Eutectoid Transformation Temperature Research Articles

INFLUENCE OF HYDROGEN SATURATION ON CHARACTERISTICS OF ZIRCONIUM

The article presents a literature review on the effect of hydrogen saturation on the properties of zirconium alloys. Zirconium alloys are an indispensable structural material for the core of nuclear reactors. During operational loads, the interaction of zirconium materials with interstitial elements (oxygen, nitrogen, hydrogen) necessarily occurs. Zirconium shows the greatest affinity for hydrogen. One of the vulnerable zirconium elements is fuel rod tubes. The safe operation of a nuclear reactor depends on their integrity. In addition, it is fuel tubes that are most exposed to hydrogen. Therefore, the systematization of knowledge about the effect of hydrogen on the properties of zirconium tubes will make it possible to better predict their operational behavior. According to scientific literature data, depending on the volume of absorbed hydrogen, either a solid solution or zirconium hydrides can form. The dependence of the absorbed hydrogen zirconium on the dilution of the hydrogen medium and temperature is shown. The chemical composition of the zirconium alloy also affects the rate and amount of absorbed hydrogen. The effect of hydrogen on the mechanical properties of zirconium alloys is presented. The differences on the fracture surface after tensile tests at room temperature are shown depending on the amount of absorbed hydrogen. Data are presented that indicate that hydrogen atoms are located in octahedral or tetrahedral interstitial voids of a hexagonal close-packed zirconium lattice. It is shown that the thermal solubility of hydrogen in α-zirconium is extremely low, its value is ~6 at. % at the eutectoid transformation temperature, and at room temperature the solubility of α-Zr hydrogen does not exceed 1·10-5 wt. %. In high-temperature β-Zr, hydrogen dissolves up to ~50 at. %. It has been established that deterioration of the properties of zirconium elements of nuclear reactors during operation due to exposure to hydrogen is likely due to a number of factors: hydrogen embrittlement, the formation of large massive accumulations of hydrides and delayed hydride cracking. It is shown that the direction of arrangement of hydrides depends on the texture of the matrix and on the stresses present in the material that act during the formation of hydrides. It has been established that hydrogen can penetrate into a metal through an oxide film, diffusing, for example, along extended defects such as dislocations and grain boundaries. It is noted that the solubility of hydrogen in zirconium depends on other penetration elements already present in the metal, for example, the solubility of hydrogen in α-zirconium depends on the soluble oxygen in the metal matrix, which is confirmed by the reduced «Zr–O–H» ternary system.

Rapid Spheroidizing Annealing via Combining Warm Deformation with Divorced Eutectoid Transformation in M50 Steel

In this work, a novel routine to realize the rapid spheroidizing annealing (SA) process has been proposed in M50 steel via combining warm deformation (WD) and divorced eutectoid transformation (DET). In this process, the sample is first subjected to partial austenitizing to realize the partial dissolution of carbides, then slowly cooled to DET temperature for WD. Finally, the sample is slowly cooled to below the eutectoid transformation temperature to complete the SA. The results show that an excellent spheroidized microstructure can be obtained via the rapid SA process within 2 h. The finest spheroidized carbide (0.295 μm), as well as an appropriate hardness (217 HV), is achieved when the WD and DET processes are conducted at 760 °C. This should be attributed to that the WD not only accelerates the DET but also can break the primary carbides, leading to the uniformly distributed and refined carbides. In addition, the proposed WD that realizes rapid SA shows the excellent roundness of spheroidized carbides compared with the traditional cold/hot deformation which needs a long-term SA process. This work provides a highly efficient routine to simultaneously realize the SA and shape forming, which is of great engineering significance for the manufacturing of bearings.

Tribological Wear of as Cast Zn-4Al Alloy Cooled at Various Rates from the Eutectoid Transformation Temperature

Tribological Wear of as Cast Zn-4Al Alloy Cooled at Various Rates from the Eutectoid Transformation Temperature

Effect of Cooling Rate at the Eutectoid Transformation Temperature on the Corrosion Resistance of Zn-4Al Alloy.

The main purpose of this work was to experimentally determine the effect of the cooling rate during the eutectoid transformation on the corrosion resistance of a hypoeutectic Zn-4Al cast alloy in 5% NaCl solution. This was considered in relation to the alloy microstructure. For this purpose, metallographic and electrochemical studies were performed. It was found that the faster cooling promoted the formation of finer (α + η) eutectoid structures, which translated into a higher hardness and lower corrosion current density. In the initial stage of corrosion processes the eutectoid structure in the eutectic areas were attacked. At the further stages of corrosion development, the phase η was dissolved, and the α phase appears to be protected by the formation of corrosion products.

