Conventional Thermomechanical Treatment Research Articles

Microstructure and properties of high strength and high conductivity Cu-0.48Cr-0.20Nb-0.27Zn alloy treated by a new combined thermo-mechanical treatment

A Cu-0.48Cr-0.20Nb-0.27Zn (wt.%) alloy with excellent comprehensive performance is designed and prepared by adding Zn element and combining it with a novel combined thermo-mechanical treatment. A high-temperature short-time treatment is introduced to the traditional thermo-mechanical treatment. Results show that the hardness, tensile strength, yield strength, elongation, and electrical conductivity of Cu-Cr-Nb-Zn alloy reach 180HV, 580 MPa, 535 MPa, 7.6%, and 80.2%IACS, respectively. Compared with conventional thermo-mechanical treatment, the alloy after the new process has increased by 15HV, 35 MPa, 40 MPa, 1.3%, and 2.0%IACS, respectively. Submicron grains and nanoscale twins emerge following high-temperature short-time treatment (850 °C/5min), and the grains undergo substantial refinement. Besides, this process promotes the precipitation of solid solution Cr atoms, thereby significantly improving the electrical conductivity of the alloy. Two distinct crystal structures (C14 and C15) of Cr2Nb with diameters ranging from 80 to 200 nm are observed. The alloy's strength is greatly increased in the following aging process, which precipitates a significant quantity of nanoscale Cr phase. All of the above results have certain reference significance for the optimization design of Cu-Cr-Nb alloy composition as well as performance improvement.

Effect of Ag and Cu addition on the strength and electrical conductivity of Al-Mg-Si alloys using conventional and modified thermomechanical treatments

The effect of adding Ag and Cu on the strength and electrical conductivity of Al-Mg-Si conductor alloys was investigated using conventional and modified thermomechanical treatments. In the conventional thermomechanical treatment, the addition of Ag and Cu moderately increased the strength from 296 MPa to 305–316 MPa above the minimum required electrical conductivity (52.5% IACS) compared to that of the base alloy. However, the modification of the thermomechanical treatment could maximize the efficiency of Ag and Cu addition at strengths above 52.5% IACS, exhibiting that the strength was increased from 317 MPa to 341–348 MPa with the Ag and Cu additions. All alloys under the modified thermomechanical treatment (MTMT) showed a superior strength and electrical conductivity compared to their counterparts under the conventional thermomechanical treatment (CTMT), resulting in a wider window in the high end of strength and electrical conductivity. In addition, MTMT led to a shorter post-aging time to reach the minimum required EC, compared to CTMT. The precipitate characteristics under both thermomechanical treatment conditions were analyzed and quantified using differential scanning calorimetry and transmission electron microscopy (TEM). The TEM results revealed that the alloys under MTMT had a higher precipitate number density by more than three times compared to their counterparts under CTMT, leading to the higher strength levels in the MTMT alloys. Strength and electrical resistivity models were then applied to understand the multiple contributions of the main strengthening mechanisms and microstructure features to the mechanical strength and electrical conductivity. • Modified thermomechanical treatment is proposed to enhance both strength and electrical conductivity (EC). • The effects of Ag and Cu additions on the strength and EC in Al-Mg-Si conductor alloys are studied. • The efficiency of the Ag and Cu additions is maximized using the modified thermomechanical treatment in terms of strength and EC. • The strength and electrical resistivity models are applied.

Ultra-fine-grained and gradient FeCrAl alloys with outstanding work hardening capability

FeCrAl alloy is a promising accident tolerant fuel cladding material for next generation nuclear reactors. Grain refinement may lead to FeCrAl alloys with improved mechanical properties. However, the addition of Al retains the ferrite phase and makes the grain refinement difficult through conventional thermomechanical treatments. In this study, a model FeCrAl alloy, C35M, was processed by surface mechanical grinding treatment. Microscopy studies show the formation of gradient microstructures, consisting of surface nanolaminate layer and ultra-fine-grained sublayer. In situ micropillar compression tests reveal the gradient FeCrAl alloy has a high flow stress, exceeding 1.4 GPa, and excellent work hardening capability. A modified Kocks-Mecking model was utilized to explain the grain size-dependent work hardening behavior. This study sheds lights on the design of gradient ferritic steels with high-strength and good work hardening ability for various industrial applications.

