Conventional Thermomechanical Processing Research Articles

Effect of annealing treatment on texture evolution, microstructure and mechanical properties of Mg–Mn–Ce alloy

Magnesium alloys have received a lot of attention in aerospace, automotive, electronic communications and other fields due to their low density, high specific strength and damping performance. However, magnesium alloy sheets prepared by conventional thermomechanical processing usually have a strong basal texture, resulting in poor formability. In this paper, Mg–Mn–Ce alloys with good plastic forming ability were prepared by studying and optimizing the annealing process. Under the low-temperature annealing condition, it can promote the static recrystallization and grain refinement of Mg–Mn–Ce alloy, and also eliminate the internal stress. With the increase of annealing temperature, a large amount of dispersed Mg12Ce as a stable second phase effectively hindered the grain boundary migration, while the nanoscale α-Mn forms a fine second phase to inhibit grain growth, and both of them together greatly stabilize the deformation structure of the alloy, and maintain a good grain organization under the condition of annealing temperature of 300 °C and holding time of 30 min. The present work also improves the Arrhenius constitutive equation for the Mg–Mn–Ce alloy with respect to the degree of deviation, which significantly improves the accuracy of the model in predicting the thermoforming behavior. Meanwhile, to address the shortcomings of the traditional forming method of magnesium alloy, the processing process of spreading the centrifugal casting tube flat is proposed, the forming process is simulated, and the location of easy breakage in the forming process is predicted. These findings provide microanalytical support for the improvement of forming properties of magnesium alloys by annealing.

Functional Grading Between Soft-Magnetic Fe-Co/Fe-Ni Alloys and the Effect on Magnetic and Microstructural Properties.

Producing soft magnetic alloys by additive manufacturing has the potential to overcome cracking and brittle fracture issues associated with conventional thermomechanical processing. Fe-Co alloys exhibit high magnetic saturation but low ductility that makes them difficult to process by commercial methods. Ni-Fe alloys have good ductility and high permeability in comparison to Fe-Co, but they suffer from low magnetic saturation. Functional grading between Fe-Co and Ni-Fe alloys through blown powder directed energy deposition can produce soft magnetic materials that combine and enhance properties beyond the strengths of the individual magnetic materials. This work focuses on the microstructure, crystal structure, and magnetic properties of functionally graded Fe49Co49V2/Ni80Fe16Mo4 coupons. The grading between the two materials is found to refine the microstructure, thereby improving the mechanical hardness without the use of a nonmagnetic element. Postbuild thermal treatments are found to recrystallize the microstructure and increase the grain size, leading to improved magnetic properties. Analysis of crystal structures provides an understanding of the solubility limits and phase equilibria between the BCC (Fe-Co) and FCC (Ni-Fe) structures. Success in functional grading of soft magnets may provide a pathway toward improving energy conversion efficiency through strategic combinations of high saturation and high strength materials.

Microstructure Design of Multiphase Compositionally Complex Alloys II: Use of a Genetic Algorithm and a Vanishing Cracked Particle Model

Achieving high strength, ductility, and toughness via microstructure design is challenging due to the interrelated dependencies of strength and ductility on microstructural variables. As a natural extension of the microstructure design work in Bhattacharyya and Agnew (Microstructure design of multiphase compositionally complex alloys I: effects of strength contrast and strain hardening, 2024), an optimization framework to obtain the microstructure that maximizes the toughness is described. The strategy integrates a physics-based crystal plasticity model, which accounts for damage evolution within the reinforcement through a “vanishing cracked particle” model that is governed by Weibull statistics, and a genetic algorithm-based optimization routine. Optimization constraints are imposed in the form of bounds on the microstructure parameters such that they are most likely attainable by conventional thermomechanical processing. Various matrix strain hardening behaviors are considered, as well as the strength contrast between the two phases and fracture behavior of the reinforcement. It is shown that the addition of a fine-grained (hard) reinforcing phase is preferred as is a matrix that exhibits sustained strain hardening such as is observed under TRIP/TWIP scenarios. Finally, the Pareto-optimal set of solutions for several scenarios are presented which offer new insights into the linkages between microstructure and mechanical properties.