Structural and Mechanical Properties of Ti⁻Co Alloys Treated by High Pressure Torsion.

The microstructure and properties of titanium-based alloys can be tailored using severe plastic deformation. The structure and microhardness of Ti–4 wt.% Co alloy have been studied after preliminary annealing and following high pressure torsion (HPT). The Ti–4 wt.% Co alloy has been annealed at 400, 500, and 600 °C, i.e., below the temperature of eutectoid transformation in the Ti–4 wt.% Co system. The amount of Co dissolved in α-Ti increased with increasing annealing temperature. HPT led to the transformation of α-Ti in ω-Ti. After HPT, the amount of ω-phase in the sample annealed at 400 °C was about 80­85%, i.e., higher than in pure titanium (about 40%). However, with increasing temperature of pre-annealing, the portion of ω-phase decreased (60–65% at 500 °C and about 5% at 600 °C). The microhardness of all investigated samples increased with increasing temperature of pre-annealing.

Non-Toxic Environment for Ferritic Nitrocarburising Process

Experimental research aimed to find a solution for replacing components with high toxicity (or generating such components as a result of reactions occurring in the environment at processing temperatures) from the environments used for ferritic nitrocarburising process (FNCP) with non-hazardous components, but extremely active during the process. In the temperature range in which this type of processing is applied (lower than the eutectoid transformation temperature in the Fe-N phase diagram), the most commonly used media are liquid or gaseous; liquid ones contain toxic components (sodium or potassium cyanates/cyanides), and gaseous ones require complex equipments. Packing is extremely rarely used, but in this case pack-mix contain toxic components (15 � 20 wt.% sodium or potassium ferrocyanide). Urea also called carbamide (CO (NH2)2) is the active component in the pack-mixing proposed to be used for FNCP. Carbamide is used in low temperature cyanidation thermochemical heat treatment (liquid FNC), together with sodium or potassium carbonates, resulting in very toxic reaction products (sodium or potassium cyanates). Compared to cyanidation, in the version proposed in the paper, the carbamide does not react with carbonates because they are not found in the composition of the environment but decomposes in the presence or absence of oxygen (by a disproportionation reaction) with the formation of some gas molecules interesting for the process. It has been concluded that the use of carbamide together with two other components, activated charcoal (having a triple role - dispersing, storage, surface saturation) and respectively ammonium chloride as surface reaction activator, is an effective solution for achieving the desired goals by applying this type of thermochemical processing to a wide range of products made of quality steels up to alloy miscellaneous steels.

Diffusive and displacive phase transitions in Ti–Fe and Ti–Co alloys under high pressure torsion

The high pressure torsion (HPT) of Ti–4 wt. % Fe and Ti–4 wt. % Co alloys has been studied. Before HPT, the samples were annealed above and below the temperature of eutectoid transformation. After quenching, the first samples contained a mixture of α and β phases. The latter samples contained a mixture of α-Ti saturated with Fe or Co and respective intermetallic phases TiFe. During HPT the high-pressure ω-phase formed in all samples. The amount of ω-phase was higher when the initial samples contained the α+β mixture then in the case of α+TiFe or α+Ti2Co mixture. Most probably, the martensitic β→ω transformation occurs easier than the α→ω one. The amount of ω-phase was higher in Ti–Fe alloy than in the Ti–Co alloy for both initial states. It seems that cobalt additions disadvantageously change the lattice parameters in Ti for martensitic β→ω and α→ω transformations. The phase transformations with mass transfer (diffusive ones) also take place by HPT of Ti–Fe and Ti–Co alloys. Both α-based solid solutions partially decomposed after HPT. After HPT, the lattice parameters of α-phase containing cobalt or iron changed towards those of pure α-Ti. It has been observed for the first time that the composition of TiFe phase also changes after HPT.

