Conventional Thermomechanical Processing Research Articles

Fine grained AZ31 produced by conventional thermo-mechanical processing

Abstract The magnesium alloy AZ31 was processed by severe hot rolling and annealing. This processing was optimised to produce recrystallised grain sizes as small as 2.2 μm. The texture of the processed plate was similar to that of the as-received material, with a strong basal alignment in the normal direction. Tensile testing of the fine grained material showed an increase in the strength and elongation compared to the as-received plate. It is concluded that the improved mechanical properties of the fine grained material results from a refinement in the grain size and the elimination of shear bands that pre-exist in the as-received material.

Critical Comparison of Novel and Conventional Processing for Dual-Phase Steels

The final properties of an industrial product depend on the processing route of the material. Hence there is an impetus to study different processing routes to obtain the most desirable final properties. In the present study, low carbon steels have been subjected to the novel deformation induced ferrite transformation (DIFT) technique to produce dual-phase microstructures that are composed of ultra-fine ferrite with martensite and/or bainite as a second transformation product. In this thermomechanical processing technique, the steels have been rapidly cooled from the austenitization temperature to the deformation temperature (which is at least 25°C above the Ar3 temperature) to produce highly undercooled austenite, followed by heavy deformation, and subsequently rapidly cooled thereby facilitating transformation to fine grained ferrite. Comparing the final microstructures obtained by this route with those attained by conventional thermo-mechanical processing, it can be concluded that significant ferrite grain refinement is attainable by the novel DIFT technique thereby emphasizing its potential to achieve improved mechanical properties.

Nano‐Scale Design of TiAl Alloys Based on β‐Phase Decomposition

AbstractPhase decomposition and ordering reactions in β/B2‐phase containing TiAl alloys were utilized to establish a novel, previously unreported, type of laminate microstructure. The characteristic constituent of this microstructure are laths with a nanometer‐scale substructure that are comprised of several stable and metastable phases. Microstructural control can be achieved by conventional thermomechanical processing and leads to a structurally and chemically very homogeneous material with excellent mechanical properties. The physical metallurgy of this novel type of alloy has been assessed by transmission electron microscope investigations and mechanical testing.

Microstructure, creep, and tensile behaviour of a Ti–15Al–33Nb (at.%) beta+orthorhombic alloy

The microstructural evolution, creep and tensile deformation behaviour of a Ti–15Al–33Nb (at.%) alloy was studied. Monolithic sheet material was produced through conventional thermomechanical processing techniques comprising non-isothermal forging and pack rolling. Electron microscopy studies showed that depending on the heat-treatment schedule, this alloy may contain three constituent phases including: β (disordered body-centred cubic), α2 (ordered hexagonal close-packed based on Ti3Al) and O (ordered orthorhombic based on Ti2AlNb). Heat treatments at all temperatures above 990°C, followed by water quenching, resulted in fully-β microstructures. Below 990°C, Widmanstätten O-phase or α2-phase precipitated within the β grains. The fine-grained as-processed microstructure, which exhibited 90 vol.% β-phase, exhibited excellent strength (UTS = 916 MPa) and ductility (ϵf>12%). After heat treatment, greater volume fractions of the orthorhombic phase precipitated and resulted in lower ϵ f values with UTS values ranging between 836–920 MPa. However, RT elongations of more than 2% were recorded for microstructures containing up to 63 vol.% O-phase. Specimens subjected to 650°C tensile experiments tended to exhibit lower strength values while maintaining higher elongation-to-failure. Tensile creep tests were conducted in the temperature range 650–710°C and stress range 49–275 MPa. The measured creep exponents and activation energies suggested that grain boundary sliding operates at intermediate stress levels and dislocation climb is active at high stresses. Microstructural effects on the tensile properties and creep behaviour are discussed in comparison to a Ti–12Al–38Nb O + β alloy.

Microstructure, creep, and tensile behavior of a Ti–21Al–29Nb(at.%) orthorhombic+B2 alloy

The creep and tensile deformation behavior of a Ti–21Al–29Nb (at.%) alloy were studied. Monolithic sheet materials were produced through conventional thermomechanical processing techniques. Heat treatments at all temperatures above 1050 °C, followed by water quenching, resulted in fully-B2 microstructures. Below 1050 °C, either equiaxed or Widmanstätten O-phase precipitated within the B2 grains. RT elongation-to-failure values of less than 2% were recorded for aged microstructures containing 72–78 volume percent O phase. Tensile-creep experiments were conducted in the temperature range 650–710 °C and stress range 48–250 MPa. The measured creep exponents and activation energies suggested that the creep mechanisms were dependent on stress and microstructure. Microstructural effects on the tensile properties and creep behavior are discussed and the data was compared to that for other Ti 2AlNb-based alloys.