The microstructural effects on the mechanical response of polycrystals: A comparative experimental-numerical study on conventionally and additively manufactured metallic materials

Metal additive manufacturing (AM) provides an unprecedented opportunity to explore extreme microstructural regimes and their associated effects on the mechanical response of polycrystals. In this work, a single alloy (composition) belonging to the high-Mn steels family was processed by a conventional thermo-mechanical treatment and two different AM processes. This resulted in three different materials with strikingly diverse microstructures with respect to the microstructural size and polarity. The influence of disparities in the statistical microstructural features arising from the different processing conditions on the (macroscopic) mechanical response was investigated through an experiment-simulation approach. The (as-processed/as-built) microstructures were analyzed on the meso-scale for derivation of a set of parameters, namely meso-structure descriptors, which adequately describe the meso-structural heterogeneity features. To link the microstructure and mechanical properties, we used a physics-based crystal plasticity modeling approach in the framework of full-field polycrystal homogenization. The meso-structure descriptors together with the submeso-scale/constitutive microstructural parameters were used to simulate the (anisotropic) mechanical response of the aforementioned materials. Then, for a reduced-order quantitative assessment of the effect of microstructural polarity on the anisotropic mechanical response, the plastic anisotropy indices were defined and calculated using the simulation results. This study provides new insights into the primary microstructural effects (the size and polarity effects) on the mechanical (stress and strain hardening) response of polycrystals as well as the propensity of micro-mechanisms accommodating polycrystal plasticity in highly different microstructural regimes, particularly those typically obtained by metal AM.

Ultrafine-Grained Two-Phase High-Entropy Alloy Microstructures Obtained via Recrystallization: Mechanical Properties

A metallurgical methodology has been set up to design alloys with improved mechanical properties such as strength-ductility synergy, via improved strain hardening. To this end, a multi-concepts approach including High-Entropy-Alloys (HEA), grain refinement, chemical contrast and using the transformation induced plasticity (TRIP) deformation mechanism has been implemented. The use of a conventional thermomechanical treatment comprising cold rolling followed by annealing at 650°C, produced two high entropy alloys with submicron alpha (hcp) and beta (bcc) phases. For Ti35-5-5 alloys, stress-induced martensitic transformation, and its subsequent reverse transformation during annealing at 650°C, led to a well-recrystallized state. A microstructure consisting of alpha and beta equiaxed grains was obtained, whose size increased within the submicron domain from about 300 nm to 600 nm with the holding time between 15 to 300 min. Contrariwise, for Ti35-6.5-6.5, which does not deform by the TRIP effect, the same thermomechanical treatment does not produce the recrystallization. Rather precipitation of platelets in the recovered beta matrix occurred. As for the mechanical properties, the yield strengths of the alloys with dual-phase microstructure lie between 950 and 1150 MPa for Ti35-5-5 and between 850 and 950 for Ti35-6.5-6.5 counterpart, for annealing times ranging from 15 min (higher yield strength) to 300 min (lower yield strength). This corresponds to a very large increase in the yield strength compared to that of the as-cast alloys, displaying values of about 400 MPa and 725 MPa for Ti35-5-5 and Ti35-6.5-6.5, respectively. Reasonable ductility was obtained for the alloys with optimized microstructures, that both display a tensile ductility of about 12% after annealing for 300 min at 650°C

Developing an industrial-scale ECAP Mg-Al-Zn alloy with multi-heterostructure for synchronously high strength and good ductility

Although equal channel angular pressing (ECAP) effectively improves the mechanical properties of Mg alloys, it suffers from several problems, like complexity, being time-consuming, and being feasible for small sample sizes, which limit its potential application in industrial fields. Herein, we developed a compact processing route that combined industrial-scale ECAP with partial pre-homogenization and post-aging processes (rather than the commonly used full pre-homogenization and post-aging processes) to improve the processing efficiency. Using this compact processing route, we obtained an industrial-scale AZ91 ECAP alloy with a superior combination of high strength and good ductility. A multi-heterostructure with a heterogeneous grain structure and heterogeneous precipitates characterized the microstructure of this alloy. The heterogeneous grain structure comprised coarse grains and fine grains, and the heterogeneous precipitates included precipitate-dense grains and precipitate-sparse/free grains. The optimal mechanical properties stemmed from the simultaneous effects from the heterogeneous grain structure, heterogeneous precipitates, grain refinement, fine and dense γ-phase particles, and basal plane texture component. The present result highlights the impact of the initial second phase morphology and size on the microstructure evolution during subsequent ECAP or conventional thermo-mechanical treatments, which provides a new approach to control the microstructure of Mg alloys.