Evolution of bimodal-structure and achieving ultra-high yield strength in the as-extruded ZK70 alloy via Gd addition

Achieving ultra-high tensile yield strength over 400 MPa in RE modified Mg–Zn-based alloys using conventional thermomechanical processing is a challenging task. In this study, we successfully produced a Mg–7Zn–2Gd–0.5Zr (wt. %) alloy with exceptional strength through traditional hot-extrusion techniques. This alloy exhibits a tensile yield strength of 440 MPa and an acceptable ductility of 4.5%, surpassing a majority of previously reported Mg–Zn-RE-based alloys. Our findings demonstrate that the addition of Gd in Mg–Zn-based alloys effectively retards dynamic recrystallization (DRX) during extrusion, leading to the formation of a distinctive bimodal structure comprising approximately 13.8% coarse un-recrystallized (unDRXed) grains and about 86.2% fine DRXed grains. Furthermore, the alloying effect of Gd enhances particle density, including micron-sized particles and dynamic nano-precipitates. These dynamic precipitates play a crucial role in impeding dislocation movement during extrusion, thereby contributing to the formation of bimodal structure. Additionally, both nano-precipitates and segregation of Zn and Gd atoms towards grain boundaries facilitate the formation of fine DRXed grains through pinning effect. Consequently, the significantly increased tensile yield strength observed in our Gd-modified alloy can be attributed to multiple strengthening mechanisms: fine DRXed grains, strong texture exhibited by unDRXed grains, numerous dynamic precipitates, high-density residual dislocations, and co-segregation of solutes. However, the formation of bimodal structure worsens the yield asymmetry of the Gd-modified alloy due to its strong basal texture. These results provide a valuable insight for further development efforts aimed at achieving ultra-high-strength Mg–Zn-RE-based alloys.

The Role of Grain Size in the Mechanical Properties of Metals

It is now well established that the grain size is the fundamental microstructural feature of all polycrystalline materials. In practice, a very wide range of grain sizes will be needed in order to fully evaluate the effect of grain size on the mechanical properties of metals. For many years this was a significant limitation because it was not possible to use conventional thermomechanical processing to produce materials with submicrometer or nanometer grain sizes. Recently, this problem has been addressed by developing alternative processing techniques based on the application of severe plastic deformation. This overview demonstrates that, although the flow stress increases with decreasing grain size at low temperatures and decreases with decreasing grain size at high temperatures, this clear dichotomy in behavior may be adequately explained by using a single theoretical flow mechanism based on the occurrence of grain boundary sliding.

Overview: Using Severe Plastic Deformation in the Processing of Superplastic Materials

In tensile testing, polycrystalline materials generally fail at relatively low total elongations but under some limited conditions it is possible to achieve exceptionally high elongations of up to and exceeding 400%. This superplastic condition is important scientifically but also it has important uses through the industrial development of superplastic forming operations. This overview traces the historical development of this superplastic effect and it provides a summary of the main characteristics of the superplastic flow process. Conventional thermomechanical processing is not able to produce exceptionally small grain sizes within the submicrometer or nanometer range but this limitation was effectively overcome through processing using severe plastic deformation (SPD). The advantages of SPD processing are discussed and examples are presented. Finally it is demonstrated that the experimental data may be easily and effectively displayed through the construction of deformation mechanism maps based on combinations of stress, grain size and temperature.

Homogeneous α-precipitation and enhanced plasticity in Ti–6Cr–5Mo–5V–4Al high-strength metastable titanium alloy with heterogenous β-structure