Grain boundary engineering for superplasticity in steels

The microstructure with suitable boundary characters for superplasticity is summarized for the steels which consist of two phases, i.e., ferrite (bcc α) + austenite (fcc γ) or ferrite (α) + cementite (orthorhombic θ -Fe3C). In (α + γ) duplex alloys, a conventional thermomechanical processing (solution treatment + heavy cold rolling + aging) produces the (α + γ) duplex structure through the competition of recovery/recrystallization of matrix and precipitation. In Fe-Cr-Ni (α + γ) duplex stainless steels with high γ fractions (40–50%), α matrix undergoes recovery to form α subgrain boundaries and γ phase precipitates on α subgrain boundaries with near Kurdjumov-Sachs relationship during aging. By warm deformation, the transition of α boundary structure from low-angle to high-angle type occurs by dynamic continuous recrystallization of α matrix and, simultaneously, coherency across α/γ boundary is lost. Contrarily, α phase first precipitates in deformed γ matrix in Ni-Cr-Fe based alloy during aging. Subsequently discontinuous recrystallization of γ matrix takes place and the (α + γ) microduplex structure with high-angle γ boundaries is formed. The formation of those high-angle boundaries in (α + γ) microduplex structure induces the high strain rate superplasticity. In an ultra-high carbon steel, when pearlite was austenitized in the (γ + θ) region, quenched and tempered at the temperature below A1, an (α + θ) microduplex structure in which most of α boundaries are of high-angle type is formed through the recovery of the fine (α ′ lath martensite + θ) mixture during tempering. Such (α + θ) microduplex structure with high angle α boundaries exhibits higher superplasticity than that formed by heavy warm rolling or cold rolling and annealing of pearlite which contains higher fraction of low angle boundaries.

Developing a Superplastic Forming Capability in Nanometals

Exceptionally high superplastic ductilities may be achieved in metallic alloys when the microstructure consists of very small and reasonably equiaxed arrays of grains separated by grain boundaries having high angles of misorientation. It is now well established that processing through the application of severe plastic deformation provides the potential for achieving submicrometer or nanometer grain sizes in bulk solids, where these grain sizes are generally significantly lower than those attained using conventional thermomechanical processing. This paper describes the development of a superplastic forming capability in three different aluminum-based alloys by processing using the technique of equal-channel angular pressing (ECAP) in which a material is subjected to a high imposed strain by pressing repetitively through a die. Processing by ECAP can be used to achieve superplastic ductilities in these alloys because precipitates are present within the matrix and they serve to retain the ultrafine grain sizes at elevated temperatures.

Crystallographic textures in rolled and annealed Fe-Ga and Fe-Al alloys

The feasibility of obtaining [001] preferred texture in polycrystalline Fe85Ga15 and Fe85Al15 magnetostrictive alloys containing 1 mol pct NbC using a low-cost conventional thermomechanical processing approach is shown in this work. Thermomechanical processing conditions examined consisted of a sequence of hot rolling, two-stage warm rolling at 400 °C with intermediate anneal at 900 °C and texture anneal in the temperature range of 900 °C to 1300 °C for time periods up to 24 hours. Textures present prior to and after texture annealing were characterized using orientation imaging microscopy in a scanning electron microscope. In the case of Fe85Ga15 alloy with 1 mol pct NbC, the deformation-induced texture after a two-stage warm rolling consists of mixed {100} 〈110〉 and {111} 〈110〉 type partial textures. Texture annealing of the Fe85Ga15 alloy with 1 mol pct NbC at 1150 °C for 2 hours changes the texture to a predominant texture that is close to {001}〈100〉. On increasing the annealing time to 24 hours, the texture shifts toward {110}〈100〉. While texture anneal at both 1150 °C and 1300 °C produces a [001] or near-[001] preferred orientation along the rolling direction in the Fe85Ga15 alloy with 1 mol pct NbC, 1150 °C-24 hour treatment was found to provide the strongest [001] orientation among the conditions examined. Similar trends are observed for the case of Fe85Al15 alloy with 1 mol pct NbC.