Microstructure and mechanical properties of V–Me(Cr,W)–Zr alloys as a function of their chemical–thermal treatment modes

Formation of regularities of the nanometric oxide precipitates and defect microstructure in vanadium-based low activation alloys V–Cr–Zr–(C,N,O) and V–Cr–W–Zr–(C,N,O) as a function of the regimes of their thermochemical treatment was investigated. Several methods of internal oxidation which provide formation of the nanosized ZrO2 particles of controllable dispersion, ensure the nanometric size of the heterophase structure to be maintained up to the temperatures as high as 1300–1400 °С, and allow the recrystallization temperature to be increased up to ≥1400 °С were proposed. Formation of such microstructure contributes to dispersion- and substructural hardening and results in more than twofold increase in the yield stress of these alloys both at room and elevated (800 °С) temperatures, compared to the conventional thermo-mechanical treatment.

Optimizing the strength and ductility of spark plasma sintered Al 2024 alloy by conventional thermo-mechanical treatment

In the present paper, we have developed an effective thermal-mechanical process for achieving both high strength and good ductility in a spark plasma sintered Al 2024 alloy. After conventional solid solution treatment, room temperature cold rolling and artificial aging at 175°C for 3h, the engineering yield strength and ultimate tensile strength of the Al2024 alloy are 529.6 and 583.1MPa, respectively, while maintaining a remarkable elongation to failure of 16.2%. The superior of the mechanical properties was discussed.

Effects of heat treatment processes on microstructure and creep properties of a high nitrogen 15Cr–15Ni austenitic heat resistant stainless steel

Abstract Conventional thermo-mechanical treatment (CTMT) and modified thermo-mechanical treatment (MTMT) process were applied for manufacturing a high nitrogen niobium-stabilized 15Cr–15Ni austenitic alloy. CTMT process consists of 5 h of solution treatment at 1270 °C followed by water quenching and subsequent aging at 820 °C for 50 h. MTMT process differs from CTMT process in hot plastic deformation performed immediately after the solution treatment at 1270 °C and longer aging time. Microstructure and creep properties of the steel obtained by both processing routes were investigated. Creep rupture tests at 750 °C showed double increase in rupture time brought about by MTMT process. Examination of crept microstructure by transmission electron microscopy revealed that the improved creep properties in MTMT process were mainly due to improved distribution uniformity of fine nano-sized carbonitride precipitates in the austenitic matrix and that MTMT process has no effects on the number density and distribution of copper precipitates present in the steel. However, the creep ductility in MTMT process drastically reduced comparing to CTMT process. The higher density of grain boundaries due to finer grain recrystallized microstructures and the formation of higher volume fraction of coarser M23C6 precipitates at the boundaries are believed to be the main reason for the lower creep ductility in MTMT process.

Achievable Strength of Nanostructured Composites with Co-Deformable Components

Large amount of research has been undertaken on the effects of conventional thermomechanical treatment and chemistry variations on the mechanical properties of nanostructured bulk materials developed for wire rod and sheet products. The thermomechanical treatments are selected to refine as much as possible the microstructure to achieve high strength. In most of the cases, the alloy additions are deliberated added to be beneficial to the mechanical properties of the nanostructured materials, especially the tensile strength. In addition to refine the microstructure, both the thermomechanical treatments and chemistry variations may also alter the shape and distribution of the strengthening phases. This article describes the nanostructured composites with face-centered cubic (fcc) copper or body-centered cubic (bcc) ferrite as matrix and discusses several factors that affect the mechanical strength of such materials.

