https://material.karlov.mff.cuni.cz/people/janecek/studenti/FyzMetNano/Estrin_AM61_2013_782.pdf

Extreme grain refinement by severe plastic deformation: A wealth of challenging science

Processes of severe plastic deformation (SPD) are defined as metal forming processes in which a very large plastic strain is imposed on a bulk process in order to make an ultra-fine grained metal The objective of the SPD processes for creating ultra-fine grained metal is to produce lightweight parts by using high strength metal for the safety and reliability of micro-parts and for environmental harmony In this keynote paper, the fabrication process of equal channel angular pressing (ECAP), accumulative roll-bonding (ARB), high pressure torsion (HPT), and others are introduced, and the properties of metals processed by the SPD processes are shown Moreover, the combined processes developed recently are also explained Finally, the applications of the ultra-fine grained (UFG) metals are discussed

Severe Plastic Deformation (SPD) Processes for Metals

DAMASK – The Düsseldorf Advanced Material Simulation Kit for modeling multi-physics crystal plasticity, thermal, and damage phenomena from the single crystal up to the component scale

Since 1990, TiNi and TiNiX (X=Cu, Pd, Hf) thin films have been made by sputtering The motivation for fabricating sputter-deposited TiNi-base shape memory alloy thin films originates from the great demand for the development of powerful microactuators which can drive micromachines, because actuation force and displacement are greatest in shape memory alloys amongst many actuator materials Stable shape memory effect and superelasticity, which are equivalent to those of bulk alloys, have been achieved in the sputter-deposited TiNi thin films Narrow transformation temperature hysteresis and high transformation temperatures were also achieved in TiNiCu and TiNi(Pd or Hf) thin films, respectively In the meantime, unique microstructures consisting of nonequilibrium nanoscale precipitates and nonequilibrium compositions in the matrix have been found in Ti-rich TiNi thin films which were fabricated from amorphous condition by annealing at a considerably low temperature Several micromachining processes have been proposed to fabricate some prototypes of microactuators utilizing TiNi thin films The present paper will review the recent development of the above mentioned topics relating to sputter-deposited TiNi-base shape memory alloy thin films

Martensitic transformation and shape memory behavior in sputter-deposited TiNi-base thin films

Recent developments in the study of hydrogen embrittlement: Hydrogen effect on dislocation nucleation

Effects of Mo and W on creep strength of low Cr heat resistant steels have been investigated from the stand points of the phase stability of precipitates during long-term exposure at the elevated temperatures. The study on 2.25%Cr-Mo-V-Nb (Mo-steel) and 2.25%Cr-W-V-Nb (W-steel) with a same amount of Mo-equivalent has shown that the substituting W for Mo retards the evolution of microstructure, and thus remarkably improves the creep rupture strength. TEM observations have suggested that the most important precipitates strongly affecting the stability of microstructure are M 6 C type carbide and MC type carbide in both Mo-steel and W-steel. It is found that the M 6 C carbide precipitates with concentration of Mo and W during long-term aging. Therefore, the amounts of solute Mo and W supersaturated in matrix have reduced to the thermal equilibrium levels with changing the substructure from bainite lath structure to sub-grain liked structure. The kinetics of Mo- and W-partitioning between matrix and M 6 C carbide has been successfully expressed by a Johnson-Mehl-Avrami type equation and estimated the activation energy for diffusion of solutes; 125 kJ/mol in Mo-steel and 219 kJ/mol in W-steel. Thus the diffusion rate of W is suggested to be definitely lower than that of Mo. Furthermore, Mo- and W-partitioning to MC carbides have also been confirmed to affect the coherency and the growth rate of MC carbides. TEM observations on the strain image have shown that the MC carbide in Mo-steel has already lost the coherency with matrix. On the other hand, the finer MC carbide in W-steel has kept the coherent relationship with matrix even after long-term aging. It is concluded that W-steel is superior to Mo-steel in the stabilization of bainite lath structure and precipitates at elevated temperatures, resulting in the higher creep strength.

Effect of Mo and W on the Phase Stability of Precipitates in Low Cr Heat Resistant Steels