Formation of Ultrafine Ferrite by Strain-induced Dynamic Transformation in Plain Low Carbon Steel

Strain-induced dynamic transformation (SIDT) is of great advantage to obtaining ultrafine-grained ferrite in low carbon steels partly by early impingement of ferrite nuclei which are very rapidly and concurrently formed due to the deformation and partly by random crystallographic orientation distribution of ferrite grains. The SIDT fraction increases with the increase of strain. There is, however, a critical strain under which no SIDT occurs. In order to apply this refining mechanism to actual production lines, it is necessary to reduce the critical stain and enhance the kinetics of SIDT. It has been known that refining prior austenite grain is the most effective for this purpose. The influences of deformation temperature and strain rate on SIDT were also examined. Because SIDT is a kind of softening mechanism of strained austenite, it competes with dynamic recrystallization (DRX) at below Ae3 temperature. Comparing the critical strains of SIDT and DRX, SIDT is predominant softening mechanism, which enables us to utilize it for grain refinement. This provides an important clue to overcome the limitation of conventional thermomechanical control process (TMCP) in grain refinement area.

Effect of precursor materials on Ic and Ic– H behaviors of Ag-clad (Bi,Pb)-2223 tapes

Thirty seven filamentary Bi-2223 Ag sheathed tapes have been fabricated by the PIT method using precursors which were prepared by the two-powder process. The different types of the precursors are the following: (a) a mixture of 2212 and CaCuO 2 (powder A), (b) a mixture of 2201 and CaCuO 2 (powder B). A nearly pure 2223 phase can be obtained after anneals at 818 °C in 8.5% O 2 in less than 40 h and 60 h for powder A and powder B, respectively. However, a small amount of impurity phases, especially the 2201 phase, still appears in the oxide core for tape B (fabricated from powder B) using a conventional thermomechanical process. An additional low-temperature annealing procedure at the end of the last thermal cycle was used to convert any residual 2201 in tape B into 2212, thereby improving the grain connectivity and overall I c of tape B dramatically. But, this additional annealing stage had no distinct effect on the phase evolution and the critical current of the tape A (fabricated from powder A). Using both types of precursor mixtures, we obtain tapes with high phase purity, well-aligned grains, and reproducible critical current densities. The maximum I c values (77 K, 0 T) of fully processed tapes with 0.20 mm thickness are 52 A and 42 A for powder A and powder B respectively.

Superplastic deformation of a fine-grained Zn–0.3wt.%Al alloy at room temperature

The aims of this study are firstly to obtain a fine and stable microstructure in a quasi-single phase Zn–Al alloy and secondly to characterize the superplastic deformation behavior of this alloy performing load relaxation and tensile tests, as well as a microstructural study using a scanning and a transmission electron microscopy. A very fine and stable microstructure with an average grain size of about 1 μm was produced by modifying the conventional thermomechanical treatment process for commercial Al alloys to obtain a remarkable elongation of 1400% at room temperature, which is the largest ever reported for this alloy. The flow curves of log σ versus log ε ̇ constructed from load relaxation test and a series of tensile tests conducted under various initial strain rates appeared to have a sigmoidal shape typical for superplastic materials. Even after the specimen was elongated up to 100% at room temperature, no evidence for grain growth was noticed. From the observations of microstructural changes and the scratch offsets on the surface of a deformed specimen, the dominant deformation mechanism could be identified as the grain boundary sliding (GBS), possibly accommodated by plastic flow due to dislocation glide near grain boundaries. At a still higher strain rate, the accommodation mechanism of GBS appears to change from dislocation activity near grain boundaries to deformation twinning at stress concentrations such as triple junctions and grain boundary ledges. When the grain size was increased by an order, predominant deformation mechanism appeared to be dislocation activity evidenced by the cell structure reducing the tensile elongation substantially to around 100%.

Microstructure controlling by heat treatment and complex processing for Ti 2AlNb based alloys

The phase evolution and microstructure controlling process were studied for Ti 2AlNb based alloys. Monolithic sheet materials were produced through conventional thermomechanical processing techniques comprising non-isothermal forging and pack rolling. Phase evolution studies showed that depending on the heat treatment schedule, Ti 2AlNb based alloys may obtain various constituent phase, including B2, α 2, O phases. This study indicated that substitution of Nb with Ta improved the B2 transus temperature. Heat treatments in different phase fields produced different microstructures. It was difficult to remove the prior large B2 grain boundary when solid soluted above B2 transus temperature. In order to optimize the microstructure, a complex processing method was proposed: prior heat treatment was applied to obtain a fine and homogenous O+B2 lath structure, followed by thermomechanical processing at suitable temperature. By means of this method, duplex microstructure with equiaxed α 2/O particles and fine O phase laths in B2 matrix, and without prior large B2 grain boundary, was secured. A systematic means of microstructure controlling was suggested.