Effects of alloying elements and cooling rates on the high-strength pearlite steels

ABSTRACTThe effects of the alloying elements of Cr, Mn and the cooling rates after hot deformation on the microstructures and mechanical properties of pearlite steels were studied. Results show that increasing Cr and decreasing Mn significantly increase the eutectoid transformation temperature of steel. The grain sizes of prior austenite of the steels after hot deformation are ∼12 µm. However, the high-Cr–low-Mn steel exhibits a finer interlamellar spacing and some better mechanical properties than that of the high-Mn–low-Cr steel. A full pearlite microstructure with an interlamellar spacing of 97 nm was obtained on the former steel, which exhibits a hardness of HRC49, a tensile strength of 1700 MPa and an elongation of 19%.

Refinement and fracture mechanisms of as-cast QT700-6 alloy by alloying method

The as-cast QT700-6 alloy was synthesized with addition of a certain amount of copper, nickel, niobium and stannum elements by alloying method in a medium frequency induction furnace, aiming at improving its strength and toughness. Microstructures of the as-cast QT700-6 alloy were observed using a scanning-electron microscope (SEM) and the mechanical properties were investigated using a universal tensile test machine. Results indicate that the ratio of pearlite/ferrite is about 9:1 and the graphite size is less than 40 μm in diameter in the as-cast QT700-6 alloy. The predominant refinement mechanism is attributed to the formation of niobium carbides, which increases the heterogeneous nucleus and hinders the growth of graphite. Meanwhile, niobium carbides also exist around the grain boundaries, which improve the strength of the ductile iron. The tensile strength and elongation of the as-cast QT700-6 alloy reach over 700 MPa and 6%, respectively, when the addition amount of niobium is 0.8%. The addition of copper and nickel elements contributed to the decrease of eutectoid transformation temperature, resulting in the decrease of pearlite lamellar spacing (about 248 nm), which is also beneficial to enhancing the tensile strength. The main fracture mechanism is cleavage fracture with the appearance of a small amount of dimples.

The Novel Technique of Grain Refinement in the Aluminum-Free Ultrahigh Carbon Steel

In order to achieve high strength and toughness of the aluminum-free ultrahigh carbon steel (UHCS), the effect of a novel three-parameter combination technique including heating temperature, cooling rate, and isothermal eutectoid transformation time on the phase composition, grain morphology, and mechanical properties is investigated by X-ray diffraction analysis, optical microscopy, and mechanical tests. The result shows that when the steel is cooled from 860 °C to the eutectoid transformation temperature with the rate of 1.0 °C s−1 and held for 1.5 h, the excellent mechanical properties are obtained due to the grain refinement of carbide and matrix.

Thermal stability of B2 CuZr phase, microstructural evolution and martensitic transformation in Cu–Zr–Ti alloys

The effects of minor Ti addition on the thermal stability of B2 CuZr phase, the microstructure and the martensitic transformation (MT) in Cu50Zr50−xTix (x = 0, 2.5, 7.5 and 10 at%) alloys were investigated. It was found that the crystallization products, i.e. Cu10Zr7 and Cu(Ti, Zr)2, of Cu–Zr–Ti amorphous alloys transform to B2 CuZr phase at high temperatures. The corresponding eutectoid transformation temperature gradually increases with increasing Ti content, implying the decrease of the thermodynamic stability of the B2 CuZr phase. The microstructures of Cu–Zr–Ti martensitic alloys were proven to contain B2 CuZr, CuZr martensite, Cu10Zr7, Cu(Ti, Zr)2, and ZrTiCu2 crystals. Dilatometric measurements reveal that the MT temperature reduces with increasing Ti content, which would be of electronic nature. With increasing thermal cycles, the MT temperature gradually decreases while the reverse MT temperature increases, which results from the enhancement of the dislocation density, the partial decomposition of the equiatomic CuZr crystals and the partially reversible MT.

Microstructure feature of friction stir processed ductile cast iron

In the present investigation, the ductile cast iron was successfully fiction stir processed at a tool rotational rate of 1000rpm and a traveling speed of 90mm/min, and an obvious defect-free processed zone was obtained. The macro- and micro-structures, phase evolution, thermal history and microhardness of the processed zone were investigated. The results revealed that the high density of ultra-refined and nearly equiaxed graphite particles (0.5–1μm) uniformly were distributed in most parts of the stir zone. The flake-shaped or tadpole-shaped graphites distributed along flow line due to the flow of the plasticized material in the thermo-mechanically affected zone. The distribution of fine graphite particles at various locations of the stir zone exhibited different plasticized material flow characteristics. The gradual increase of shear zone from the advancing side to the retreating side was clearly detected in the edge of the stir zone just beside the thermo-mechanically affected zone due to the shear flow of softened material. The numerous dense martensite structures were formed in the processed zone as the peak temperature exceeded the eutectoid transformation temperature (A1), which in turn favored the improvement of hardness of the base material. The microhardness test results show that the maximum hardness of the stir zone reached 817HV0.2.

