Structure formation upon the β→α transformation in a pseudo-single crystal of the Co-29.7% Ni alloy

Optical microscopy, scanning and transmission electron microscopy, X-ray diffraction, and dilatometry have been used to investigate structure formation upon the β→α (fcc-hcp) transformation in a Co-29.7% Ni alloy. During cooling of an fcc single crystal of the Co-29.7% Ni alloy to the temperature of the β→α phase transformation, the single crystal, because of the polymorphic fcc-hcp transformation, is converted into a two-phase (β + α) pseudo-single crystal. The growth direction of the crystal coincides with the crystallographic direction [112]_β. It is shown that four variants of orientations of the α-martensite crystals are realized during the β→α transformation in a single crystal; the entire volume of the initial crystal is divided into packets, which contain plates of the α phase of predominantly one orientation. The thickness of the α-martensite plates determined by the method of electron microscopy is from 0.1 to 1.0 μm. After a cycle of the α→β→α transformations, in the pseudo-single crystal investigated there is observed a restoration of the initial macro- and microstructure; i.e., the fact of structural heredity is confirmed. The calculated value of the volume changes (ΔV/V) in the Co-29.7% Ni alloy upon the fcc-hcp transformation is close to that experimentally determined from dilatometric data. The hysteresis in the temperature of the onset of the β→α transformation in this alloy is 195 K.     

Microstructures of Ti50Al45Mo5 alloy powders produced by plasma rotating electrode process

Microstructures of Ti₅₀Al₄₅Mo₅ (at.%) alloy powders produced by the plasma rotating electrode process (PREP) were investigated. The powders have inhomogeneous structures, which consist of dendrites and rounded grains. The dendrites, which show a “rosettelike” morphology, are formed on the powder surface and around the rounded grains. The rosettelike dendrites are of hexagonal α ₂ (D0₁₉) phase even though the dendrites have an equiaxial morphology, and a small amount of β ₂ (B2) phase is also contained inside. It is suggested that the solidification to α (hcp-A3) phase occurred by the peritectic reaction between the primary β (bcc-A2) dendrites and the liquid: L+β→L+β+α. The rounded grains, on the other hand, are of β ₂ phase in which acicular α or α ₂ laths are precipitated with the Burgers orientation relationship. Antiphase domain boundaries in the β ₂ matrix are intersected by α(α ₂) laths. It is interpreted that the α(α ₂) laths were formed by the solid-state transformations: β ₂→β ₂+α→β ₂+α ₂. The formation of the two different microstructures in the powder particles is rationalized in terms of the changes in local composition of the liquid phase during the rapid solidification process.     

Microstructures of Ti<Subscript>50</Subscript>Al<Subscript>45</Subscript>Mo<Subscript>5</Subscript> alloy powders produced by plasma rotating electrode process

Microstructures of Ti₅₀Al₄₅Mo₅ (at.%) alloy powders produced by the plasma rotating electrode process (PREP) were investigated. The powders have inhomogeneous structures, which consist of dendrites and rounded grains. The dendrites, which show a “rosettelike” morphology, are formed on the powder surface and around the rounded grains. The rosettelike dendrites are of hexagonal α ₂ (D0₁₉) phase even though the dendrites have an equiaxial morphology, and a small amount of β ₂ (B2) phase is also contained inside. It is suggested that the solidification to α (hcp-A3) phase occurred by the peritectic reaction between the primary β (bcc-A2) dendrites and the liquid: L+β→L+β+α. The rounded grains, on the other hand, are of β ₂ phase in which acicular α or α ₂ laths are precipitated with the Burgers orientation relationship. Antiphase domain boundaries in the β ₂ matrix are intersected by α(α ₂) laths. It is interpreted that the α(α ₂) laths were formed by the solid-state transformations: β ₂→β ₂+α→β ₂+α ₂. The formation of the two different microstructures in the powder particles is rationalized in terms of the changes in local composition of the liquid phase during the rapid solidification process.     

Structural properties in flame quenched Cu−9Al−4.5Ni−4.5Fe alloy

The flame quenching process has been employed to modify the surfaces of a commercial marine propeller material, aluminum bronze alloy (Cu−9Al−5Ni−5Fe), and the material’s microstructure and hardness properties have been studied. The thermal history was accurately monitored during the process at various surface temperatures and holding times. XRD and EDX analyses have shown that aboveT _β temperature the microstructure consisting of α and κ phases changes into α and β’ martensite due to an eutectoid reaction of α+β→κ and a martensitic transformation of β→β’. The β’ martensite phase formed has a face-centered cubic (FCC) crystal structure with typical twinned structure. The hardness of the flame-quenched layer having the α+β’ structure is similar to or lower than that of the α+κ structure, depending highly on the size and distribution of β’ and κ phases. It is noted that the sliding wear resistance of the flame-quenched layer is enhanced with the formation of β’ martensite.     

