Dynamic restoration and microstructural evolution during hot deformation of a P/M Al6063 alloy

Asgharzadeh, H.,Simchi, A.,Kim, H.S. Elsevier 2012 Materials science & engineering. properties, micro Vol.542 No.-

<P><B>Highlights</B></P><P>► Restoration mechanisms in a P/M Al6063 alloy were studied. ► Microstrucural changes during hot deformation were analyzed by TEM and EBSD. ► The activation energy of the hot deformation was determined. ► Correlations between Zener–Hollomon parameters and the size of subgrains and recrystallized grains were established. ► DRV prevailingly occurred at high <I>Z</I> values DRX was the dominant mechanism at the low <I>Z</I> values.</P> <P><B>Abstract</B></P><P>Hot deformation behavior of Al6063 alloy produced by direct powder extrusion was studied by means of uniaxial compression test in the temperature range between 300 and 450°C and strain rate range between 0.01 and 1s<SUP>−1</SUP>. Electron backscatter diffraction (EBSD) technique and transmission electron microscopy (TEM) were utilized to study the microstructure of the material before and after the hot deformation. The microstructure of the extruded alloy consisted of elongated grains within a subgrain structure and small grains free of low angle grain boundaries (LAGBs). An equiaxed duplex microstructure consisting of large substructured grains and fine grains separated by high angle grain boundaries (HAGBs) were observed after hot deformation. Evaluation of the hot-deformation activation energy using a hyperbolic sine law yielded a value of 198kJmol<SUP>−1</SUP>. The kinetic analysis and microstructural changes suggest that dynamic restoration mechanisms, i.e., dynamic recovery (DRV) and dynamic recrystallization (DRX), are operative to cause flow softening during hot compression. Correlations between Zener–Hollomon parameters and the size of subgrains and recrystallized grains were established. It was found that DRV prevailingly occurred at high <I>Z</I> values (<I>Z</I>>1.12×10<SUP>16</SUP>) while DRX was the dominant mechanism at the low <I>Z</I> values.</P>

Enhancement of thermoelectric properties in CuI-doped Bi₂Te_2.7Se_0.3 by hot-deformation

Cho, Hyunyong,Kim, Jin Hee,Back, Song Yi,Ahn, Kyunghan,Rhyee, Jong-Soo,Park, Su-Dong Elsevier 2018 Journal of alloys and compounds Vol.731 No.-

<P><B>Abstract</B></P> <P>We investigated the thermoelectric properties of CuI-doped Bi<SUB>2</SUB>Te<SUB>2.7</SUB>Se<SUB>0.3</SUB> compounds, fabricated by a repetitive hot-deformation process. The degree of texture of the materials was enhanced by the number of hot-deformation processings, and experimentally verified by X-ray diffraction measurements. Very interestingly, the enhanced texture induced by the hot-deformation produced a moderate reduction in electrical resistivity by improving electron mobility, while the Seebeck coefficient remained almost unchanged. The corresponding power factor at room temperature was significantly improved, from 3.1 mW m<SUP>−1</SUP> K<SUP>−2</SUP> to 4.1 mW m<SUP>−1</SUP> K<SUP>−2</SUP> after two successive hot-deformation processings, and consequently a high value of Z T of 1.07 was achieved at 423 K. This demonstrates that tuning the texture of Bi<SUB>2</SUB>Te<SUB>3</SUB> based materials by multiple hot-deformation processing steps can be an effective approach for increasing Z T .</P> <P><B>Highlights</B></P> <P> <UL> <LI> We investigated thermoelectric properties the CuI-doped Bi<SUB>2</SUB>Te<SUB>2.7</SUB>Se<SUB>0.3</SUB> by hot-deformation. </LI> <LI> Anisotropic texture is enhanced by hot deformation process. </LI> <LI> CuI doping and enhanced texture improves electron mobility. </LI> <LI> CuI doping is effective to increase power factor by increasing carrier density. </LI> <LI> It achieved high power factor and high ZT 1.07 at 423 K in n-type materials. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

