiber dominant components on the surface and center of the processed zone respectively.

Hierarchical multi-phase microstructural architecture for exceptional strength-ductility combination in a complex concentrated alloy via high-temperature severe plastic deformation

Modification of a coarse B2 phase and realization of hierarchical multi-phase microstructures were achieved in an Al0.5CrFeCoNi complex concentrated alloy (CCA) using high-temperature severe plastic deformation (HTSPD). Under HTSPD conditions, a thermodynamically stable B2 phase dissolved, while a thermodynamically unstable coarse Al3Ni phase formed. Al supersaturation in the matrix after HTSPD led to the formation of fine B2 precipitates during the subsequent aging treatment. The combination of Al3Ni for enhanced work hardening and fine B2 precipitates for strength resulted in excellent strength-ductility of 1400MPa and 20%.

Effect of temperature of isothermal multidirectional forging on microstructure development in the Al-Mg alloy with nano-size aluminides of Sc and Zr

The effect of deformation temperature on structural changes in the homogenized ingot of a complex-alloyed aluminum alloy 1570C (Al-5% Mg-0.2% Sc-0.1% Zr) during hot isothermal multidirectional forging (IMF) was investigated. The alloy with equiaxial grains of 25 μm and coherent precipitates of Al3(Sc, Zr) 5–10 nm in diameter was subjected to IMF at the temperatures of 325° and 450 °C (∼0.6 and 0.8 Tm, respectively) at a strain rate of 10−2 s−1 to a total strain of ε = 8.4 with a pass strain of 0.7. It has been shown that grain refinement takes place during IMF at both temperatures. The formation of new fine (sub)grains surrounded by low- and high-angle boundaries started in the vicinity of the original grain boundaries in the earlier stages of deformation. With increasing strain, the volume fraction and boundary misorientations of such crystallites increased; that results in almost complete replacement of the original structure with a new fine-grained one via continuous dynamic recrystallization. The alloy grain refinement was controlled by the interaction of lattice dislocations and/or grain boundaries with precipitates, effectively inhibiting migration of the boundaries as well as dynamic recovery, leading to rearrangement of lattice dislocations, and their annihilation. Herewith, the kinetics of the fine-grained structure formation was practically independent on the processing temperature, resulting in roughly similar microstructures after the same strains, which characterized by the closed mean misorientation angles and fractions of high-angle boundaries. However, the size of (sub)grains processed at 325 °C was significantly lower than at 450 °C, i.e., 1.2 and 2.5 μm, respectively. This indicated that parameters of the developed structure were thermally activated and controlled by precipitates. The features of microstructure formation and the nature of grain refinement of a complex-alloyed aluminum alloy under high-temperature severe plastic deformation conditions are discussed.

Comparative analysis of microstructures formed in highly alloyed aluminum alloy during high-temperature equal-channel angular pressing and multidirectional forging

A comparative analysis of structural changes in the high-strength aluminum 7475 alloy subjected to equal channel angular pressing (ECAP) and multidirectional forging (MDF) at a temperature of 400°C (~0.75 Tm) is performed. It is shown that both ECAP and MDF of the present alloy led to significant grain refinement. The main mechanism of structural changes is mainly related to the formation of deformation bands such as geometrically necessary boundaries and microshear bands, which propagate in various directions and fragment initial grains, thus developing in “continuous” dynamic recrystallization. Comparison between ECAP and MDF performed to roughly the same strain values reveals the formation of similar micro-structures, which are characterized by close values of the average misorientation angles and the fractions of high-angle boundaries. It is also found that, irrespective of the deformation scheme, the grain size developed in both ECAP and MDF obeys a common dependence (unique function) on Zener–Hollomon parameter Z. Microstructure evolution in the highly alloyed aluminum alloy during high-temperature severe plastic deformation and the effect of severe plastic deformation techniques on grain refinement are discussed in detail.

Формирование мелкозернистой структуры в процессе высокотемпературной интенсивной деформации высоко¬прочного алюминиевого сплава (обзор)

Формирование мелкозернистой структуры в процессе высокотемпературной интенсивной деформации высоко¬прочного алюминиевого сплава (обзор)

Effect of High-Temperature Severe Plastic Deformation on Microstructure and Mechanical Properties of IF Steel

Extensive research work has been carried out on interstitial-free steel to understand its response to deformation; particularly, the behavior during severe plastic deformation (SPD). However, most of these studies were mainly undertaken in the ferritic regime. The present investigation reports the initial results of our attempt to employ accumulative roll bonding (ARB), one of the variants of SPD, at a high temperature (950 °C). A considerable grain refinement has been observed, which may be attributed to the severity of deformation and recrystallisation at high temperatures. Nanoindentation tests have been performed at various stages of ARB process to understand the evolution of mechanical properties.

