Papers by Keyword: Thermomechanical Treatment (TMT)

Abstract: The article represents results of influence of different severe plastic deformation (SPD) techniques on TiNi alloys. It is demonstrated that strength and shape memory effect (SME) of TiNi can be significantly enhanced due to formation of ultrafine-grained (UFG) and nanocrystalline (NC) structures by SPD. Influence of equal channel angular pressing (ECAP), high pressure torsion (HPT), multi-step SPD deformations (ECAP plus cold rolling) on structure, mechanical and functional properties of TiNi alloys is considered. There are represented first results of influence of equal channel angular pressure-Conform (ECAP-C) on TiNi alloys, which is a perspective technology for industrial fabrication of UFG metals and alloys.

Abstract: Mechanical behavior of structural nitrogen-containing steels with various structures and compositions, including the same steels with different summary C+N content and C/N ratio were studied using pressing tests in a wide temperature range, tensile tests, impact bending tests, hardness measurements and shock-wave loading resistance. The tempering and aging under load processes after quenching or thermomechanical treatment with various regimes have been investigated using optical and electron microscopies, X-ray diffraction analysis, calorimetric and dilatometric analyses. Hot strain resistance of the austenite is determined essentially by the steel composition, while the final structure and mechanical properties of hot-deformed austenite are determine mainly by hot deformation conditions. The higher the nitrogen content and C/N ratio, the higher hot strain resistance was and earlier the softening processes start, especially recrystallization process. The nitrogen microalloying of low-alloyed structural steels changes kinetics of the martensite tempering. Application of the high temperature thermomechanical treatment or combined thermomechanical strengthening with following tempering under load allows the use of these steels in a high-strength state after low-temperature tempering.

Abstract: Binary TiNi alloys containing 50.0 at.% Ni and 50.7 at.% Ni were tested under bending. Shape memory effect (SME) and two-way SME (TWSME) parameters were measured. Low-temperature thermomechanical treatment (LTMT) by drawing with true strain e ≈ 0.6 and post-deformation annealing (PDA) was performed in the temperature range 300600С for 0.3  1 hr permitted obtaining different types of initial austenite structure. The wires with 0.3 and 0.45 mm in diameter were studied. The curves of element distribution through the surface layer were graphed. The SME training procedure was carried out in bending with the constrained strain 0.7 and 8 %. Thickness, structure and element composition of the oxide film strongly depend on PDA regimes. In all cases titanium quantity in the boundary layer correlates with the oxygen quantity, diffused from the surface. Evidently, it is caused by their chemical reaction and oxide formation. Nickel atoms remain in uncombined state, and their migration is directed from the surface deep into the sample. LTMT permits decreasing the film thickness more than by 10 times. Surface state strongly affects all studied parameters: characteristic temperatures, recovery strain and TWSME value. Increasing of wire diameter from 0.3 to 0.45 mm leads to significant changes of studied parameters. Forecasting of said influence is difficult because of its ambiguity. Therefore, the oxide film must be eliminated by etching in order to obtain the reliable results

Abstract: The Ti-21.8Nb-6Zr and Ti-19.7Nb-5.8Ta (at.%) shape memory alloys are thermomechanically treated by cold drawing and post-deformation annealing at 550-600°C forming a nanosubgrained structure in the β-phase. Cyclic mechanical testing using a “loading-unloading” mode with 2% tensile strain in each half-cycle reveals the non-perfect superelastic behavior of both alloys during the very first cycles of testing, which becomes perfect during further mechanocycling. The Young’s modulus of thermomechanically-treated alloys is low (about 45 GPa), and it decreases during mechanocycling (n=10 cycles) down to 25-35 GPa, approaching the Young’s modulus of cortical bone tissues. The Young’s modulus obtained in the 10th cycle is stable or changes only slightly during a further 40-day pause at room temperature and then during repeated mechanocycling. The residual strain per cycle, the transformation yield stress and the mechanical hysteresis decrease during mechanocycling. Subsequent to a 40-day pause at room temperature, they restore their initial values. Repeated mechanocycling is accompanied by a repeated decrease of these parameters.