Morphology and coarsening kinetics of MC-type carbide (MC-carbide) precipitating during the tempering process have been investigated in V- and Nb-bearing Cr-Mo martensitic steels. Detailed transmission electron microscopy (TEM) observations show that the addition of V and Nb stabilizes the B1-type MC-carbide instead of L’3-type M2C-carbide. The morphology of the MC-carbide is characterized as disk-like with Baker and Nutting orientation relationships with the matrix. When the specimens are fully solution treated followed by quenching, the MC-carbide precipitates as a multicomponent system with continuous solid solution of VC, NbC, and MoC. The V-, Nb-, and Mo-partitioning control the lattice parameter of MC-carbide and consequently affect the coherency between MC-carbide and the matrix. The coherent MC-carbide grows into an incoherent one with the progress of tempering. The numerical analysis on TEM observations has shown that the coarsening kinetics of MC-carbide is equated to (time)1/5 criteria, while the coarsening kinetics of the coexisting cementite is equated to (time)1/3 criteria. It is thus suggested that the Ostwald ripening of MC-carbide is controlled by pipe diffusion of V, Nb, and Mo along dislocations. It has been confirmed that the coarsening rate of the multicomponent MC-carbide is affected by V, Nb, and Mo content. Applying the thermodynamic solution database, the rate equation for MC-carbide coarsening can be expressed as a function of V, Nb, and Mo content, and the activation energy for pipe diffusion can be estimated as ΔQ
 v: ΔQ
 Nb: ΔQ
 Mo=1:3.9:0.6.

Coarsening kinetics of multicomponent MC-type carbides in high-strength low-alloy steels

As a process to manufacture ultrafine-grained hot strip or plate of which ferrite grain size was about 1 μm, heavy reduction hot rolling at relatively low temperature, such as larger than 70% reduction per pass at lower than 700°C, had been presented. However, it is quite difficult to apply the process to commercial production because the rolling load is very large. A new process of multi-pass rolling at extremely short interval has been proposed and developed in order to manufacture ultrafine-grained hot strip at ordinary rolling load. Authors named it "Super Short interval Multi-pass Rolling (SSMR) process". Ultrafine-grained hot strip of 1.2 mm thick was successfully manufactured in laboratory with the simple chemical composition of 0.15%C-0.74%Mn. Its grain size was 0.9 μm at 50 μm beneath the surface and 1.2 μm on the average over the thickness. The rolling load in SSMR was smaller than 30 kN/mm and it was relatively small owing to relatively small reduction, smaller than 50 % per pass, and relatively high temperature, higher than 820°C. Consequentially it was confirmed that the rolling load in SSMR process could be reduced to half compared to the previously presented process of heavy reduction hot rolling at low temperature.

Super Short Interval Multi-pass Rolling Process for Ultrafine-grained Hot Strip

A noble grain refinement method to attain ferrite grain size of 1 μm in plain C-Mn steel by multi-pass hot rolling in a stable austenite temperature region has been explored. 0.1%C-1%Mn and 0.16%C-0.7%Mn steels were finish-rolled at about Ae 3 and then rapidly cooled. Special emphases have been placed on the interval, At, between the finish of rolling and the start of water spray cooling of more than 1000°C·s -1 in cooling rate and the interval between the last two rolling passes. By reducing At from 0.5 to 0.05 s alone, the grain sizes near the sheet surface and in the central region were markedly reduced to 1.3 and 2.2 μm respectively. By reducing At and the rolling pass interval together, a further reduction in grain size was then achieved. Consequently, the steel sheet in which the ultrafine grain structure of about 1 μm in grain size penetrated from the sheet surface to the depth of a quarter of the sheet thickness and the grain size in the central region was well below 2 μm was obtained. The transformation to the ultrafine ferrite is considered to be static and from deformed y. The detailed grain refinement mechanisms are discussed based on the microstructure observation of quenched C-Mn steels and a fcc Ni-30%Fe alloy as well as the calculation of prior y texture by the misorientation distribution function method.

Grain Refinement of C–Mn Steel to 1 μm by Rapid Cooling and Short Interval Multi-pass Hot Rolling in Stable Austenite Region

A low alloy steel for oil country tubular well which has a yield stress of 110 ksi or above, and excellent sulfide stress cracking resistance The low alloy steel comprises, by weight, 02 to 035% carbon, 02 to 07% chromium, 01 to 05% molybdenum, 01 to 03% vanadium, 0 to 05% silicon, 0 to 1% manganese, 0 to 01% aluminum, 0 to 01% niobium, 0 to 005% titanium, 0 to 0005% boron, 0 to 01% zirconium, 0 to 1% tungsten, 0 to 001% calcium, 0025% or less phosphorus, 001% or less sulfur, 001% or less nitrogen, and 001% or less oxygen The low alloy steel further comprises a total amount of precipitated carbides between about 2 to 5% by weight, and a ratio of the MC type carbide to the total amount of the precipitated carbides is between about 8 to 40% by weight

Kaori Miyata

Papers

Effect of Mo and W on the Phase Stability of Precipitates in Low Cr Heat Resistant Steels

Coarsening kinetics of multicomponent MC-type carbides in high-strength low-alloy steels

Super Short Interval Multi-pass Rolling Process for Ultrafine-grained Hot Strip

Grain Refinement of C–Mn Steel to 1 μm by Rapid Cooling and Short Interval Multi-pass Hot Rolling in Stable Austenite Region

Low alloy steel for oil country tubular goods and method of making