Formation of long-period stacking ordered phase only within grains in Mg–Gd–Y–Zn–Zr casting by friction stir processing

Friction stir processing (FSP) was applied to Mg–Gd–Y–Zn–Zr casting, resulting in significant dissolution of large grain boundary eutectic β-Mg3RE and long-period stacking ordered (LPSO) phases. A fine-grained structure with a grain size of ∼2.4μm was developed in the FSP sample. Besides, fine LPSO lamellae with a width of ∼5 to ∼200nm were distributed only within the grains in the FSP sample, which is different from that observed in both solution-treated and other plastically deformed counterparts. Formation of such special structure is attributed to the high temperature severe plastic deformation generated by FSP. The FSP Mg–Gd–Y–Zn–Zr exhibited high mechanical properties with a yield strength of ∼345MPa and an elongation of ∼21.5%. This study provides an effective way for mechanical property optimization in LPSO-contained magnesium alloys.

Refinement of second phase dispersions in iron aluminide intermetallics by high-temperature severe plastic deformation

Severe plastic deformation has been carried out in some iron aluminide alloys by repeated high-temperature forging and used to refine the dispersion of cast-in second phase particles. The second phases considered range from brittle Laves phases to tougher boride phases, with both apparently refined in the same way by the forging. It appears that refinement is mostly determined by the initial morphology of the second phase particles, with elongated particles being broken towards an equiaxed shape. A relatively uniform distribution of fine second phase particles, such as would be needed for good high-temperature creep resistance, requires very high levels of imposed plastic strain on initially fine elongated particles.

High-Temperature Severe Plastic Deformation of Ferritic Steel by Torsion

An ultra-low carbon IF steel was heavily deformed up to an equivalent strain of 36 at various high temperatures of ferrite single-phase region and various strain rates. Effects of temperature and strain rate on the microstructures evolved in torsion deformation were clarified. On the other hand, it was found that homogeneous ultrafine grained structures were not obtained by the present torsion deformation though very high strain was applied. The coarser grain sizes than those obtained by conventional severe plastic deformation (like ARB) were due to the deformation at higher temperature and lower strain rate, but lower fraction of high-angle grain boundaries in the torsion specimen was suggested to be attributed to the characteristics of monotonic torsion (or simple shear) deformation including the way of strain evaluation.

Alloy composition, deformation temperature, pressure and post-deformation annealing effects in severely deformed Ti–Ni based shape memory alloys

Structure formation in Ti-48.5, Ti-50.0, Ti-50.7 at.% Ni and Ti-47 at.% Ni-3 at.% Fe shape memory alloys depending on deformation temperature (−196 to 400 °C) and pressure (4–8 GPa) under conditions of high-pressure torsion and post-deformation annealing (200–400 °C) was studied using transmission electron microscopy and X-ray diffraction methods. The tendency to form an amorphous structure depends on the relative values of the deformation temperature and martensite start ( M s) temperature as follows: it is strongest in initially martensitic alloy, intermediate in a premartensitic austenite, and the weakest in initially thermally stable austenitic alloy. Lowering of the deformation temperature in the range below the martensite finish ( M f) temperature facilitates amorphization. Raising of the deformation temperature in the austenitic temperature range suppresses amorphization. The upper limiting deformation temperature for partial amorphization of the alloy having the highest M s is located about 300 °C. The upper limiting deformation temperature for actually nanocrystalline structure formation is located about 350 °C for non-aging Ti–Ni alloys and somewhat higher than 400 °C for aged Ti–Ni alloy. The thermomechanical conditions of the equal-channel angular pressing for obtaining actually nanocrystalline structure are recommended. Isothermal martensitic transformation is observed in the Ti-48.5 at.% Ni alloy as a result of keeping for 10 year at room temperature after high-temperature severe plastic deformation. Increasing the pressure suppresses the tendency to form an amorphous structure. The nanocrystalline structure formed under post-deformation annealing from the amorphous structure remains finer than the nanostructure formed as a result of severe plastic deformation through the annealing temperature range covering a nano-grain size scale.

Modifications de structure et de propriétés de composés intermétalliques par déformation plastique intense

Abstract The processing of intermetallic compounds by severe plastic deformation (SPD) methods is reviewed. In Ni3Al-based massive compounds or multiphase materials this leads to a disordered nanocrystalline structure. In TiAl, high-temperature SPD by multi-step forging produces sub-microcrystalline materials, but room-temperature SPD by high-pressure torsion results in a partial phase transformation. This is also the case for the σ phase of multiphase FeCrCo alloys.