Ab initio study of the intermetallics in Nb–Si binary system

First-principles calculations were carried out for the Nb3Si, αNb5Si3, βNb5Si3 and NbSi2 intermetallic compounds and their constituent elements. The enthalpies of formation were computed for all the intermetallic compounds, as well as the eutectoid transformation temperature for Nb3Si → Nb + αNb5Si3, the transition temperatures of βNb5Si3 to αNb5Si3, and the range of stability of the NbSi2. There was good agreement between the results of the present study and the Nb–Si phase diagram. The elastic constants, bulk, shear and Young's moduli, Poisson's ratio and Debye temperature were also calculated for the intermetallic compounds and Nb and Si. The Cauchy pressures, Pugh's index and Poisson's ratio indicate that the high temperature phases (βNb5Si3 and Nb3Si) should be more ductile (with Nb3Si being the most ductile) than the αNb5Si3 and NbSi2 (with the latter being the most brittle).

Thermomechanical process optimization of U-10 wt% Mo – Part 1: high-temperature compressive properties and microstructure

Nuclear power research facilities require alternatives to existing highly enriched uranium alloy fuel. One option for a high density metal fuel is uranium alloyed with 10wt% molybdenum (U–10Mo). Fuel fabrication process development requires specific mechanical property data that, to date has been unavailable. In this work, as-cast samples were compression tested at three strain rates over a temperature range of 400–800°C to provide data for hot rolling and extrusion modeling. The results indicate that with increasing test temperature the U–10Mo flow stress decreases and becomes more sensitive to strain rate. In addition, above the eutectoid transformation temperature, the drop in material flow stress is prominent and shows a strain-softening behavior, especially at lower strain rates. Room temperature X-ray diffraction and scanning electron microscopy combined with energy dispersive spectroscopy analysis of the as-cast and compression tested samples were conducted. The analysis revealed that the as-cast samples and the samples tested below the eutectoid transformation temperature were predominantly γ phase with varying concentration of molybdenum, whereas the ones tested above the eutectoid transformation temperature underwent significant homogenization.

Effect of Divorced Eutectoid Transformation Temperature on Behaviour of Cementite Growth in GCr15 Steel

Cementite spheroidization of GCr15 bearing steel includes granulating and growth of cementite. The effects of divorced eutectoid transformation (DET) temperatures on growth of cementite were studied. It was found that the particles of cementite are fine and uniform when DET occurs at 690～720°С, and alloy elements mostly precipitate at different temperatures during cementite growth of GCr15 bearing steel; Cr and Si mostly precipitate in the eutectoid temperature upper limit, while Mn is in the eutectoid temperature lower limit.

Effect of 0.770 wt%H addition on the microstructure of Ti–6Al–4V alloy and mechanism of δ hydride formation

Ti–6Al–4V alloy was hydrogenated at 750 °C below the eutectoid transformation temperature with 0.770 wt%H, and its microstructural evolution and phase transformations were investigated by the use of optical microscopy (OM), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The results show that, in comparison to the as-received alloy, the contrasts of equiaxed α phase and transformed β phase under optical microscope in the hydrogenated one were reversed. The XRD peaks of β phase after hydrogenation moved to lower angles obviously because of the lattice expansion of β phase with the solution of hydrogen atoms, and the XRD peaks of α phase were broadened due to microscopic internal stress. In addition, XRD and TEM analysis revealed that the face-centered cubic (FCC) δ hydride could precipitate not only in α phase but also in β phase. Furthermore, the mechanisms of δ hydride formation were discussed, and the results showed that the hydrogen diffusion has a significant effect on the δ hydride formation.

Phase transformations in Pu–Ga and Pu–Al alloys: Effects of pressure and temperature on kinetics of δ-phase decomposition

Pressure effected evolution of equilibrium and metastable binary Pu–Ga (Al)-phase diagrams is discussed. With an increase in pressure δ-solid solution region decreases and disappears, which leads to a change of phase diagram type. In experiments and via thermodynamic calculations boundaries of δ-phase region were established at pressure varying from the atmospheric one to ∼1 GPa. Temperature of δ-phase eutectoid transformation versus pressure was determined; a fragment pressure–temperature–composition ( PTC) diagram was constructed, parameters of four-phase equilibrium were found. Influence of static pressure on δ-phase decomposition kinetics is demonstrated, feasibility of decomposing effected by loading with high intensity waves is assessed.