Abstract: Superplastic behavior of certain metals and alloys having very fine grains, very large tensile elongations are obtained within certain temperature ranges at low strain rates. These alloys can be formed into complex shapes by superplastic forming, a process that employs common metalworking techniques. This paper aims to study the formability characteristic for aluminium material (Al 7075) by considering variable parameters such as sheet thickness of 1.75 and 1.5 mm and the temperature of 520°C and 530°C.

Abstract: Using dilatometric technique, it is the purpose of present research to quantify the kinetics of austenite decomposition during cooling after isothermal hot compression. To reach this aim, based on lattice parameters and thermal expansion coefficients of different phases, a new model was proposed. In this model, the contributions of both isotropic and non-isotropic dilatations were considered. The model was applied to 22MnB5 steel to predict formation of bainite and martensite. Furthermore, the model was developed in order to determine the final fraction of deformation induced ferrite (DIF).

Abstract: The thermomechanical treatment of a 2A12 aluminum alloy was researched and the influence of pre-ageing on microstructure and hardness was analyzed emphatically. The results reveal that the hardness of specimen increases when they are pre-aged, the hardness value rises at first and then decreases, reaching the maxmum value when pre-aged at 180°C×30min . After plastically deformed at 450°C, the hardness keeps on increasing, and the grains are equiaxed polygon structure. After all the workpieces are aged in the end, the small particles of the second phase precipitates completely and disperses within the original phase matrix, the particles interact with dislocations in upper state that formed during plastic deformation and lead to a great increase in hardness compared with as-received. the best pre-aging parameter is 180°C×30min.

Abstract: The thermomechanical treatment of a 2A12 aluminum alloy was researched and the influence of deformation ratio above recrystallization temperature on microstructure and hardness was analyzed emphatically. The result reveals that when the deformation ratio is low at first, the hardness of specimen decreased because the effect of recrystallization softening overwhelms the effect of deformation strengthening. However, hardness increased with increasing deformation ratio. In addition, the microstructure became more homogeneous and grains were refined obviously by metallography microscope observation with increasing deformation ratio. The second phase precipitates dispersedly to strengthen the alloy.

Abstract: Fe-8Mn-6Si-13Cr-6Ni-12Co shape memory alloys are characterized by a good corrosion resistance and a modest shape memory effect. Experimental tests of the corrosion resistance of Fe-8Mn-6Si-13Cr-6Ni-12Co have been studied and compared with the Fe-32Mn-6Si alloy using weight loss, free corrosion and polarization resistance tests. The shape memory effect measurement of the Fe-8Mn-6Si-13Cr-6Ni-12Co and Fe-32Mn-6Si alloys has been also tested after 5% of deformation. The results of corrosion experimental tests show that the Fe-8Mn-6Si-13Cr-6Ni-12Co SMA is not very active in the Na2SO4 solution at pH4 at room temperature. In addition, its corrosion resistance is better than the Fe-32Mn-6Si and almost similar to the 316L stainless steel. To improve the shape memory effect of the Fe-8Mn-6Si-13Cr-6Ni-12Co, a thermomechanical treatment has been applied by 8% prestrain in tensile and followed by heat treatment at 1320K for 1 hour. The results show an improvement in the shape memory effect after 5% of deformation in tensile test.

Abstract: Aluminum alloys are commonly used as a material for heat exchangers due to their higher thermal conductivity and specific strength among various metallic materials. Twin roll strip casting process is considered to produce the high quality and low manufacturing cost aluminum alloy fin stock for automobile heat exchangers. Thermomechamical treatment has carried out to obtain optimum processes for initial cold rolling, intermediate annealing and final cold rolling, which can meet the requirements for high strength and high thermal conductivity after brazing heat treatment. Additionally the effect of copper element variation was considered to determine the optimum content of copper element in Al-Zn-Mn-Si-Fe-Cu based alloys produced by twin roll strip casting process. Mechanical properties and thermal condutivity have been evaluated before and after simulated brazing process. The nuclei of recrystallization might be formed along shear deformation bands during initial cold rolling and should be grown during intermediate annealing to enhance the permeation of molten brazings for the following brazing process. Final cold rolling has allowed strain hardening and controlling of sagging amount as fin stock materials of heat exchanger. In the present study the suitable thermomechnical treatment and optimum copper content was suggested to balance the properties of strength, thermal conductivity, brazing behaviour, corrosion resistance and sagging resistance in Al-Zn-Mn-Si-Fe-Cu based alloys produced by twin roll strip casting process.