Low-temperature Thermomechanical Treatment Research Articles

Synergistic effects of cold rolling and age hardening on the hardness and tensile characteristics of AA6061 hybrid composites

The present study involves the fabrication of aluminium alloy 6061 matrix hybrid composites with varying weight fractions of silica sand and copper particles by employing the conventional stir casting method. The combined influence of age hardening (AH) and low temperature thermomechanical treatment (LTMT) on the hardness and tensile properties of AA6061 hybrid composites was investigated. The uniform dispersion of the particles in the matrix was confirmed by microstructure analysis and the improvement of Brinell hardness values. The composites exhibited higher tensile strength and hardness than the base alloy. Both AH and LTMT enhanced the properties of the hybrid composites and a comparison between them revealed the best results for LTMT hybrid composites. The LTMT hybrid composite with 3 wt% silica sand and 3 wt% copper (3S3C) subjected to 12% rolling deformation and aged at 100 °C had the highest hardness and tensile strength of 144.26 HV and 290 MPa respectively. The hardness and tensile strength of AA6061-3S3C hybrid composite subjected to LTMT in peak aged condition showed an improvement of 125 and 97% respectively when compared with those of AA6061 alloy. Fracture surface analysis of the thermomechanical treated composites in peak aged condition showed a mixed mode of failure dominant with the ductile fracture.

Effect of low and high temperature ECAP modes on the microstructure, mechanical properties and functional fatigue behavior of Ti-Zr-Nb alloy for biomedical applications

A Ti-18Zr-15Nb (at%) shape memory alloy was subjected to a high-temperature thermomechanical treatment (HTMT) combining an equal channel angular pressing (ECAP) at 500 °C for n = 4–8 passes and a short-time post-deformation annealing (PDA) at 600 °C. The phase composition, microstructure, texture, mechanical and functional properties were studied. The functional fatigue behavior observed in this study was compared to that resulted from the reference low-temperature thermomechanical treatment (LTMT) by ECAP at 200 °C for 3 passes + PDA (600 °C for 5 min). The ECAP at 500 °C (n = 4) led to the formation of a highly deformed, dynamically polygonised substructure of β-phase with a crystallographic texture close to the [101] direction. In this state, the alloy exhibited an excellent combination of the static functional and mechanical properties: a relatively high strength (UTS = 670 MPa), a sufficient ductility (δ = 13.3 %), a low Young’s modulus (E < 40 GPa), and a high superelastic recovery strain (εrsemax= 3.1 %). An increase in the number of passes during ECAP to n = 8 led to a greater substructural hardening of the material, grain/subgrain refinement, and the release of α-phase. This significantly increased the alloy strength (UTS = 897 MPa), but reduced its ductility (δ = 5.9 %) and suppressed martensitic transformation. The alloy after both the HTMT (ECAP at 500 °C, n = 4) and TMT (ECAP at 200 °C, n = 3) processes exhibited an equally excellent functional fatigue resistance accompanied by a superior superelastic behavior with small accumulated strains. However, the HTMT process appears to be more technologically advanced, as it eliminates the need for PDA and reduces the risks of specimen cracking during processing.

Experimental analysis of hardness and tensile characteristics of copper reinforced AA6061 stir cast composites subjected to thermal and deformation assisted heat treatments

Aluminium alloy 6061 based composites reinforced with a varied weight percentage of copper particulates are fabricated utilizing a liquid metal stir casting technique. The collective influence of copper reinforcement, age hardening and low temperature thermomechanical treatment on AA6061 was investigated. The age hardened composites displayed better hardness and ultimate tensile strength than as cast composites. Thermomechanical treatment of the composites further enhanced the mechanical properties and hence showed better results over age hardened composites. The study revealed that an increase in the deformation enhanced the hardness and strength of the composite while the aging time to achieve the peak hardness was reduced. Both age hardened and thermomechanically treated composites with 6 wt.% copper reinforcement indicated the best peak hardness and UTS values at the lower aging temperature of 100 °C. The thermomechanically treated composite with 6 wt.% Cu, 15% deformation aged at 100 °C showed 80 and 69% increase in hardness and UTS, respectively, over as cast composite. Fracture surface analysis of the as cast, age hardened, and thermomechanical treated composites showed a mixed mode of fracture dominant with the brittle failure.