A long-standing conflict concerning metastable β-titanium (Ti) alloys is that high strength can be achieved through precipitation strengthening of α-phase while plasticity is largely compromised and even lost completely. Such strength-plasticity trade-off severely limits their structural applications. Here, an attempt to address the issue has been made in Ti–6Cr–5Mo–5V–4Al (Ti6554) high-strength metastable β-Ti alloy. Heterogeneous β-structure is architected by partial static recrystallization (SRX) through properly manipulating solution treatment in the hot-rolled bars. The resulting microstructure is composed of the recrystallized ‘new’ equiaxed β-grains (βe) and the remnant elongated β-fibers (βf). This β-morphology dramatically affects the subsequent αs-precipitation behavior during the aging process. αs-precipitation is rather uniform throughout βf-grains because the pre-existing dislocations left by hot rolling provide copious nucleation sites. However, αGB-films are preferentially formed along βe-grain boundaries, which leads to a boundary-affect zone (BAZ) and thereby renders αs-precipitation in a size gradient from β-grain boundaries to grain interiors. As a result, an ultra-high strength of ∼1700 MPa is achieved whilst the ductility can be up to be 5% in the β-hetero aged samples, which is superior to the β-homo aged siblings that have the same strength level but fully succumb to macroscopic brittleness. The enhanced plasticity in the β-hetero aged samples originates from much better deformation compatibility conferred by the aged βf-grains. However, the soft feature of BAZ around βe-grain boundaries in the β-homo aged samples causes strain localization, which eventually induces intergranular brittle fracture. These findings provide an effective strategy for enhancing plasticity of high-strength metastable β-Ti alloys by structural design, where homogeneous αs-precipitates can be easily constructed by combining conventional thermomechanical processing and heat treatment.

Effects of grain size on the precipitation pathway and mechanical properties of (CoCrFeNi)94Ti2Al4 high entropy alloy

(CoCrFeNi)94Ti2Al4 (Co23.5Cr23.5Fe23.5Ni23.5Ti2Al4, at. %) high entropy alloy (HEA) was fabricated through arc-melting and conventional thermomechanical process. After cold-rolling with 90% thickness reduction and varied annealing treatment (900 °C/30 min, 1000 °C/30 min), (CoCrFeNi)94Ti2Al4 specimens with varied grain size (d) were obtained. Subsequently, the fine-grained (FG, d∼10.5 μm) and coarse-grained (CG, d∼57.9 μm) specimens exhibit the obviously different microstructure after aging heat treatments. The CG specimen mainly shows the homogeneous γ′-precipitates in the γ-matrix. In contrast, FG specimen displays the more complicated microstructure consisting of γ-matrix, intra-granular γ′ precipitates, inter-granular blocky-Heulser and discontinuous γ′-precipitates. After 650 °C/30 h aging treatment, the room-temperature (RT) yielding strength and tensile strength of FG specimen are enhanced by 97% and 42%, compared with CG specimen counterpart, along with a reasonable uniform elongation ∼20.8%. At the higher tensile temperature of 600 °C, the flow stress of FG specimen rises to a maximum value, followed by flow softening with increasing strain, whereas the flow softening does not occur for CG specimen. Considering the thermodynamic and kinetic factors, the effects of temperature and grain size on precipitation pathway and mechanical properties were discussed.

Enhanced grain refinement and texture weakening in Al–Mg–Si alloy through a novel thermomechanical processing

A novel thermomechanical processing including solution treatment, first rolling with large deformation and overaging based on particle-stimulated nucleation of recrystallization (PSN) was proposed to obtain an Al–Mg–Si alloy sheet with fine grain structure, weak texture and high plastic strain ratio. The difference between conventional thermomechanical processing and novel thermomechanical processing with two overaging time parameters was investigated by texture and microstructure characterization and tensile tests. The results show that the finest grain structure with an average size of 17 µm, the weakest recrystallization texture with a volume fraction of 8.4 %, the highest average plastic strain ratio r of 0.69 and the lowest planar anisotropy Δr of − 0.001 were acquired by the novel thermomechanical processing with a long overaging time. In comparison with conventional thermomechanical processing, the novel thermomechanical processing can generate completely different particle distributions and tends to develop a weak texture consisting of the CubeND {001}<310>orientation. Increasing the overaging time is effective in coarsening the particles and thus refining the grain structure, weakening the texture, and improving the average r and Δr values. The genetic characteristics of texture evolution and the particle distribution determine the final recrystallization texture.