분무주조 고속도공구강의 고온변형 거동에 관한 연구

하태권,정재영 한국소성∙가공학회 2018 소성가공 : 한국소성가공학회지 Vol.27 No.2

In the present study, the mechanical behavior of the spray-formed high speed steel was investigated employing the internal variable theory of inelastic deformation. Special attention was focused on the effect of the microstructure evolution during the hot working process, such as the distribution of carbides to provide a basic database for the production condition of high speed steels with excellent properties. The billets of high speed steel ASP30TM were fabricated by a spray forming, and the subsequently hot-rolled and heat-treated process to obtain uniformly distributed carbide structure. As noted the spray-formed high speed steel showed relatively coarser carbides than hot-rolled and heat-treated one with fine and uniformly distributed carbide structure. The step strain rate tests and high temperature tensile tests were carried out on both the spray-formed and the hot-rolled specimens, to elucidate their high temperature deformation behavior. The spray-formed high speed steel showed much higher flow stress and lower elongation than the hot-rolled and heat-treated steel. During the tensile test at 900oC, the interruption of the deformation for 100 seconds was conducted to reveal that the recovery was a main dynamic deformation mechanism of spray formed high speed steel. The internal variable theory of the inelastic deformation was used to analyze data from the step strain rate tests, revealing that the activation energies for hot deformation of as-spray-formed and hot-worked steels, which were 157.1 and 278.9 kJ/mol, and which were corresponding to the dislocation core and lattice diffusions of -Fe, respectively

Comparison of Hot-deformation Behavior and Magnetic Properties between Nd-Fe-B HDDR and MQU-F Powder

Jae-Gyeong Yoo,Hee-Ryoung Cha,Dong-Hwan Kim,Yang-Do Kim,Jung-Goo Lee 한국자기학회 2020 Journal of Magnetics Vol.25 No.1

Hot-deformation behavior of Nd-Fe-B HDDR powder was investigated in order to understand the difference in texturing mechanism of HDDR and MQU-F powder during hot-deformation. The composition of the HDDR powder was the same as that of MQU-F powder. However, the grain size of HDDR powder (~300 nm) was eight times larger than that of MQU-F powder (~40 nm). After being subjected to hot-pressing at 700 °C under 200 MPa in a vacuum, the grains of the magnets made from HDDR powder and MQU-F powder have globular and platelet shapes, respectively. The remanence was 12.2 kG after die-upsetting process at the strain of 0.5. And it increased up to 13 kG at the strain of 1.4 although the grains still maintained globular-like shapes. The remanence of 13 kG was almost the same as that obtained the magnet from MQU-F powder at the same deformation condition. These results indicate that the hot-deformation behavior is quite different between HDDR and MQU-F powder.

Al-10%Si-2%Cu-1%Mg-0.5%Mn 합금 주괴의 고온변형

張畯然,文仁淇 대한금속재료학회 2002 대한금속·재료학회지 Vol.40 No.8

Hot compression test was performed on Al-10%Si-2%Cu-1%Mg-0.5%Mn ingot in the temperature range 440∼500℃ and in the strain rate range 0.1∼10/sec in order to understand the hot deformation features and to explore the possibility of uniform distribution of eutectic Si particles. A typical stress-strain curve was well established where the stress increases sharply in the beginning of the deformation, and then decreases gradually in the strain rate of 0.5∼10/sec. Apparent activation energy of 65 ㎉/㏖ and stress exponent of 8.4 were evaluated from the peak stress. Deformation at low strain rate and high temperature is favorable in view of low flow stress and relatively uniform distribution of Si particles. Hard Si eutectics between primary Al grains begin to deform after soft Al grains are easily elongated in an early stage of the deformation. This type of deformation mode strongly demands at least 0.7 true strain to break the net-work structure of eutectic Si effectively. It is suggested that suitable heat treatment prior to the hot deformation be employed to ensure uniform distribution of fine Si particles.