Low-temperature superplastic deformation of EK61 and EP975 wrought nickel-based superalloys with an ultrafine-grained structure

It is shown that thermomechanical treatment (TMT) of the EK61 superalloy with a strain rate (ε∙) of 10−3 s−1 and a gradual decrease in the treatment temperature from (0.92 – 0.65)Tδ leads to the transformation of the initial coarse-grained structure into an ultrafine-grained (UFG) mixed type structure. Such a mixed UFG microstructure consists of γ-phase grains and δ-phase particles 0.3 μm in size and relatively large particles of the δ-phase up to 2 μm in size. The EK61 superalloy having such a microstructure exhibits low-temperature superplasticity in the temperature range of 700 – 900°C, the maximum characteristics of which are achieved at 850°C and ε∙ =10−3 s−1. Low-temperature TMT in the temperature range of (0.84 – 0.8)Тγ' also leads to the formation of an UFG structure in the EP975 superalloy. The UFG microstructure of the EP975 superalloy consists of γ-phase grains and γ'-phase particles 0.5 μm in size. And there are relatively large particles of the γ'-phase up to 3.5 μm in size of a globular-shaped form. The EP975 superalloy with such a structure exhibits low-temperature superplasticity in the temperature range of 900 –1000°C, the maximum characteristics of which are achieved at 1000°C and ε∙ =10−3 s−1.

Mechanical characterization of low temperature thermomechanical treated AA6061-silica sand composites

In the present work efforts are made to confirm the effects of reinforcement and thermomechanical treatment (TMT) on the mechanical properties of aluminium matrix composite (AMC) produced through stir casting technique. Silica sand particles with varying weight percentages were used to fabricate the composites and the extracted particle analysis revealed the retention of a minimum of 97% of reinforcement particles. The composites are subjected to conventional age hardening and low temperature thermomechanical treatment (LTMT) to enhance their mechanical properties. The mechanical characterization of the composites subjected to heat treatment processes include hardness and tensile tests. Nearly 20–30% increase in peak hardness, 30% decrease in aging time and 15% increase in ultimate tensile strength (UTS) was observed in LTMT treated composites as compared to age hardened composites. The best results of peak hardness and UTS of 139.76 HV and 274 MPa respectively were achieved by composite containing 6 wt.% silica sand reinforcement subjected to 15% deformation and aged at 100 °C.

Optimization of a thermomechanical treatment of superelastic Ti-Zr-Nb alloys for the production of bar stock for orthopedic implants

Ti-19Zr-14 Nb (at%) shape memory alloy was subjected to low- and high-temperature thermomechanical treatments to fabricate long-length bar stock for the production of load-bearing orthopedic implants. The phase composition, structure, texture, uniaxial tensile and three-point bending fatigue behavior were studied. Low-temperature thermomechanical treatment (LTMT), combining cold rotary forging and post-deformation annealing at 550 °C led to the formation in the peripheral zone of the bar cross-section of a statically recrystallized fine-grained structure with a relatively strong [001] crystallographic texture and of a mixed statically recrystallized and polygonized substructure, in the central zone of the cross-section. In this state, the alloy exhibited an excellent combination of the static functional and mechanical properties: relatively high strength (UTS=680 MPa), low Young’s modulus (E < 40 GPa), and high superelastic recovery strain (εrsemax=3.4%). High-temperature thermomechanical treatment (HTMT), consisting in hot rotary forging at 700 °C, led to the formation of a dynamically polygonized substructure with a uniform crystallographic texture close to the [101] direction; this direction corresponding to the maximum theoretical recovery strain limit. As compared to the LTMT bar stock, its HTMT equivalent manifested slighly inferiour static properties but superior fatigue resistance in bending, thus being an optimal candidate for the production of orthopedic implants.