ECAP with back pressure for optimum strength and ductility in aluminium alloy 6016. Part 2: Mechanical properties and texture

The simultaneous increase in strength and ductility of aluminium alloy 6016 processed by equal channel angular pressing (ECAP) was investigated. A complete study of microstructure, texture and mechanical properties after ECAP processing with and without back pressure was carried out for the O temper. The simultaneous increase in strength and ductility of AA6016-O with number of ECAP passes was explained by the use of back pressure during ECAP. A maximum ductility of ∼100% was obtained at the temperature of 200 °C and strain rate of 10 −4 s −1, which is a significant improvement on the ductility exhibited by AA6016 (∼89%) after a conventional thermomechanical treatment at a much higher temperature of 500 °C. The mechanical behaviour was interpreted in the context of the textures developed in the material. A significant amount of texture rotation due to applied back pressure was found.

Optimisation of post-drawing treatments by means of neutron diffraction

The mechanical properties and the durability of cold-drawn eutectoid wires (especially in aggressive environments) are influenced by the residual stresses generated during the drawing process. Steelmakers have devised procedures (thermomechanical treatments after drawing) attempting to relieve them in order to improve wire performance. In this work neutron diffraction measurements have been used to ascertain the role of temperature and applied force – during post-drawing treatments – on the residual stresses of five rod batches with different treatments. The results show that conventional thermomechanical treatments are successful in relieving the residual stresses created by cold-drawing, although these procedures can be improved by changing the temperature or the stretching force. Knowledge of the residual stress profiles after these changes is a useful tool to improve the thermomechanical treatments instead of the empirical procedures used currently.

New nano-particle-strengthened ferritic/martensitic steels by conventional thermo-mechanical treatment

For increased fusion power plant efficiency, steels for operation at 650 °C and higher are sought. Based on the science of precipitate strengthening, a thermo-mechanical treatment (TMT) was developed that increased the strength from room temperature to 700 °C of commercial nitrogen-containing steels and new steels designed for the TMT. At 700 °C increases in yield stress of 80 and 200% were observed for a commercial steel and a new steel, respectively, compared to commercial normalized-and-tempered steels. Creep–rupture strength was similarly improved. Depending on the TMT, precipitates were up to eight-times smaller at a number density four orders of magnitude greater than those in a conventionally heat treated steel of similar composition.

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%.

Tensile properties and fracture toughness of TiAl alloys with controlled microstructures

The objective of this study is to improve the mechanical properties by careful control of both microstructure and alloy additions in two-phase TiAl alloys based on Ti-47Al-2Cr-2Nb (at%). Hot extrusion at temperatures above Tα produces refined lamellar structures, whose microstructural features can be further controlled by subsequent heat-treatment at and above 900 °C. The mechanical properties of the alloys with lamellar structures depend on three factors: colony size, interlamellar spacing, and alloying additions. The tensile elongation at room temperature is strongly dependent on lamellar colony size, showing increasing ductility with decreased colony size. The strength at room and elevated temperatures is sensitive to interlamellar spacing, showing increasing strength with decreased colony spacing. The fracture toughness at room temperature can be substantially improved by heat-treatment at 1320 and 1350 °C. The tungsten addition at a level of 0.2% improves the tensile strength, whereas the silicon addition at a level of 0.3% reduces the castability of the TiAl alloys. The TiAl materials produced by hot extrusion are much superior to those produced by conventional thermomechanical treatments.

Key Microstructures Controlling the Mechanical Properties of Two-Phase TiAl Alloys with Lamellar Structures

ABSTRACTThe objective of this study is to identify key microstructural parameters which control the mechanical properties of two-phase γ-TiAl alloys with lamellar structures. TiAl alloys with the base composition of Ti-47Al-2Cr-2Nb (at. %) were prepared by arc melting and drop casting, followed by hot extrusion at temperatures above the oc-transus temperature, Tα. The hot extruded materials were then heat treated at various temperatures above and below Tα in order to control microstructural features in these lamellar structures. The mechanical properties of these alloys were determined by tensile testing at temperatures to 1000° C. The tensile elongation at room temperature is strongly dependent on grain size, showing an increase in ductility with decreasing grain size. The strength at room and elevated temperatures is sensitive to interlamellar spacing, showing an increase in strength with decreasing lamellar spacing. Hall-Petch relationships hold well for the yield strength at room and elevated temperatures and for the tensile elongation at room temperature. Tensile elongations of about 5% and yield strengths around 900 MPa are achieved by controlling both colony size and interlamellar spacing. The mechanical properties of the TiAl alloys with controlled lamellar structures produced directly by hot extrusion are much superior to those produced by conventional thermomechanical treatments.