Martensite and its reverse transformation in nanocrystalline bulk Co Research Papers (original) (raw)
Atom Probe Tomography (APT) was used to analyze the carbon distribution in a heavily cold drawn pearlitic steel wire with a true strain of 6.02. The carbon concentrations in cementite and ferrite were separately measured by a sub-volume... more
Atom Probe Tomography (APT) was used to analyze the carbon distribution in a heavily cold drawn pearlitic steel wire with a true strain of 6.02. The carbon concentrations in cementite and ferrite were separately measured by a sub-volume method and compared with the literature data. It is found that the carbon concentration in ferrite saturates with strain. The carbon concentration in cementite decreases with the lamellar thickness, while the carbon atoms segregate at dislocations or cell/grain boundaries in ferrite. The mechanism of cementite decomposition is discussed in terms of the evolution of dislocation structure during severe plastic deformation.
Austenite formation, which originated from a fined-grained ferrite plus carbide microstructure, was observed during tensile testing at 973 K (60 K below Ae1, the equilibrium austenite–pearlite transformation temperature). Scanning... more
Austenite formation, which originated from a fined-grained ferrite plus carbide microstructure, was observed during tensile testing at
973 K (60 K below Ae1, the equilibrium austenite–pearlite transformation temperature). Scanning electron microscopy, electron
backscatter diffraction and atom probe tomography results reveal the mechanism of austenitic transformation below Ae1. The initial
fine-grained microstructure, in combination with the warm deformation process, determines the occurrence of strain-induced austenite formation below Ae1. The initial fine-grained microstructure essentially contains a higher dislocation density to facilitate the formation of Cottrell atmospheres and a larger area fraction of ferrite/carbide interfaces which serve as austenite nucleation sites. The warm deformation promotes the Ostwald ripening process and the increase in dislocation density, and hence promotes the accumulation of local high carbon concentrations in the form of Cottrell atmospheres to reach a sufficiently high thermodynamic driving force for austenite nucleation. The critical carbon concentration required for the nucleation of austenite was calculated using classical nucleation theory, which correlated well with the experimental observations.
High-strength (1.2–1.5) C–(2–2.5) Mn–(1.5–2) Si–(0.8–1.5) Cr steels (mass%) consisting of martensite and carbides exhibit excellent superplastic properties (eg strain rate sensitivity m≈ 0.5, elongation≈ 900% at 1023K). A homogeneous... more
High-strength (1.2–1.5) C–(2–2.5) Mn–(1.5–2) Si–(0.8–1.5) Cr steels (mass%) consisting of martensite and carbides exhibit excellent superplastic properties (eg strain rate sensitivity m≈ 0.5, elongation≈ 900% at 1023K). A homogeneous martensitic starting microstructure is obtained through thermomechanical processing (austenitization plus 1.2 true strain, followed by quenching). Superplastic forming leads to a duplex structure consisting of ferrite and spherical micro-carbides.
In an Fe-9 at.% Mn maraging alloy annealed at 450°C reversed allotriomorphic austenite nanolayers appear on former Mn decorated lath martensite boundaries. The austenite films are 5-15 nm thick and form soft layers among the hard... more
In an Fe-9 at.% Mn maraging alloy annealed at 450°C reversed allotriomorphic austenite nanolayers appear on former Mn decorated lath martensite boundaries. The austenite films are 5-15 nm thick and form soft layers among the hard martensite crystals. We document the nanoscale segregation and associated martensite to austenite transformation mechanism using transmission electron microscopy and atom probe tomography. The phenomena are discussed in terms of the adsorption isotherm (interface segregation) in conjunction with classical heterogeneous nucleation theory (phase transformation) and a phase field model that predicts the kinetics of phase transformation at segregation decorated grain boundaries. The analysis shows that strong interface segregation of austenite stabilizing elements (here Mn) and the release of elastic stresses from the host martensite can generally promote phase transformation at martensite grain boundaries. The phenomenon enables the design of ductile and tough martensite.