No ball milling needed: Alternative ODS steel manufacturing with gas atomization reaction synthesis (GARS) and friction-based processing

Oxide dispersion strengthened (ODS) steels are promising structural materials for future fusion reactors. The high-density (∼1023/m3) of highly stable Y-(Ti)-O nano-oxides provide high sink strength for radiation resistance and high-temperature (> 650 °C) creep strength. Concomitantly, helium management is enabled by trapping high density (∼1023/m3) of small (< 3 nm) helium bubbles in the vicinity of nano-oxides. However, conventional route of making ODS steels involves prolonged ball milling, canning, degassing, and laborious thermo-mechanical processing (TMP). Such route, especially the batch-by-batch ball milling step, faces persistent challenge with scalability and high costs. Gas atomization reaction synthesis (GARS) method has demonstrated the potential of making precursor ODS steel powders without ball milling, but the nano-oxide density was around 1021/m3 in the final consolidated form by conventional TMP. Taking advantage of GARS precursor powder, we use friction-based processing, including friction consolidation and extrusion, to manufacture ODS steel with further improved nano-oxide characteristics. Preliminary results showed that Y/Ti/O species were intimately mixed and rapidly reacted to form nano-oxides with a number density of ∼1022/m3.

Serrated flow stress and nano-precipitation in (CoCrFeNi)94Ti2Al4 high entropy alloy

(CoCrFeNi)94Ti2Al4 high entropy alloy (HEA) was fabricated through drop-casting and conventional thermo-mechanical processing. After cold rolling and recrystallization heat treatments, the tensile properties were investigated at room temperature (RT), 573 K, and 873 K, respectively. At RT, the stress-strain curve exhibits non-serration. However, with the increasing tensile temperature from 573 K to 873 K, the serration evolves from type A + B to type C, and the maximum stress drop rises from 15.0 MPa to 24.7 MPa. At 873 K, C-type serration first appears at the yield strain, but finally disappears when the plastic strain above 12%. Transmission electron microscopy (TEM) analysis indicates that the simultaneous effects of twinning boundary and high density dislocations promote the dynamic precipitation of nanoscale L21-Heusler phase during tensile deformation.

Exploiting tube high-pressure shearing to prepare a microstructure in Pb-Sn alloys for unprecedented superplasticity

Superplastic Pb-Sn alloys were produced via tube high-pressure shearing (t-HPS), in a single step starting from elemental solid bulks. A Pb-40 wt% Sn alloy showed an exceptional superplastic elongation as high as ∼1870% at a strain rate of 1.0 × 10−3 s−1 at room temperature, thereby elevating the optimum strain rate for maximum elongation under these conditions by more than one order of magnitude over conventional cast Pb-Sn alloys. This unprecedented room temperature superplasticity is attributed to the equiaxed grains having uniform sizes of the order of one micrometer, and in particular to the well-mixed domains of Pb and Sn in nearly equal proportions. This microstructure cannot be attained in cast eutectic or hypoeutectic alloys through conventional thermomechanical processing, but instead it is a direct outcome of t-HPS-generated compositional patterning at room temperature.

New nanoscale artificial pinning centres for NbTi superconductors

The efficiency of new artificial pinning centres have been investigated in NbTi using the oxide dispersion strengthening (ODS) concept more typically applied to steels and Ni-superalloys. NbTi alloys containing a dispersion of nanoscale Y2Ti2O7 particles have been successfully manufactured through a powder metallurgy route by mechanical alloying Nb, Ti and Y2O3 nanopowders using high energy ball milling and subsequent consolidation and annealing. The microstructure and superconducting properties of the NbTi-Y2O3 have been characterised after each processing step. After 40 h of ball milling, the Y2O3 particles are dissolved into the nanostructured NbTi matrix formed by mechanical alloying of Nb and Ti. On annealing at temperatures above 800 °C the dissolved Y and O react with Ti from the matrix, and fine Y2Ti2O7 particles (<5 nm) are precipitated. These NbTi-Y2Ti2O7 composites showed excellent superconducting performance, with JC of 5.2 kAmm−2 at 4.2 K, 5 T, considerably higher than Y2O3-free bulks manufactured using the same process and NbTi wires manufactured using the conventional thermomechanical process. The results suggest that ODS powder processing is a promising new route to manufacture NbTi alloys containing artificial pinning centres that give superior superconducting properties.

