At the beginning of civilization, metals have been mankind`s most essential materials due to their ductility and strength. One of the crucial problems in metals is a counter balance between ductility and strength; high strength generally leads to poor ductility in metals. Thus ASML`s essential objective is to investigate the alloys with a remarkable balance between strength and ductility, using cutting-edge analyses of the relationship between microstructure and mechanical properties.
Research Areas of ASML
Advanced High-Strength Steels Design
For the past decades, the automobile industry has been improving fuel efficiency, passenger safety, and reduction in greenhouse gas emission by adopting advanced high strength steels (AHSS) with both high strength and high ductility. In general, AHSS is classified as the first-generation AHSS (1st-G AHSS), the second-generation AHSS (2nd-G AHSS), and the third-generation AHSS (3rd-G AHSS).
The 1st-G AHSS, such as dual-phase (DP), complex-phase (CP), and transformation-induced plasticity (TRIP) steels, possesses the high strength of over 600 MPa but the relatively low ductility of below 20%. This low ductility restricts the application of the 1st-G AHSS to automobile parts with complex shapes. The 2nd-G AHSS, such as high Mn TRIP and twinning-induced plasticity (TWIP) steels, shows a remarkable combination of high strength of over 700 MPa and large uniform ductility exceeding 50%. The extraordinary tensile properties of the 2nd-G AHSS result from the high strain hardening rate due to the formation of strain-induced martensite or mechanical twins in the austenite phase during plastic deformation. However, the 2nd-G AHSS has difficulties in mass production, material cost, and welding, which are caused primarily by a lot of alloying elements greater than approximately 17 wt.%. Therefore, nowadays, the 3rd-G AHSS, such as lightweight steel, quenching and partitioning (Q&P) processed steel and medium Mn steel has been attracted because it has a good trade-off between material cost and mechanical properties.
One of the objectives of ASML is to investigate the next generation AHSS showing better mechanical properties than 3rd-G AHSS even maintaining reasonable materials cost. This concept is based on optimal using of various strengthening mechanisms simultaneously.
Hydrogen is known to be one of the most harmful elements in steel because the presence of that in steels might cause a detrimental effect on mechanical properties. The hydrogen can permeate in steels from the production process (e.g. pickling, electroplating, and welding), product assembling, and service environment exposure. Even a very low concentration of hydrogen (less than 1 ppm) can cause a significant decrease in ductility and premature failure of steels and this phenomenon is the so-called hydrogen embrittlement (HE). It is known that the diffusible hydrogen is the suspect causing HE, and there are several proposed mechanisms for HE, such as internal pressure model, hydrogen-enhanced decohesion (HEDE), hydrogen-enhanced local plasticity (HELP) and hydrogen-enhanced strain-induced vacancy (HESIV).
In the internal pressure model, the trapped hydrogen atoms at internal defects, such as dislocations and voids, form molecular hydrogen, increase internal pressure, and then assist the growth of voids and cracks. The HEDE mechanism can be explained that the dissolved hydrogen lowers the cohesive bonding energy of the metal lattice due to increase in interatomic distance, resulting in intergranular or transgranular fractures. The HELP mechanism is based on a stress shielding effect to increase the mobility of dislocations. Dissolved hydrogen forms Cottrell atmospheres around the dislocations, reduces the stress field and interaction of individual dislocations, and then increases the dislocation mobility, leading to localized plasticity and premature fracture. The HESIVE mechanism is usually observed in low-alloyed steel and it can be explained that the hydrogen in steel enhances the strain-induced creation and agglomeration of vacancies, promoting the ductile fracture process.
Our lab.`s objective is to clarify the HE mechanism of steels with various microstructures and showing various deformation mechanism like TRIP and TWIP through the state-of-the-art analyses by thermal desorption analysis (TDA), scanning Kelvin probe microscope (SKPFM) and post-mortem microstructural analysis such as electron backscattered diffractometer (EBSD) and electron channeling contrast imaging (ECCI) method. Through our research, we expect to reveal the reason for HE in steels.