Phase transformation of a CuZnAl alloy during friction

Changes in compositions and microstructures in the surface layer of a Cu-Zn-Al shape memory alloy during friction were investigated by means of scanning electron microscopy, electron probe microanalysis (EPMA), X-ray diffraction, and transmission electron microscopy. It has been found that in friction, single M18R martensite β′ transforms into α+β, with the shape and distribution of the new phases varying with the conditions of friction. In mild friction, Zn and Al diffuse over short distances along the sliding direction, resulting in a structure in which needlelike β and α alternate along the sliding surface. In severe friction, where higher surface temperatures and temperature gradients are expected, Zn and Al make long-distance uphill diffusion from inside toward the surface. Consequently, Zn and Al are concentrated on the surface and depleted in the subsurface, and the phase layers present from the outmost surface inward are β, α, α+β, cross-martensite, and the matrix.     

Partial phase diagram of the Ti-Al binary system

The equilibrium temperature-composition coordinates of the β/(β + α), (β + α)/α,α/(α + γ), and (α γ @#@)/γ phase boundaries were determined for the binary Ti-Al phase diagram through the temperature range 1150 to 1400 °C by means of diffusion couples with subsequent EPMA examination. This was supplemented with microstructural examination of a two-phase alloy. No peritectoidal decomposition reaction of the type α→ β + γ reported by Murray [88Mur] at 1280 °C was found. Results indicate that the a phase persists to temperatures well above 1400 °C. The phase boundaries from the present work are in agreement with the Mc Cullough [88Mcc] binary phase diagram.     

Scientific aspects of the choice of the regulated structural state and texture of “marine” titanium alloys

Conclusions  

Tunnel scanning microscopy and spectroscopy in studying Fe-Cr stainless steels

Scanning tunnel microscopy (STM) and scanning tunnel spectroscopy (STS) are used in studying highly pure iron and chromium and Fe-x%Cr (2.4 ≤ x ≤ 60 wt %) alloys at the air boundary, as well as some of the alloys in 0.01 N H₂SO₄. The obtained probability coefficients (α) of the electron tunnel transfer from a specimen to the needle and the slopes (β) of the logarithmic U _t /log (I _t ) dependences as functions of the chromium content in Fe-Cr alloys confirm the critical compositions of the alloys containing ∼6.5 and 10–13% chromium, which is in accord with the results of the steady-state and transient electrochemical measurements. A correlation between these critical compositions of the alloys and sharp changes in the histograms of α and β values is observed. Pronounced extreme properties of the Fe-25.2% Cr alloy surface, which corresponds to the inclusion of Cr atoms in tetrahedral voids of the alloy crystal lattice, are noticed. Original Russian Text ? E.V. Trofimova, E.V. Kasatkin, I.I. Reformatskaya, 2006, published in Zashchita Metallov, 2006, Vol. 42, No. 3, pp. 245–255. The work was financially supported by the Russian Foundation for Basic Research, project no. 04-03-32337.     

Structure of commercial α + β brasses

The chemical composition of phases and the structure of commercial α + β brasses are studied. The effect of heat treatment modes on the amount and morphology of α-and β′-grains and the effect of the content of iron on the composition and quantitative characteristics of silicides are determined. Practical recommendations on the choice of treatment mode and correction of chemical composition for obtaining optimum process and operating properties are suggested. __________ Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 2, pp. 23–29, February, 2007.     

Processing and properties of mechanically alloyed Ni(Fe)Al-Al2O3-AlN

Ni(Fe)Al powders containing a homogeneous distribution of in-situ formed A1N and A1₂O₃ dispersoids were produced by a mechanical alloying process in a controlled atmosphere using a high energy attrition mill. The powders were successfully consolidated by either the hot pressing or hot extrusion process. The phase information investigated by TEM, XRD and a modified TGA analysis reveals that Fe can be soluble up to 19.2% to the NiAl phase(β) at room temperature after the MA process. Subsequent thermomechanical treatment under specific conditions was tried so as to induce a secondary recrystallization to improve the high temperature properties. However, no clear evidence of abrupt grain coarsening was obtained, presumably due to the restricted condition for the boundary break-away mechanism in this material. The mechanical properties in terms of strength at room temperature as well as at high temperatures were shown to have improved through the addition of AlN, and room temperature ductility was also shown to have improved by the precipitation of the second phase of α in this material.     