Characterization of Hot Deformation Behavior and Processing Map of As‑Cast H13 Hot Work Die Steel

Yahui Han,Changsheng Li,Jinyi Ren,Chunlin Qiu,En LI,Shuaishuai Chen 대한금속·재료학회 2021 METALS AND MATERIALS International Vol.27 No.9

The hot-working behavior of as-cast H13 hot work die steel was investigated in the method of isothermal compression testinvolving the wide deformation temperatures of 900–1150 °C and strain rates of 0.01–10 s−1, with the true strain to 0.8, onthe MMS-200 thermo-mechanical simulator. Two characteristic parameters involving the critical strain for DRX initiation(c ) and the strain for peak stress (p ) were identified. The ratio of critical strain to peak strain ranged from 0.26 to 0.6, whichdecreased with the increase in temperature and decrease in strain rate. Processing maps were established using dynamicmaterial model at strains of 0.2, 0.4, 0.6, 0.8. The power dissipation maps were not significantly affected by the strain, whilethe instability maps were sensitivity with the strain when it was over 0.4. The area of instability domain at strain of 0.8 wasthe largest. The instable characteristics contained the mixed grain structure, adiabatic shear band, intense deformation inserious deformation area and brittle elemental segregation area. The chief effect on the power dissipation was the strain rate,the optimum hot working parameters at strain of 0.8 (910–985 °C, 1010–1150 °C and 0.01–0.05 s−1) were determined. Inthis filed, the original coarse as-cast grains were gradually refined by dynamic recrystallization mechanism and the DRXgrain numbers had a significant increase with the increase of power dissipation efficiency.

Effect of Low-Cycle Fatigue on the Hot Ductility of Plain Carbon Steel

( Sang Chul Seo ),( Hyun Jung Kim ),( Byung Ho Park ),( Kwang Suk Son ),( Sung Keun Lee ),( Sun Bae Kang ),( Dong Gyu Kim ) 대한금속재료학회 ( 구 대한금속학회 ) 2006 METALS AND MATERIALS International Vol.12 No.3

In a continuous casting steelmaking operation, the surface of a slab is under a condition that can be characterized as high-temperature, low-cycle fatigue in which the tensile and compressive stress is repeatedly developed. For this reason, for the evaluation of the hot ductility of a slab, considering the fatigue deformation is more feasible before a tensile or compressive test. In this study, the effects of low-cycle fatigue on the hot ductility of steels with a carbon content of 0.06-0.8 wt.% are investigated at various temperatures. For a carbon content of 0.06 %, there were no significant differences between the RA values from a simple tensile test and those from a tensile test after fatigue deformation. The tendency of ductility deterioration with fatigue deformation is evident in 0.1 %C steel, and is due to the deformation-induced ferrite film that forms around the prior austenite grain. Conversely, high carbon steel containing 0.8 %C did not show a recovery of hot ductility in a low temperature region, and the specimen on which the tensile was measured after fatigue showed a higher hot ductility in the low temperature region, which is thought to result from the pearlite refinement effects. As the results obtained in this work showed noticeable differences in the hot ductility of carbon steel through the test conditions, it is suggested that for more accurate data, fatigue deformation be adopted in which the temperature range in an unbending operation is determined in the steelmaking factory.

Effect of Hot-compaction Temperature on the Magnetic Properties of Anisotropic Nanocrystalline Magnets

W. Li,H. J. Wang,M. Lin,B. Lai,D. Li,W. Pan 한국자기학회 2011 Journal of Magnetics Vol.16 No.3

The effect of the hot-compaction temperature on the microstructure and magnetic properties of anisotropic nanocrystalline magnets was investigated. The hot-compaction temperature was found to impact both the magnetic properties and the microstructure of die-upset magnets. The remanence of the isotropic precursor increases slightly with the improved hot-compaction temperature, and the grains start to grow on the flake boundary at higher hot-compaction temperatures. After hot deformation, it was found that the change in the magnetic properties was the inverse of that observed with the hot-compaction temperature. Microstructural investigation showed that die-upset magnets inherit the microstructural characteristics of their precursor. For the die-upset magnets, hot pressed at low temperature, scarcely any abnormal grain growth on the flake boundary can be seen. For those hot pressed at higher temperatures, however, layers with large equiaxed grains could be observed, which accounted for the poor alignment during the hot deformation, and thus the poor magnetic properties.

