L.P. Karjalainen

Nanoscale Deformation Behavior of Phase-Reversion Induced Austenitic Stainless Steels: The Interplay Between Grain Size from Nano-Grain Regime to Coarse-Grain Regime

We have used the recently adopted concept of phase reversion to obtain grain size from the nanograined/ultrafine-grained (NG/UFG) to fine grain (FG) regime by varying temperature–time annealing sequence of cold deformed metastable austenite. The phase-reversion induced NG/UFG structure was characterized by high strength-high ductility combination. The concept of phase reversion involves severe cold deformation of metastable austenite to generate strain-induced martensite. Upon annealing, martensite transforms back to austenite through a diffusional reversion mechanism with NG/UFG, sub-micron grains (SMG) or FG structure, depending on the annealing condition. Depth-sensing nanoindentation experiments were combined with electron microscopy to elucidate the dependence of grain size from nanograin/ultrafine-grain (NG/UFG) to coarse grain (CG) regime on the deformation mechanisms. There was distinct transition in the deformation mechanism from intense mechanical twinning and stacking faults in NG/UFG structure to strain-induced martensite formation at the intersection of shear bands in the CG structure. The transition in the deformation mechanism is discussed in terms of increase in austenite stability with decrease in grain size.

Nonuniform distribution of carbonitride particles and its effect on prior austenite grain size in the simulated coarse-grained heat-affected zone of thermomechanical control-processed steels

The spatial distribution of carbonitride particles in the simulated coarse-grained heat-affected zone (HAZ) of Nb-Ti microalloyed thermomechanical control-processed (TMCP) steels was investigated using a scanning transmission electron microscope (STEM). It was found that the particles in quenched coarse-grained HAZ were frequently distributed in a nonuniform way, forming clusters and arrays of particles. This nonhomogeneity is defined by the grouping tendency of particles and described by the closeness of the average number density (the mean particle number per unit area) to the average local number density (the mean particle number per unit area, excluding the examined areas without particles). A high concentration of Nb (0.04 mass pct in this article) promoted the formation of carbonitride particle arrays and clusters because of its high segregation tendency at grain and subgrain boundaries during the cooling of a slab. Some of these particles remain undissolved at the peak temperature of a welding thermocycle and may result in sympathetic nucleation of new particles on them. The effectiveness of the particle groups to restrict grain growth is discussed.

Dependence of cellular activity at protein adsorbed biointerfaces with nano- to microscale dimensionality

Protein adsorption is one of the first-few events that occur when a biomedical device comes in contact with the physiological system. The adsorption process is subsequently followed by communication with cells and formation of tissue. Given the strong interest in nanostructured surfaces, we describe here the impact of grain structure from nanograined (NG) regime to coarse-grained (CG) regime on the self-assembly of proteins (bovine serum albumin) and consequent functional response of osteoblasts. The objective is accomplished using the innovative concept of phase reversion that enables a wide range of grain size (from NG to CG regime) to be obtained using an identical set of parameters, besides additional attributes of high strength/weight ratio and wear resistance. Depending on the grain structure a consistent and significant change in the adsorption characteristics of protein was observed at biointerface, such that the cell density was statistically different. The high surface coverage and leaf-like conformation of adsorbed protein on NG surface as compared to bare branch-like structure with low surface coverage on the CG surface, was beneficial in favorably modulating cellular activity (osteoblast functions: cell attachment, proliferation, actin, vinculin, and fibronectin expression). This is the first report that elucidates the impact of grain structure from NG to CG regime on cellular activity.

Effect of Nitrogen Content on Grain Refinement and Mechanical Properties of a Reversion-Treated Ni-Free 18Cr-12Mn Austenitic Stainless Steel

Martensite reversion treatment was utilized to obtain ultrafine grain size in Fe-18Cr-12Mn-N stainless steels containing 0 to 0.44xa0wtxa0pctxa0N. This was achieved by cold rolling to 80xa0pct reduction followed by reversion annealing at temperatures between 973xa0K and 1173xa0K (700xa0°C and 900xa0°C) for 1 to 104xa0seconds. The microstructural evolution was characterized using both transmission and scanning electron microscopes, and mechanical properties were evaluated using hardness and tensile tests. The steel without nitrogen had a duplex ferritic-austenitic structure and the grain size refinement remained inefficient. The finest austenitic microstructure was achieved in the steels with 0.25 and 0.36xa0wtxa0pct N following annealing at 1173xa0K (900xa0°C) for 100xa0seconds, resulting in average grain sizes of about 0.240xa0±xa00.117 and 0.217xa0±xa00.73xa0µm, respectively. Nano-size Cr2N precipitates observed in the microstructure were responsible for retarding the grain growth. The reversion mechanism was found to be diffusion controlled in the N-free steel and shear controlled in the N-containing steels. Due to a low fraction of strain-induced martensite in cold rolled condition, the 0.44xa0wtxa0pct N steel displayed relatively non-uniform, micron-scale grain structure after the same reversion treatment, but it still exhibited superior mechanical properties with a yield strength of 1324xa0MPa, tensile strength of 1467xa0MPa, and total elongation of 17xa0pct. While the high yield strength can be attributed to strengthening by nitrogen alloying, dislocation hardening, and slight grain refinement, the moderate strain-induced martensitic transformation taking place during tensile straining was responsible for enhancement in tensile strength and elongation.

Development of Stainless Steels with Superior Mechanical Properties: A Correlation Between Structure and Properties in Nanoscale/Sub-micron Grained Austenitic Stainless Steel

A review of the structure–property–performance relationship in a technologically important cold-rolled and annealed metastable austenitic stainless steel (AISI 301LN SS) is presented. AISI 301LN SS is cold-rolled to 63% reduction and subsequently annealed at 600–1,000°C from 1 to 100 s. Cold-rolled and annealed samples are studied through X-Ray Diffraction (XRD), Superconducting Quantum Interference Device (SQUID), transmission electron microscopy (TEM) and tensile testing to understand the morphology of the cold-rolled AISI 301LN and the annealed martensite to austenite reversion, the formation of nano/submicron grain sizes and the mechanical properties achieved. Tests show that cold-rolled samples annealed at 600 and 700°C exhibit partial α′→ γ reversion, while for the case of 800–1,000°C annealing treatments, the reversion from α′-martensite to γ-austenite is almost complete, along with rapid austenite grain growth. Tensile tests performed on AISI 301LN nano/submicron grained SS reveal a high yield strength of ~700 MPa, which is twice the typical yield strength of conventional fully annealed AISI 301LN SS. An analysis of the relationship between yield strength and grain size in these nano/submicron grained SS indicates a classical Hall–Petch behavior, despite the temperature dependence observed due to an interplay between fine grained austenite, solid solution strengthening, precipitate hardening and strain hardening.