Oxide-dispersion strengthened ferritic martensitic steels such as ODS-Eurofer grade are good candidates for structural applications in future fusion power reactors. Long-term annealing treatments in vacuum were carried out in cold-rolled... more
Oxide-dispersion strengthened ferritic martensitic steels such as ODS-Eurofer grade are good candidates for structural applications in future fusion power reactors. Long-term annealing treatments in vacuum were carried out in cold-rolled samples (80% reduction in thickness) from 1 h up to 4320 h (6 months) at 800 C, i.e. the maximum temperature in the ferritic phase field, to follow its softening behavior. The microstructural stability of this steel was mapped using several characterization techniques including scanning
electron microscopy, transmission electron microscopy, electron backscatter diffraction, Vickers microhardness testing, X-ray diffraction texture measurements, low-temperature electrical resistivity, and magnetic coercive field measurements. ODS-Eurofer steel displays good microstructural stability. Discontinuous
recrystallization occurs at the early stages of annealing resulting in a low volume fraction of recrystallized grains. Extended recovery is the predominant softening mechanism at this temperature for longer times.
Ferromagnetic shape memory alloys (FSMAs) are the novel materials exhibiting shape memory effect (SME) and magnetism simultaneously. They show magnetic field-induced strains at room temperature greater than any electrostrictive,... more
Ferromagnetic shape memory alloys (FSMAs) are the novel materials exhibiting shape memory effect (SME) and magnetism simultaneously. They show magnetic field-induced strains at room temperature greater than any electrostrictive, magnetostrictive or piezoelectric material, and faster frequency response than temperature driven shape memory alloys. Among various FSMA materials, Ni-Mn-X (X =Ga, In, Sn, Sb) have gained considerable interest due to their multifunctional properties such as shape memory effect, magnetocaloric effect, magnetoresistance, etc., associated with first order martensite to austenite structural transition. FSMA spread its application in broad area from aerospace industry to medical application, but not vividly use; because of its high cost. Ni-Mn-Sn FSMAs shows low cost of manufacturing due to its low value of constituting elements. This paper investigates the behavior of Ni-Mn-Sn Heusler FSMA by varying the weight percentage of Sn. Three alloys i.e. Mn 50 Ni 50-x Sn x (x = 5, 7.5, and 10) were produced as bulk polycrystalline ingots by arc melting. In order to identify structural phases X-ray diffraction (XRD) measurements were conducted at room temperature using Cu K α radiation. By Differential Scanning Calorimetric (DSC) study it is found that, the transformation temperatures gradually decreases as increasing the Sn content, which shows it can apply in higher working temperature range than that of Ga-FSMAs. ____________________________________________________________________________________
A nanostructured surface layer of Co with the thickness of about 20 m, considered as a bulk sample, was prepared by means of surface mechanical attrition treatment (SMAT). The average grain sizes of the samples prepared by 30 and 60 min... more
A nanostructured surface layer of Co with the thickness of about 20 m, considered as a bulk sample, was prepared by means of surface
mechanical attrition treatment (SMAT). The average grain sizes of the samples prepared by 30 and 60 min SMAT are determined as 26 and
23 nm, respectively, by X-ray diffraction, and confirmed by transmission electron microscopy. Differential scanning calorimetry analysis for the
above samples and a coarse-grained sample reveals that start temperature As of the (hcp)→ (fcc) reverse martensitic transformation can be
described as: TAS = 456–293/d (in ◦C, 15 nm≤d≤100 nm, d is grain size). The nanocrystalline high-temperature (fcc) phase with grain size
smaller than about 35 nm obtained by heating SMAT samples for proper duration exhibits thermal stability during cooling from 500 ◦C to ambient
temperature even at −196 ◦C. However, these thermally stable nanocrystalline (fcc) phase samples can undergo the (fcc)→ (hcp) martensitic
transformation when treated by SMAT again. Thermal stability of the nanocrystalline low-temperature phase (hcp) was observed in SMAT Co,
that is, when the grain sizes are smaller than 15 nm, the reverse transformation will not occur until to 815 ◦C.