Microstructure and Strengthening/Toughening Mechanisms of Heavy Gauge Pipeline Steel Processed by Ultrafast Cooling

Heavy gauge pipeline steels experience a low qualification in drop-weight-tear test properties because of the low cooling capability of conventional thermomechanical controlled processing. To solve this problem, a new-generation thermomechanical-controlled processing technology based on ultrafast cooling was applied to prepare heavy gauge pipeline steels. The microstructure, strengthening and toughening mechanisms of 25.4 mm X70 and 22 mm X80 pipeline steels that were processed by ultrafast cooling were studied. The microstructures of the 25.4 mm X70 and 22 mm X80 pipeline steels consisted of bainitic ferrite, M-A island and acicular ferrite with a large fraction above 85%. The grain size and high-angle grain boundary fraction of X70 pipeline steel were 2.7 μm and 43%, respectively, whereas those of the X80 pipeline steel were 2.4 μm and 45%, respectively. The strengthening and toughening mechanisms were studied for the ultrafast cooling method. The main strengthening mechanism for 25.4 mm X70 pipeline steel was solution and grain-refining strengthening and precipitation strengthening with contributions of ~456 MPa and ~90.5 MPa, respectively. In the 22 mm X80 pipeline steel, the main strengthening mechanism was the solution and grain-refining strengthening, and dislocation strengthening with contributions of ~475 MPa and ~109.8 MPa, respectively.

Long-Term Creep Behavior of a CoCrFeNiMn High-Entropy Alloy

The potential of high-entropy alloys (HEAs) to meet or exceed austenitic stainless steel performance with the additional benefit of improved hot corrosion/oxidation resistance makes FCC HEAs attractive for use in energy applications. While shorter-term creep tests have been reported in the literature on HEAs, not all methodologies utilize repeatable techniques. This manuscript reports on over 23,500 accumulated hours of tensile creep testing with adherence to ASTM standards on a melt-solidified ingot of CoCrFeNiMn HEA converted to wrought plate using conventional thermo-mechanical processing techniques. The typical standard creep analyses are reported, i.e., Larson–Miller parameter, Monkman–Grant relationship, activation energy for creep, and creep stress exponents were calculated and compared to previously reported short-term creep tests. Additionally, characteristics of creep fracture and microstructural evolution are reported with cursory dislocation mechanisms investigated.

Control of hydrogen-induced failure in metastable austenite by grain size refinement

To promote the use of metastable austenite for use in hydrogen structures, grain refinement is used. Fe-16Cr-10Ni is a metastable austenitic alloy whose grain size can be controlled by conventional thermo-mechanical processing. With a minimum grain size just below 1 µm, the material was exposed to hydrogen gas at different pressures and tensile tests were performed. SEM/EBSD in-situ tensile tests were also performed. Hydrogen affected all grain sizes, with evidence of intergranular and transgranular fracture at higher hydrogen contents in the larger grains. Smaller grains retained a high yield stress with good elongation, while the largest grains were most embrittled. Cracking was preceded with significant phase transformation and occurred by microvoid coalescence in fine grains and along grain boundaries in coarse grains (with traces of ductility), without indication of failure of austenite-martensite interfaces.