Cost-Effective Superplastic Alloys
Superplasticity refers to the ability to have extremely large ductility (> 400%) through grain boundary sliding, when the alloy with fine grain sizes (< 10 μm) deforms at temperatures typically above the half of melting temperature (K). The remarkable ductility by superplasticity facilitates the excellent formability of metals with complex shapes at rapid production rates. Although the Zn, Ni, Al and Ti alloys are suggested to conventional superplastic alloy owing to their excellent ductility, they are not suitable for actual structural materials due to several essential problems; the high material cost (Ni and Ti alloys) and low essential mechanical properties including strength after superplastic forming (Zn and Al alloys).
To overcome the issues, superplastic ferrous alloys have been studied. One of the steels investigated frequently is the duplex stainless steels containing 23-34 wt.% Cr, 4-22 wt.% Ni and a few other alloying elements, such as Mo, Ti, N and Mn. A cold-rolled duplex stainless steel exhibited a mixed microstructure of equiaxed ferrite and austenite with ultrafine grain sizes of ~0.8 - 2.0 μm leading to remarkable total elongations to failure up to 2500% at the intercritical temperature ranging from 800 °C to 1100 °C. As was consistent with the conventional superplastic metals, high elongation of duplex stainless steel was due to active grain boundary sliding (GBS) at the interfaces of ferrite and austenite phases during deformation.
Ultrahigh C steels with 0.8-4.0 wt.% C and a few other alloying elements including Mn, Cr and V are also examined repeatedly as alternative superplastic steels. Subsequent to thermomechanical processing such as warm working and cyclic hot rolling-annealing, ultrahigh C steels contain a mixed microstructure of fine cementite particles in sizes of ~0.1 - 0.3 μm and equiaxed austenite having sizes of 0.2 - 2.0 μm at temperatures ranging from ~700 °C to 1000 °C so that this class of steels exhibits remarkable total elongations up to ~1000%. The optimum concentration of C for superplasticity is estimated as ~1.3 - 1.8 wt.% because the high C concentration causes coarsening of cementite which acts as nucleation sites for voids leading to low ductility.
Comparing with non-ferrous alloys, it is true that both the duplex stainless steels and ultrahigh C steels have the advantages of the lower materials cost and the higher strength after forming. Nevertheless, under the growing demand for steel to maintain high formability with reasonable cost in modern industries, these steels are not suitable due to the facts that the material cost of duplex stainless steels is much higher than the conventional steels and the complicated treatments are necessary for ultrahigh C steels to achieve reasonable superplastic ductility. Thereby, the development of new superplastic steel is indispensable for the renewal of a strong industrial base.
Our lab`s goal is to investigate the brand-new superplastic ferrous alloy having simple production method and reasonable materials cost.
Thermo-Mechanical Process Design
The microstructure of alloys and steels could be controlled by the material`s synthesis. Generally, alloys are subjected to various hot and cold rolling or forging and annealing with various conditions for the productions. This procedure is called as a thermo-mechanical process. The different microstructures which are characterized by various thermo-mechanical process give a lot of effort on material`s properties such as mechanical properties.
Our lab`s goal is to try and apply the various thermo-mechanical process on various alloys and steels to optimize the mechanical properties.
Multi-Scale Microstructure Analysis
Revealing interrelation between microstructure and mechanical properties is the essential topic of the metallurgical engineering. Multi-scale microstructural analysis from bulk scale to atomic scale could give a clear information of the component of steel and alloys, thus it could facilitate the successful alloy design and reveal the secret of interrelation.
Our lab uses state-of-the-art microstructural analysis techniques such as not only general X-ray diffraction (XRD) and electron microscope (SEM, TEM), but also high-performance electron backscatter diffraction (EBSD), electron channeling contrast imaging (ECCI) and 3D atom probe tomograph (APT).