Thermomechanical Processing Methods Research Articles

Low temperature texture development in Nd2Fe14B/α-Fe nanocomposite magnets via equal channel angular pressing

While suitable texture has been developed in Nd2Fe14B/α-Fe nanocomposites via thermomechanical processing methods such as die upsetting by incorporating low melting point eutectic Nd-Cu additives, significant grain coarsening occurs during this process due to the high temperature and long timescales involved, resulting in a loss of exchange coupling. Equal channel angular pressing (ECAP) is a severe plastic deformation technique which has been successfully used to produce a suitable texture in single-phase Nd2Fe14B at temperatures on the order of 500°C while preserving grain sizes on the order of 20-30nm. We investigate the development of texture in a commercial Nd2Fe14B/α-Fe nanocomposite alloy with added Nd90Cu10 produced via ECAP and then characterise it using texture x-ray diffraction and magnetic measurements. It is found that initial texture can be developed in this nanocomposite system at T = 520°C via ECAP. The average grain size of Nd2Fe14B as measured via X-ray diffraction after ECAP remains below 50nm with a developed texture. The effect of varying the amount of Nd90Cu10 additive is also investigated. It is found that with decreasing Nd90Cu10, the degree of texture is reduced while the volume fraction of α-Fe increases. This work demonstrates the development of texture in nanocomposite Nd2Fe14B/α-Fe with Nd-Cu additives whilst maintaining a grain size of approximately 50nm.

Enhanced Tensile Properties of Mg Sheets by a Unique Thermomechanical Processing Method

A unique thermomechanical processing method combining fast rolling with large thickness reduction and short-time annealing (FR-STA) was developed to produce lower-cost magnesium (Mg) sheets with improved tensile properties. Sheets of Mg-3Al-1Zn-0.3Mn (wt pct) and Mg-1Zn-0.2Nd-0.2Zr (wt pct) were produced by FR incorporating large thickness reduction, exhibiting enhanced strength and improved ductility relative to slow rolling with small thickness reduction after STA.

МЕХАНИЗМЫ ОБРАЗОВАНИЯ ЗАРОДЫШЕЙ РЕКРИСТАЛЛИЗАЦИИ В МЕТАЛЛАХ ПРИ ТЕРМОМЕХАНИЧЕСКОЙ ОБРАБОТКЕ

In the last 15-20 years, mathematical models are the most important “tool” in the design and creation of technology of thermomechanical processing of metals and alloys. This is the result of appearance of new class of models based on physical theories. Single-level macrophenomenological models based on macro experiments are replaced by crystals plasticity. Founders of physical theories of plasticity are G.I.Taylor, G.H.Bishop , R.Hill, T.G.Lin. Many others researchers from the Soviet Union and Russia made a significant contribution to the development of this direction: R.Z. Valiev, Y.D. Vishnyakov, S.D. Volkov, O.A. Kaybishev, V.A. Likhachev, V.E. Panin, V.V. Rybin, T.D. Shermergor et al. Physically based approach requires deep understanding of the internal mechanisms and processes that accompany thermomechanical effects caused by inelastic deformation at different scale levels. The important one for the microstructure formation and mechanical properties of finished products obtained by thermomechanical processing methods is the process of recrystallization. At that point in this article provides an review of the existing theories of recrystallization nucleation mechanisms. Basic physical mechanisms of nucleation recrystallized grains are classified: 1) the mechanism, based on the classical theory of fluctuations, proposed by J.E.Burke and D.Turnbull; 2) R.W. Cahn’s mechanism of nucleation and growth subgrains polycrystal, formed as a result of the process polygonization; 3) P.A. Beck’s and P.R. Sperry’s mechanism grain boundary migration, initially present in the polycrystal - strain induced boundary migration (SIBM); 4) the mechanism of nucleation and growth of new grains as a result of coalescence subgrains (H.Hu, J.C.M.Li, H.Fujita ). Analysis of existing models describing the inelastic deformation at high temperatures demonstrates the need for consideration and inclusion in the models description of physical mechanisms of high-temperature processes accompanying plastic deformation.