Transient Nucleate Boiling Process Used for Obtaining Super Strong Carbon Steels and Irons

Based on self–regulated thermal process, in the paper four types of thermomechanical treatments are considered. The first is a high temperature thermomechanical treatment (HTTMT) followed by complete martensitic transformation. The second is a low temperature thermomechanical treatment (LTTMT) plus martensitic transformation. The third is the high and low temperature thermomechanical treatment (HTTMT and LTTMT) plus martensitic transformation. And the last includes HTTMT and LTTMT plus bainitic transformation to obtain super strong and ductile materials. It is shown in the paper that listed technologies are enough intensive to obtain very strong and ductile materials using plain high carbon steels. A detailed consideration of all processes in the paper will motivate engineers to perform mentioned technologies in forging shops to receive super strong and ductile materials without costly alloying that saves energy and alloying elements. The paper discusses the opportunity of preventing martensite transformation to receive fine and nano–bainitic microstructure during intensive quenching. A hypothesis is forwarded that explains possible technology used in 8th and 9th centuries in the Middle East to manufacture Damascus steel. The secret of Damascus steel could be the duration of transient nucleate boiling process needed for preventing martensite transformation during forging of steel.

Alternate Thermomechanical Heat Treatment Cycles for the Enhancement in Hardness and Tensile Properties of Commercial Grade AA6061

Numerous research incorporating precipitation hardening and/or strain hardening on Al-Mg-Si alloys has demonstrated improved mechanical properties in the alloys. However, there is a lack of research on how these processes can be combined efficiently to provide better strength and hardness for the alloys. The present study describes the effects of low temperature thermomechanical treatment (LTMT - combination of conventional age hardening and strain/work hardening) carried at two isothermal aging temperatures on the hardness and strength of the aluminium alloy 6061 (AA6061). Further, an innovative LTMT process is also proposed, which aims at further enhancing the mechanical properties of the alloy. The study revealed the increase in degree of deformation improved the strength and hardness of the alloy with reduced aging time. The effect of the degree of deformation, pre-deformation, and alternate deformation and aging cycles on the hardness and strength of the alloy was studied with the help of fracture surface analysis.

Electronic tricroscopic examination of the structure of the Ti-50.2 at. % Ni alloy

The method of electron-microscopic analysis investigated the structure of the alloy Ti-50.2 at % Ni at different stages of dissolution; low-temperature thermomechanical treatment (LTTT), post-deformation edgy (PDE), and recrystallization. The microstructure of B2-austenite, which is formed in the process of incoordination, was also investigated, including in the conditions of isothermic burning of the original recrystallized structure.

ИССЛЕДОВАНИЕ МЕХАНИЗМОВ УПРОЧНЕНИЯ В СПЛАВЕ СИСТЕМЫ Al–Mg–Si–Cu В ПРОЦЕССЕ НИЗКОТЕМПЕРАТУРНОЙ ТЕРМОМЕХАНИЧЕСКОЙ ОБРАБОТКИ

In this work, the samples of alloy 6013 of the Al–Mg–Si–Cu system after low-temperature thermomechanical treatment (LTMT) according to the scheme: quenching → aging → deformation → aging were studied by dark-field methods of transmission electron microscopy. The analysis of the structure of precipitates of hardening phases and dislocation structure in the volume of grains after different degrees of cold hardening of LHMT is carried out. It is shown that LHMT affects the formation of the structure of hardening precipitates, contributing to both heterogeneous nucleation at dislocations and homogeneous nucleation in the bulk of grains, due to the intensification of diffusion processes.

Effect of Thermomechanical Treatment Parameters on Structure, Phase Composition and Mechanical Properties of Ti-3Al-5Mo-4.5V Titanium Alloy

The structure, phase composition and mechanical properties of (α+β) — titanium alloy solution treated at 850 °C, cold-rolled at the reduction ratio in the range of 0… 45%, followed by ageing at 450, 500, 550 °C for 0.5, 1.5, 3 hours was studied using XRD, microindentation and tensile testing. The influence of strain level at cold rolling and time-temperature parameters of ageing on the formation of structure and phase composition of solution treated and water quenched Ti-3Al-5Mo-4.5V alloy was investigated and discussed in terms of tensile properties and microhardness. The parameters of low temperature thermomechanical treatment (LTMT) of the (α+β) — alloy were proposed to obtain a high-strength state.