Effect of Ce and La Substitution on the Microstructure and Magnetic Properties of Hot-deformed Nd-Fe-B Magnets

Ga-Yeong Kim,Hee-Ryoung Cha,Youn-Kyoung Baek,Young-Kuk Kim,Dong-Hwan Kim,Yang-Do Kim,Jung-Goo Lee 한국자기학회 2020 Journal of Magnetics Vol.25 No.2

Nd in hot-deformed Nd-Fe-B magnets was partially substituted by Ce and La for the purpose of reducing the materials cost and balancing the utilization of rare earth resources. Initial melt-spun ribbons with the nominal compositions of (Nd1-xMx)13.6FebalB5.6Ga0.6Co6.6 (x = 0, x = 0.2/M=Ce, x = 0.3/M=Ce, Ce+La, wt.%) were prepared by a single-roller melt-spinning method, and pulverized into powders. The powders were then hot-pressed at 973 K under 100 MPa, and deformed at 973 K until 75 % of height reduction was achieved. The magnetic properties of hot-deformed magnets were decreased with the substitution of Ce for Nd. In addition, simultaneous substitution of Ce and La for Nd resulted in much lower magnetic properties. This tendency was almost the same as initial melt-spun powders. The deterioration on magnetic properties by substation of Ce and La for Nd in hot-deformed Nd-Fe-B magnets could be attributed to not only inferior intrinsic properties of (Ce/La)₂Fe14B to Nd₂Fe14B phase but also the effect of Ce/La substitution on microstructure such as density and grain alignment during hot-deformation. Although the magnetic properties were deteriorated by substitution of Ce and La for Nd in hot-deformed Nd-Fe-B magnets, the cost performance was largely enhanced from 2.32 to 2.62 MGOe×kg/$ by 13 % when increasing the content of Ce substituted for Nd from 0 to 0.3 wt.%.

Zr–Nb Alloys and Its Hot Deformation Analysis Approaches

Kuldeep K. Saxena,Vivek Pancholi 대한금속·재료학회 2021 METALS AND MATERIALS International Vol.27 No.7

Zr and its alloys are considered as strategic materials for nuclear industry. In nuclear industries, zirconium and its alloysare mostly used to manufacture the tubes for reactors. The properties of the end component i.e. tube is fully dependent onthe material processing route and the fnal microstructure. Therefore, in the present review paper, brief description of zirconium and its alloys is provided, bringing in the efect of various phase stabilizers and the microstructures after processing. Additionally, the processing route of Zr and Zr–Nb alloys are explained in terms of primary and secondary processing. Inprimary processing, the production of usable shapes is obtained through ingot melting followed by secondary operationssuch hot rolling or forging. Further, optimization of mechanical properties can be done by controlling the microstructureusing various thermo-mechanical processes. The secondary processing such as cold working and/or annealing also helps tocontrol the fnal microstructure to large extent. The microstructure control is fully dependent on the dominant deformationmechanism during the hot deformation. The dominant deformation mechanism depends on relative ease with which followingprocesses occur; dislocation generation and glide, cross slip and climb and, difusion. The possible restoration mechanismsfor diferent Zr-alloys are discontinuous dynamic recrystallization (DDRX), continuous dynamic recrystallization (CDRX),rotational recrystallization (RRX), geometric dynamic recrystallization (GDX), dynamic recovery (DRV) accompanied bygrain growth (GG). There are various techniques which helps to understand the hot deformation behaviour of any material. These techniques are; analysis of stress–strain data, development of processing maps, development of constitute equation. Atthe end, present paper summarises the work related to processing map, fndings of processing map in terms of safe processing parameter, dominant deformation mechanism, role of activation energies during deformation of Zr–Nb alloys and theirdiferent phases (i.e. single α or β phase, or two phase α+β).