Conversion of Lignin Precursors to Carbon Fibers with Nanoscale Graphitic Domains

Lignin is one of the most abundant and inexpensive natural biopolymers. It can be efficiently converted to low cost carbon fiber, monolithic structures, or powders that could be used directly in the production of anodes for lithium-ion batteries. In this work, we report thermomechanical processing methods relevant for the conversion of lignin precursors into carbon fiber-based anode materials, the impact of lignin precursor modification on melt processing, and the microstructure of the final carbon material. Modification of softwood lignin produced functionalities and rheological properties that more closely resemble hardwood lignin thereby enabling the melt processing of softwood lignin in oxidative atmospheres (air). The conversion process encompasses melt spinning of the lignin precursor, oxidative stabilization, and a low temperature carbonization step in a nitrogen/hydrogen atmosphere. We determined resistivities of individual carbon fiber samples and characterized the microstructure by scanning electron microscopy. Neutron diffraction reveals nanoscale graphitic domains embedded in an amorphous carbon matrix. These unique structural characteristics make biomass-derived carbon fibers a suitable material for energy storage applications with enhanced electrochemical performance.

Effect of Thermomechanical Processing on the Microstructure and Mechanical Properties of Nb-Ti-V Microalloyed Steel

The paper presents the results of thermomechanical treatment via forging on the microstructure and mechanical properties of newly obtained microalloyed steel containing 0.28% C, 1.41% Mn, 0.027% Nb, 0.028% Ti, and 0.019% V. The investigated steel is assigned to the production of forged elements for the automotive industry. Conditions of forging using the thermomechanical processing method were developed based on plastometric tests. Continuous and double-hit compression tests were conducted using the Gleeble 3800 thermomechanical simulator. The samples were investigated in a temperature range from 900 to 1100 °C and a strain rate of 1 and 10 s−1. To determine the recrystallization kinetics of plastically deformed austenite, discontinuous compression tests of samples using the applied deformation were conducted in a temperature range from 900 to 1100 °C with isothermal holding of the specimens between successive deformations for 2-100 s. Observations of the microstructures of thin foils were conducted using a TITAN80-300 FEI transmission electron microscope. The applied thermomechanical treatment allows to obtain a fine-grained microstructure of the austenite during hot-working and production of forged parts. These acquire advantageous mechanical properties and guaranteed crack resistance after controlled cooling from the end plastic deformation temperature and successive tempering. Forgings produced using the thermomechanical treatment method, consecutively subjected to tempering in a temperature range from 550 to 650 °C, reveal values of YS0.2 which equal from 994 to 892 MPa, UTS from 1084 to 958 MPa, KV from 69 to 109 J, KV−40 from 55 to 83 J, and a hardness ranging from 360 to 300 HBW.

Advances in Development of Titanium Alloys for Spinal Fixation Applications-Titanium Alloys with High Fatigue Strength and Low Springback for Spinal Fixation Applications-

The fatigue strength of the newly developed β-type titanium alloy Ti-29Nb-13Ta-4.6Zr (referred to as TNTZ) is important for its applications in spinal fixation rods. Enhancements in the fatigue strength of TNTZ upon solution treatmnet and thermo-mechanical processing (aging after severe cold-caliber rolling or sever cold-swaging) have been investigated. Aging the rods at 723 K for 259.2 ks after cold-swaging is shown to be the optimal thermo-mechanical processing method for the TNTZ rod , yeilding a high 0.2% proof stress of about 1200 MPa, high elongation of 18%, and high fatigue strength of 950 MPa. High springback is undesirable for application of TNTZ spinal-fixation rods. To reduce springback, a High Youngs modulus is required; on the other hand, a low Youngs modulus is beneficial to reduce bone absorption and obtain good bone remodeling. To achieve a balance between these two factors, TNTZ has been modified by adding Cr to obtain an alloy with a high Youngs modulus at the deformed region and a low Youngs modulus throughout the undeformed region. TNTZ-8Ti-2Cr and TNTZ-16Ti-4Cr are novel alloys whose β phase is more stable than that of TNTZ. Only the β phase is present in these alloys, which exhibit relatively low Youngs moduli of <65 GPa after solution treatment, and higher Youngs moduli after cold rolling, owing to a deformation-induced ω-phase transformation. These modified TNTZ alloys show significantly less springback than the original TNTZ, based on tensile and loadingunloading bending tests.