To the definition of stress-strain state of springs during recovering when hardened

The results of experimental work on the recovering of springs are presented. A methodology is proposed for theoretical determination of the parameters of springs during recovering using low-temperature thermomechanical treatment (tension with heating), shot peening and contact hardening.

Transient nucleate boiling process as a basis for designing of high efficiency innovative technologies

It is shown in the paper that during quenching metal samples in liquid media, such as water salt solutions, nanofluids and low concentration of water polymer solutions, the main attention should be paid to critical heat flux densities and convective heat transfer coefficients which should be combined with the self – regulated thermal process (SRTP). No one can affect duration or intensity of cooling during transient nucleate boiling by adding any additives or nanoparticles to a liquid because nucleate boiling process by itself is very intensive due to acting thousands of vapor bubbles with the high frequency and intense liquid vaporization. Only two options are left which can affect cardinally cooling process via maximizing critical heat flux densities and increasing convective heat transfer coefficients. When any film boiling during quenching in still water or water solutions is completely absent, process of cooling is intensive due to intense acting thousands of vapor bubbles during nucleate boiling. These conclusions are very important for investigators who are focused on development intensive low temperature thermo-mechanical and cryogenic treatment of materials. The ideas discussed in this paper can be useful for engineers, scientists and students.

Theoretical background of method of springs recovering

During their life span, high load compression springs lose their resistance and their operating load is reduced. In the course of time, relaxation is observed. The method of compression springs recovering from hardened wire by low-temperature thermo-mechanical treatment and contact predeformation is presented. The results of experimental tests on recovering load bearing characteristics of the lot of springs according to a new technology are reported. Load bearing characteristics of recovered springs are theoretically specified. The difference between theoretical and experimental researches does not exceed 3.2 %. This method of springs parameterization is recommended to be used in the development of technology of springs recovering with the use of low-temperature thermo-mechanical hardening and contact predeformation.

Ferrite, martensite and supercritical iron: A coherent elastochemical theory of stress-induced carbon ordering in steel

A mean-field model based on the elasticity theory of point defects has been developed to investigate the role of uniform stress fields on the long-range ordering of carbon atoms in bct-iron. From an analysis of the thermodynamic equilibria, composition – temperature – stress state diagrams are derived. We demonstrate that ferrite, martensite and supercritical iron are various instances of the same bct-iron phase region. A coherent mapping of the phase transitions is drawn, identifying (i) continuous transitions such as ferrite ordering, martensite enhanced ordering and ferrite – martensite transformation, and (ii) discontinuous transitions such as temperature-induced martensite and stress-induced martensite. Our analysis is supported by rigid-lattice Monte Carlo simulations. Recently published experimental results on highly-drawn perlitic wires are re-interpreted in terms of supercritical iron, rather than strain-induced martensite. Novel low-temperature thermomechanical treatments of supersaturated ferrite are suggested, for improved nanostructure design of martensitic steels.

Microstructural and Fractographic Analysis of Plastically Deformed Al-Zn-Mg Alloy Subjected to Combined High-Cycle Bending-Torsion Fatigue

The present manuscript aims to determine the effect of various deformation levels during low-temperature thermo-mechanical treatment (LTTT) on the microstructure and fatigue strength of Zr-microalloyed Al-Zn-Mg alloy. The fatigue strength of the alloy was studied in high-cycle bending-torsion tests. The determination of the influence of plastic deformation level during LTTT on the microstructure was based on transmission electron microscopy (TEM) observations. The analysis of fracture after the fatigue tests was performed based on fractographic images using scanning electron microscopy (SEM). It was found that the major factor affecting the microstructure of the alloy, and determining the nature of fatigue fracture, is the strain level applied during LTTT.