Non-equiatomic FeNiCoAl-based high entropy alloys with multiscale heterogeneous lamella structure for strength and ductility

A new method based on conventional thermomechanical processing is proposed for design of non-equiatomic FeNiCoAlTaB (NCATB) high entropy alloy (HEA) with multiscale heterogeneous lamella (HL) structures to achieve outstanding mechanical properties. Two kinds of microstructures are produced in this study: 1) specimens annealed at 1050 °C exhibit structure composed of fine grains and ultrafine grains, which exhibits a combination of high strength (UTS ~1050 MPa) and good ductility (elongation ~23%); and 2) specimens annealed at 1200 °C which exhibit structure composed of fine grains and coarse grains, which demonstrates excellent synergy of good strength (UTS ~890 MPa) and exceptional ductility (elongation ~43%). It has been shown that shear bands formed in the cold rolling step are responsible for the fine-grain area in both specimens. Additionally, precipitation of NiAl (B2) particles also affects the formation of HL structures due to Zener pinning effect. The back-stress strengthening mechanism, responsible for high strength and ductility, is verified through combined “loading-unloading-reloading” (LUR) cyclic tensile tests and electron backscatter diffraction (EBSD) based geometrically necessary dislocation (GND) density analysis.

Toughness and ductility improvement of heavy EH47 plate with grain refinement through inter-pass cooling

In this study, a novel inter-pass cooling technology was applied during rough rolling (R-IPC) to process a 60 mm ultra-heavy EH47 steel plate with excellent toughness and ductility. The plate was compared to a plate processed through conventional thermo-mechanical controlled processing (TMCP). The microstructure evolution at 1/4-thickness and 1/2-thickness of the plates was studied, while the relationship between microstructure and mechanical properties, through tensile and Charpy V-notch impact toughness, was established. In this study, it was reported that the R-IPC steel deformation penetration was higher compared to the TMCP steel. The ferrite fractions of the R-IPC and TMCP steels were 47% and 10% at 1/4-thickness, as well as 35% and 0.5% at 1/2-thickness, while the acicular ferrite fractions at 1/2-thickness of the R-IPC and TMCP steels were 50% and 9.5%, respectively. The granular bainite width at 1/2-thickness of the R-IPC steel (14 ± 4 µm) was significantly lower than the TMCP steel (27 ± 10 µm). The M/A island volume fraction in R-IPC steel was higher than the TMCP steel at 1/4-thickness, whereas it was reduced dramatically at 1/2-thickness. The high intensity of the desired texture {112}<110> was obtained at 1/2-thickness of the R-IPC steel, while the undesired {411}<148> was obtained at 1/2-thickness of the TMCP steel, which was responsible for the R-IPC steel superior toughness compared to the TMCP steel. The tensile and yield strengths of the two rolled plates were similar, while the elongation at 1/4-thickness of the R-IPC steel was higher compared to the TMCP steel. The low temperature impact energy of the R-IPC steel was significantly higher compared to the TMCP steel, resulting from finer grain size, higher ferrite fraction and higher area of the {110} texture parallel to the impact direction.

Low Temperature Superplasticity in Ultrafine-Grained AZ31 Alloy

The mechanical behavior of an AZ31 magnesium alloy processed by high-pressure torsion (HPT) was evaluated by tensile testing from room temperature up to 473 K at strain rates between 10-5 – 10-2 s-1. Samples tested at room temperature and at high strain rates at 373 K failed without any plastic deformation. However, significant ductility, with elongations larger than 200%, was observed at 423 K and 473 K and at low strain rates at 373 K. The high elongations are attributed to a pronounced strain hardening and a high strain rate sensitivity. The results agree with reports for a similar alloy processed by severe plastic deformation. However, the level of flow stress is lower and the strain rate sensitivity and the elongations are larger than observed in this alloy processed by conventional thermo-mechanical processing.

Design of non-equiatomic high entropy alloys with heterogeneous lamella structure towards strength-ductility synergy

A non-equiatomic FeNiCoAlCrB (NCACB) high entropy alloy (HEA) with a heterogeneous lamella (HL) structure is fabricated through conventional thermomechanical processing. In the HL microstructure, fine-grain regions result from inhibited grain growth due to Zener pinning of boundaries by NiAl (B2) precipitates, while coarse-grained regions originate from grain growth within large deformation bands in the absence of precipitates. A back-stress strengthening mechanism, unique to deformation of heterogeneous microstructures, is verified through mechanical behavior and electron backscatter diffraction enabled geometrically necessary dislocation density analysis. This mechanism gives rise to the combination of both high strength and high ductility in HL-NCACB-HEA.