Nucleation and growth of the α′ martensitic phase in Pu–Ga alloys

In a Pu–2.0 at.% Ga alloy, it is observed experimentally that the amount of the martensitic α′ product formed upon cooling the metastable δ phase below the martensite burst temperature ( M b) is a function of the holding temperature and holding time of a prior conditioning (“annealing”) treatment. Before subjecting a sample to a cooling and heating cycle to form and revert the α′ phase, it was first homogenized for 8 h at 375 °C to remove any microstructural memory of prior transformations. Subsequently, conditioning was carried out in a differential scanning calorimeter apparatus at temperatures in the range between −50 and 370 °C for periods of up to 70 h to determine the holding time and temperature that produced the largest volume fraction of α′ upon subsequent cooling. Using transformation peak areas (i.e., the heats of transformation) as a measure of the amount of α′ formed, the largest amount of α′ was obtained following holding at 25 °C for at least 6 h. Additional time at 25 °C, up to 70 h, did not increase the amount of subsequent α′ formation. At 25 °C, the Pu–2.0 at.% Ga alloy is below the eutectoid transformation temperature in the phase diagram and the expected equilibrium phases are α and Pu 3Ga, although a complete eutectoid decomposition of δ to these phases is expected to be extremely slow. It is proposed here that the influence of the conditioning treatment can be attributed to the activation of α-phase embryos in the matrix as a beginning step toward the eutectoid decomposition, and we discuss the effects of spontaneous self-irradiation accompanying the Pu radioactive decay on the activation process. Subsequently, upon cooling, certain embryos appear to be active as sites for the burst growth of martensitic α′ particles, and their amount, distribution, and potency appear to contribute to the total amount of martensitic product formed. A modeling approach based on classic nucleation theory is presented to describe the formation of α-phase embryos during conditioning. The reasons why the holding times during conditioning become eventually ineffective in promoting more α′ formation on cooling are discussed in terms of the differences in the potency of the embryos created in the δ matrix during conditioning and in terms of growth-impeding volume strains in the matrix resulting from an increasing number of martensite particles, thus opposing further growth. It is suggested that the disparate amounts of the α′ formation reported in the literature following various studies may be in part a consequence of the fact that conditioning times at ambient temperatures are inevitably involved in any handling of radioactive samples prior to testing.

Effect of milling speed on an Fe–C–Mn system alloy prepared by mechanical alloying

Mechanical alloying (MA) of elemental iron, graphite and manganese powders consisting of Fe–7 mass% C–6 mass% Mn were carried out by using a planetary ball mill at different milling speed. The structural evolution of the MAed powders was characterized by X-ray diffraction (XRD), differential thermal analysis (DTA) and scanning electron microscopy (SEM). The MAed powders were hot isostatic pressed (HIPed) at 100 MPa for 1 h at lower (853 K) or higher (1053 K) than the eutectoid transformation temperature A 1. The processes of amorphization, super-saturated solid solution and particle refinement are influenced by the milling speed. These processes are faster with the increase in the milling speed. The hardness and microstructure of the HIPed compacts are influenced by the morphology of the MAed powders obtained. The powders MAed at high speed has higher hardness at the HIPed compacts than the powders MAed at low speed. Since the sintering progresses well at a temperature higher than A 1 and the phase transformation of γ-Fe into fine carbide and α-Fe occurs during cooling process, the hardness of the compacts HIPed at 1053 K higher than A 1 is greater than that at 853 K lower than A 1.

β1→γ′ transformation in Cu-Mn-Al alloys after low temperature aging

Investigation of martensitic transformation in inhomogeneous solid solutions is a difficult problem. Convenient objects for study of this question are Cu-Mn-Al alloys, properties of which were described in cited references. Accordingly the eutectoid transformation temperature drops to 400 C by addition of 6.5 wt. % manganese. An isostructural decomposition of the {beta}{sub 1}-phase takes place within a relatively low temperature region. In this article the authors considered the influence of concentration inhomogeneities in the {beta}{sub 1}-solid solution on the martensitic transformation. The change of the martensitic points in the quenched Cu-Mn-Al alloys as a result of concentration variations is investigated.