Microstructure, Mechanical Properties, and Corrosion Resistance of Thermomechanically Processed AlZn6Mg0.8Zr Alloy.

The paper presents results of the investigations on the effect of low-temperature thermomechanical treatment (LTTT) on the microstructure of AlZn6Mg0.8Zr alloy (7000 series) and its mechanical properties as well as electrochemical and stress corrosion resistance. For comparison of the LTTT effect, the alloy was subjected to conventional precipitation hardening. Comparative studies were conducted in the fields of metallographic examinations and static tensile tests. It was found that mechanical properties after the LTTT were better in comparison to after conventional heat treatment (CHT). The tested alloy after low-temperature thermomechanical treatment with increasing plastic deformation shows decreased electrochemical corrosion resistance during potentiodynamic tests. The alloy after low-temperature thermomechanical treatment with deformation degree in the range of 10 to 30% is characterized by a high resistance to stress corrosion specified by the level of PSCC indices.

Microstructure and Fatigue Properties of AlZn6Mg0.8Zr Alloy Subjected to Low-Temperature Thermomechanical Processing

The paper presents results of the investigations on the effect of the low-temperature thermomechanical treatment on the microstructure of AlZn6Mg0.8Zr alloy (7003 alloy) and the relationships between microstructure and fatigue properties and fractography of fractured samples. Fatigue life has been determined in a mechanical test at a simple state of loading under conditions of bending as well as torsion. The development of fatigue cracking has been described based on fractography investigations of the fractured samples making use of a scanning electron microscope (SEM). It was found that the factors determining the fatigue strength of the tested alloy are the microstructure as well as the type and size of the cyclic stresses. These factors determine the fractography of fatigue samples.

Effect of deformation induced precipitation on dynamic aging process and improvement of mechanical/corrosion properties AA7055 aluminum alloy

Abstract An improved low-temperature thermo-mechanical treatment (LTMT based on DYA: dynamic aging) aiming to discover the action of deformation-induced precipitation to microstructures and corrosion resistance was proposed that included a conventional T6 peak aging (120 °C/24 h) and a subsequent dynamic aging. Its influence on mechanical properties, electrochemical behavior and intergranular/exfoliation corrosion resistance of 7055 Al alloy were investigated by tensile testing, polarization curve, intergranular/exfoliation corrosion testing combined with optical microscope and transmission electron microscope observation. It shows that after the conventional T6 peak aging, the matrix precipitates were fine and dispersive while the grain boundary precipitates were coarse and continuously distributed. With the introduction of dynamic aging, the grain boundary precipitates become discontinuously distributed gradually without much coarsening of matrix precipitates. As a result, the LTMT process can improve intergranular/exfoliation corrosion resistances without sacrificing strength that could be attributed to the discontinuous grain boundary precipitates and fine dispersed matrix precipitates, respectively. The improved corrosion resistance was also confirmed by polarization curves and conductivity test. It indicates that the present LTMT process may be a good alternative to produce heat-treatable Al alloy sheets with good strength and corrosion resistance.

Dependence of the functional characteristics of thermomechanically processed titanium nickelide on the size of the structural elements of austenite

Effect of the size of the elements of the mixed structure of B2 austenite, which consists of nanosized grains and subgrains of a polygonized substructure, on the functional properties of the Ti–50.7 at % Ni alloy preliminarily subjected to a low-temperature thermomechanical treatment (LTMT) and post-deformation annealing (PDA), has been investigated. The generation of the shape-memory effect (SME) and reversible two-way SME (TWSME) was performed using bending deformation. A maximum (for the Ti–Ni alloys) value of the recovery strain er = 15.5 ± 0.5% has been obtained after annealing at 600°C for 1 h (recrystallized structure) and after LTMT + PDA at 430°C for 10 h (mixed nanocrystalline and nanosubgrain structure). The behavior of the parameters of the SME and TWSME in different structural states has been considered. A comparative study of the effect of the temperature and time of holding at a temperature upon the PDA on the formation of the microstructure and submicrostructure of the B2 austenite has been performed.