Enhancement of the strain hardening ability in ultrafine grained Mg alloys with high strength

Two thermo-mechanical processing methods involving severe plastic deformation (SPD) are proposed for the development of a novel microstructure having nanosized Mg17Al12 (β) particles distributed within the interiors of ultrafine grains in Mg–Al based alloys. The first method involves deformation-induced break-up of the β phase during SPD, while the second method involves low temperature dynamic precipitation during SPD followed by low temperature aging. The resulting processed materials exhibited very high yield strengths of over 380 MPa and distinct strain hardening properties.

Difference of Microstructure and Fatigue Properties between Forged and Rolled Ti-6Al-4V

In the Present Study, the Effects of the Microstructural Morphologies of a Ti-6Al-4V (Ti-64) Alloy on its Fatigue Behavior Were Investigated. Ti-64 Bars Were Subjected to Two Different Thermo-Mechanical Processing Methods. The First Sample, Referred to as Material-A, Had a Forged Microstructure with the Average Primary α Volume Fraction of 44%. The Second One, Referred to as Material-B, Had a Hot-Rolled Microstructure with the Average Primary α Volume Fraction of 43%. Fatigue Tests Were Performed on each Sample to Obtain S-N Curves. The Microstructure of each Sample Was Observed Using an Optical Microscopy in Order to Measure the Grain Sizes of the Primary α and Secondary α Phases. The Results of the Fatigue Tests Indicated that Material-B Demonstrates Better Fatigue Strength than Material-A. The Microstructure of the Longitudinal Section of each Material Was Also Observed to Analyze the Results of the Fatigue Tests. The Measured Diameters and Volume Fractions of the Primary α Phases of the Two Types of Materials Are Similar. On the other Hand, the Secondary α Width of each Material Is Different. It Is Found that Fatigue Strength Is Related to the Width of the Secondary α Phase.

Relation between microstructures of martensite and prior austenite in 12 wt% Cr ferritic steel

Tempered martensitic 9–12 wt% Cr ferritic steels are used as heat resistant materials in power plant, where service under conditions of high temperature and pressure for several decades is required, and an adequate resistance to creep is one of the key requirements. In this type of steels, failure has been found to occur preferentially at prior austenite grain boundaries if the prior austenite grains are coarse. It appears that the prior austenite grain boundaries can act as a site of especial weakness in the tempered martensitic microstructure. It would therefore be useful to investigate whether the properties of prior austenite grain boundaries could be modified by some appropriate thermomechanical processing method. One approach to this is to attempt to increase the fraction of annealing twins in the austenite phase and to investigate whether this has an effect on the properties of the martensite after transformation and tempering. In this study, thermomechanical treatments involving hot-rolling have been applied and the fraction of austenite twins produced determined using electron backscatter diffraction analysis. The treatment giving the highest fraction of austenite twins was identified and the effect of the increase in twin fraction on the characteristics of the martensite was investigated. It was found that the fraction of coincidence site lattice boundaries in martensite along prior austenite grain boundaries increased with increasing fraction of prior austenite twin boundaries.

A thermomechanical process for grain refinement in precipitation hardening AA6xxx aluminum alloys

A new thermomechanical processing method, consisting of conventional rolling and a continuous non-isothermal annealing process, has been designed to achieve substantial grain refinement through cost-efficient routes in heat treatable aluminum alloys. The method has been implemented on an alloy of interest for automotive applications and a highly-stable fine-grained microstructure with a very weak texture has been achieved.

Development of thermomechanical processing method to enhance twinning in commercially pure Zr

Zirconium is widely used as fuel cladding material in nuclear power reactors. Here, a thermomechanical processing technique has been developed and implemented to improve the mechanical properties of commercially pure Zr-702. In the process the twin density increased to 45 areal%. The twins were identified to be mostly of { 1 0 1 ¯ 2 } 〈 1 0 1 ¯ 1 ¯ 〉 tensile twins or { 1 0 1 ¯ 1 } 〈 1 0 1 ¯ 2 〉 compression twins. The significant increase in hardness was analyzed for its contributing factors and was attributed to the microstructural refinement caused by twin boundaries.