High Strain Rate Superplasticity in a Zn - 22% Al Alloy after Equal-Channel Angular Pressing

It is well established that superplasticity requires a small grain size and therefore there is an interest in developing processing methods having the capability of reducing the grain size below that generally produced in conventional thermo-mechanical processing. Experiments were conducted on a superplastic Zn-22%Al eutectoid alloy where the material was subjected to equal-channel angular pressing (ECAP) through 4, 8 or 12 passes at a temperature of 373 K. It is shown that this processing procedure leads to additional grain refinement such that the grain size was reduced from an initial value of ∼1.8 μm to values as low as ∼0.6 μm. Tensile testing after ECA pressing revealed a significant enhancement of the superplastic properties including both increases in the total elongations to failure and the occurrence of these high elongations at very rapid strain rates. Elongations were achieved of up to >2380 % at a strain rate of 1 s -1 The results suggest that ECAP may be a very useful procedure for attaining superplasticity at high strain rates in commercial alloys.

Coupling Automated Electron Backscatter Diffraction with Transmission Electron and Atomic Force Microscopies

Abstract Grain boundary network engineering is an emerging field that encompasses the concept that modifications to conventional thermomechanical processing can result in improved properties through the disruption of the random grain boundary network. Various researchers have reported a correlation between the grain boundary character distribution (defined as the fractions of “special” and “random” grain boundaries) and dramatic improvements in properties such as corrosion and stress corrosion cracking, creep, etc. While much early work in the field emphasized property improvements, the opportunity now exists to elucidate the underlying materials science of grain boundary network engineering. Recent investigations at LLNL have coupled automated electron backscatter diffraction (EBSD) with transmission electron microscopy (TEM)5 and atomic force microscopy (AFM) to elucidate these fundamental mechanisms. An example of the coupling of TEM and EBSD is given in Figures 1-3. The EBSD image in Figure 1 reveals “segmentation” of boundaries from special to random and random to special and low angle grain boundaries in some grains, but not others, resulting from the 15% compression of an Inconel 600 polycrystal.

Microstructure, creep, and tensile behavior of a Ti–12Al–38Nb (at.%) beta+orthorhombic alloy

The microstructural evolution, creep, and tensile deformation behavior of a Ti–12Al–38Nb (at.%) alloy were studied. Monolithic sheet materials were produced through conventional thermomechanical processing techniques comprising nonisothermal forging and pack rolling. TEM studies showed that depending on the heat-treatment schedule, this alloy contains two constituent phases including: β (disordered body-centered cubic) and O (ordered orthorhombic based on Ti2AlNb). Heat treatments at all temperatures above 800°C, followed by water quenching, resulted in fully-β microstructures. Below 800°C, fine Widmanstatten O-phase needles precipitated within the β grains. Fully-β microstructures exhibited room-temperature (RT) elongations of more than 27%. The second-phase O precipitates provided strengthening at the expense of elongation. However, RT elongations of more than 12% were recorded for aged microstructures containing 30 vol.% O-phase. Metallographic observations revealed that slip was compatible between the two phases. Tensile creep tests were conducted in the temperature range 650–705°C and stress range 50–172 MPa. The deformation observations and measured creep exponents and activation energies suggested that the creep mechanisms were dependent on stress. For applied stresses less than 123 MPa, the creep exponents were between 1.6 and 2.0 and low dislocation densities were observed. For higher stresses, the stress exponents were between 3.5 and 7.2 and higher dislocation densities were observed. The calculated activation energies for the low-stress regime were approximately half those calculated for the high stress regime. These data suggest that Coble creep operates at low stress levels and dislocation climb is active at high stresses. Microstructural effects on the tensile properties and creep behavior are discussed in the light of existing models.