Thermo-mechanical processing of high strength β-titanium alloys and effects on microstructure and properties

This paper summarizes the thermo-mechanical processing methods for high strength β-titanium alloys. The processing methods are evaluated in terms of their effectiveness in reducing the negative effects of continuous α-layers along β-grain boundaries which tend to form in high strength β-titanium alloys, but also in terms of applicability in practice. Typical resulting microstructures and mechanical properties are presented and discussed for the alloys β-Cez and Ti-6246.

Influence of Processing Method on the Grain Boundary Character Distribution and Network Connectivity

ABSTRACTThere exists a growing body of literature that correlates the fraction of “special” boundaries in a microstructure, as described by the Coincident Site Lattice Model, to properties such as corrosion resistance, intergranular stress corrosion cracking, creep, etc. Several studies suggest that the grain boundary character distribution (GBCD), which is defined in terms of the relative fractions of “special” and “random” grain boundaries, can be manipulated through thermomechanical processing. This investigation evaluates the influence of specific thermomechanical processing methods on the resulting GBCD in FCC materials such as oxygenfree electronic (ofe) copper and Inconel 600. We also demonstrate that the primary effect of thermomechanical processing is to reduce or break the connectivity of the random grain boundary network. Samples of ofe Cu were subjected to a minimum of three different deformation paths to evaluate the influence of deformation path on the resulting GBCD. These include: rolling to 82% reduction in thickness, compression to 82% strain, repeated compression to 20% strain followed by annealing. In addition, the influence of annealing temperature was probed by applying, for each of the processes, three different annealing temperatures of 400, 560, and 800°C. The observations obtained from automated electron backscatter diffraction (EBSD) characterization of the microstructure are discussed in terms of deformation path, annealing temperature, and processing method. Results are compared to previous reports on strainannealed ofe Cu and sequential processed Inconel 600. These results demonstrate that among the processes considered, sequential processing is the most effective method to disrupt the random grain boundary network and improve the GBCD.

Reinforcement redistribution in AlSiC composites under cyclic deformations

Aluminum-silicon carbide particle composites were subjected to large strain, cyclic deformations at elevated temperatures. The initial microstructure of the composites consisted of distinct elongated clusters of reinforcements oriented in the extrusion direction. After repeated cycles where the material was deformed and subsequently returned to its original shape at elevated temperatures, the distribution of particles throughout the matrix generally became more uniform, although in some cases small round clusters of reinforcements were observed. Finite element calculations suggested that under cyclic deformations, a microstructure with either a uniform distribution of particles or tight clusters of particles with a cluster aspect ratio of unity minimizes the strain energy density of the system. The results may provide insight for the development of thermo-mechanical processing methods to produce metal-matrix composites with improved mechanical properties.

Pitting and Crevice Corrosion of Iron Aluminides in a Mild Acid-Chloride Solution

Abstract Aqueous corrosion (pitting and crevice) characteristics of iron aluminides in a mild acid-chloride (Cl−) solution (200 ppm Cl− [5.5 × 10−3 M sodium chloride {NaCl}] at pH 4 [6.25 × 10−5 M sulfuric acid {H2SO4}]) were studied as functions of alloying additions and thermomechanical processing methods. Electrochemical results indicated beneficial effects of chromium (Cr) and molybdenum (Mo). Immersion tests confirmed the electrochemical results and revealed the time dependence of localized corrosion behavior. The crevice corrosion behavior of iron aluminides was investigated using electrochemical and immersion crevice tests. The difference between crevice and pitting susceptibilities was minimal for the Mo-containing iron aluminide but significant for the iron aluminides without Mo. X-ray photoelectron spectroscopy (XPS) of surfaces passivated in the mild acid-chloride solution revealed the passive films of iron aluminides were composed mainly of oxidized aluminum (Al2O3) that coexisted with chromiu...

Application of the divorced eutectoid transformation to the development of fine-grained, spheroidized structures in ultrahigh carbon steels

This paper describes a two-step thermomechanical processing method for obtaining fully spheroidized structure in UHC steels. The first step is a hot a warm working (HWW) procedure to obtain spheroidized and fine proeutectoid carbides. The second step incorporates a D ivorced E utectoid T ransformation ( DET). This treatment is called DET and includes a well-defined soaking time and soaking temperature fro austenitizing followed by air cooling. When deformation is imparted to the UHC steel, after austenitizing and during air cooling, it is called DETWAD ( D ivorced E utectoid T ransformation W ith A ssociated D eforamation).