The phase evolution and microstructural stability of an orthorhombic Ti-23Al-27Nb alloy

The phase evolution and microstructural stability were studied for an orthorhombic Ti-23Al-27Nb alloy. Monolithic sheet materials were produced through conventional thermomechanical processing techniques comprising nonisothermal forging and pack rolling. Phase evolution studies showed that, depending on the heat treatment schedule, this alloy may contain several constituent phases including: α2 (ordered close-packed hexagonal D019 structure), B2 (ordered body-centered cubic, bcc), β (disordered bcc), and O (ordered orthorhombic based on Ti2AlNb). Differential thermal analysis studies indicated that the B2 transus temperature was 1070 °C. Heat treatment and transmission electron microscopy studies showed that the α2+B2 phase field extended between 1010 and 1070 °C. From 875 to 975 °C, a two-phase O+B2 field existed. Sandwiched between these two-phase regimes was a narrow three-phase α2+B2+O field. Below 875°C, an O+β field existed. All heat treatments at or above 875°C, followed by quenching, resulted in equiaxed microstructures. However, below 875 °C, the B2 phase transformed into a mixture of O and bcc phases with lenticular morphologies. Cellular precipitation of O+β platelets at O/B2 and α2/B2 grain boundaries occurred depending on solutionizing and aging temperatures, which is explained by the compositional gradient between the bcc phases.

Superplasticity of (α+γ) Duplex Stainless Steel with Elongated Secondary Phase

Effect of initial (α+γ) microstructure on the superplasticity of a hot forged Fe-26Cr-8Ni (α+γ) duplex stainless steel with elongated secondary γ precipitates was examined. The hot forged specimens contain the (α+γ) duplex structure with γ phase elongated along the longitudinal direction of the forged bar. Heavy cold rolling of hot forged specimens promotes recrystallization of γ and α during heating to and holding at 1273K (tensile deformation temperature), resulting in the formation of finer, more equiaxed (α+γ) microduplex structures. The specimens heavily cold rolled after forging exhibit large superplastic elongation nearly the same as the (α+γ) microduplex specimen produced by a conventional thermomechanical processing (solution treatment + heavy cold rolling + aging) although hot forged specimens show much smaller elongation.

The role of Bi-2212 in Bi-2223 Ag-sheathed tapes fabricated by a controlled melt process

We have studied the phase composition, transport current and flux pinning properties in Bi-2223 Ag-sheathed tapes fabricated by a controlled melt method (CM) and conventional thermo-mechanical process(CT), respectively. The highest Jc (77K, 0T) value of 36.8kA/cm2 (Ic = 27.5 A) was obtained in CM sample. The results of I–V curve showed a drastic voltage drop occurred at a certain current in CM samples, indicating that above a certain value of the self-field of the Ag-sheathed tape, Bi-2212 phase, which becomes a nonsuperconducting phase, might act as effective flux pinning centers.

Deformation state effects on the Jc of BSCCO tapes

Developing long-length, high- J c superconducting tapes has been a major world-wide effort in recent years because of their potential applications in power-transmission lines, motors, and other devices. The superconducting tape is usually produced by co-deforming a ductile silver sheath containing a superconducting oxide. Since the conventional thermomechanical process has failed to yield sufficient J c values for most liquid-nitrogen temperature applications, new approaches are needed to improve the criticalcurrent density, J c. This study investigated the feasibility of improving J c by increasing the shear and compressive stresses in the silver-sheathed Bi 2Sr 2Ca 2Cu 3O 10 + x (BSCCO-2223) tapes during the rolling. To investigate the effects on the J c of the stress state during rolling, specific stress states were imposed by rolling the BSCCO tapes embedded at different locations within thick steel blocks. Pure compression loading was achieved in the center plane of the blocks, while a combined compression-shear loading state was produced away from the center plane. Higher compressive hydrostatic stress at the tape edge was obtained by confining the tape width. Tapes deformed with a shear stress component exhibited higher J c values than tapes subjected to pure compression. In addition, the compressive hydrostatic stress reduced the porosity in the oxide near the tape edge and, as a consequence, increased the J c value.

The grain boundary character distribution effect on the flow stress of polycrystals: The influence of crystal lattice texture

The grain size of a polycrystalline aggregate has an important effect on its properties. Several properties of materials, such as the yield and flow stresses, ductility and hardness have been suggested to show a linear dependence on (average size of grains) −1 2 . A number of models which justify the grain size effect are based on the assumption that the grains in a polycrystal have the same size and the grain boundaries the same properties. Grain boundaries are characterized by differences in their structure and, as a result, in their properties. An experimental study of the grain boundary strengthening effect in a 316L austenitic stainless steel has been carried out on a series of specimens obtained employing a powder-forming process and produced by conventional thermo-mechanical processing. The Brinell hardness was measured and analysed as a function of (grain size) −1 2 . The experimental data, in general, confirm the Hall—Petch relationship. A significant difference is observed in the values of the K H constants. This difference is discussed in terms of the influence of crystal texture on the strengthening effect of the grain boundaries.