Refining of the microstructure of cast intermetallic alloy Ti-43% Al-X (Nb, Mo, B) with the help of heat treatment

The microstructure and high-temperature mechanical properties of cast alloy Ti-43% Al-X (Nb, Mo, B) belonging to the new class of β-hardening γ-TiAl + α₂-Ti₃Al alloys is investigated. The alloy in the cast state and after a heat treatment promoting structure refinement is studied by the methods of light and scanning electron microscopy, microscopic x-ray spectrum analysis, and differential thermal analysis. The mechanical characteristics are evaluated in tensile tests in air at 1000 and 1100°C. __________ Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 2, pp. 38–41, February, 2006.     

Densities of solid aluminum-magnesium (Al-Mg) alloys

Densities of solid Al and Al-Mg alloys were measured by the dilatometric method for seven compositions at mole fractions of magnesium: 0.015, 0.05, 0.1, 0.125, 0.15, 0.20, and 0.25. A curvilinear dependence of density on temperature (room temperature up to 800 K) was observed for all investigated alloys. Results are described by polynomials of the second degree. The molar volumes of Al-Mg alloys were calculated from the density measurements. It has been found that the densities of solid Al-Mg alloys show negative deviations from linearity and the molar volumes exhibit positive deviations from ideal behavior for all samples in the experimental concentration range. The density of the β(Al₃Mg₂) phase along the (α(Al) + β)/β boundary was calculated and is described by a temperature-dependent polynomial.     

Densities of solid aluminum-magnesium (Al-Mg) alloys

Densities of solid Al and Al-Mg alloys were measured by the dilatometric method for seven compositions at mole fractions of magnesium: 0.015, 0.05, 0.1, 0.125, 0.15, 0.20, and 0.25. A curvilinear dependence of density on temperature (room temperature up to 800 K) was observed for all investigated alloys. Results are described by polynomials of the second degree. The molar volumes of Al-Mg alloys were calculated from the density measurements. It has been found that the densities of solid Al-Mg alloys show negative deviations from linearity and the molar volumes exhibit positive deviations from ideal behavior for all samples in the experimental concentration range. The density of the β(Al₃Mg₂) phase along the (α(Al) + β)/β boundary was calculated and is described by a temperature-dependent polynomial.     

Effects of alloying elements on the mechanical properties and corrosion behaviors of 2205 duplex stainless steels

The effects of alloying elements on the microstructure, mechanical properties, and corrosion behaviors of duplex stainless steels (DSSs) have been investigated in this study. Experimental alloys were prepared by varying the concentrations of the constituent elements in DSSs. Hot ductility test, tensile test, charpy impact test, and corrosion test were performed to evaluate the properties of the experimental alloys. The results showed that the extent of edge cracking of DSSs increased with the increasing value of the crack sensitivity index (CSI). The higher the hot ductility index (HDI) was, the better the hot ductility of DSSs achieved. Austenite (γ) stabilizer generally caused a decrease in the strength and an increase in the charpy impact absorbed energy of the stainless steel. On the contrary, ferrite (α) former exerted its beneficial effect on the strength but became detrimental to the toughness of DSSs. The presences of sulfur and boron also caused a decrease in the impact energy, but nitrogen and carbon hardly affected the toughness within the concentration range tested in this study. The value of pitting nucleation potential (E _np ) of different nitrogen contents in 3.5 wt.% NaCl solution at room temperature was almost the same, but the value of pitting protection potential (E _pp ) among these alloys was increased with increasing the content of nitrogen. The susceptibility to stress corrosion cracking (SCC) of DSSs was high when tested in boiling 45 wt.% MgCl₂ solution. On the other hand, the time to failure of the experimental steels in 40 wt.% CaCl₂ solution at 100 °C was longer than that in MgCl₂ solution. Nitrogen could affect the SCC behavior of DSSs in CaCl₂ solution through the combinative effects by varying the pitting resistance and the slip step dissolution. An optimum nitrogen (N) content of 0.15 wt.% was found where the highest SCC resistance could be obtained. Although γ phase exhibited better resistance to SCC, cracks were found to penetrate through α and γ grains or to propagate along the α/γ interface. As a result, a mixed transgranular plus intergranular mode of fracture surface was observed.     

Effect of Phase Composition on the Resistance of Hydrogen-Charged Ti – 6% Al Alloy to Compressive Strain

Effect of hydrogen (0.004 – 1.0%) on the ductility and strain resistance of titanium alloy Ti – 6% Al in a temperature range of 400 – 1050°C at a deformation rate of 5 × 10^{– 3}, 5 × 10^{– 2}, and 5 × 10^{– 1} sec^{– 1} is studied. The relation between the quantitative and qualitative phase compositions forming in the alloy as a result of its alloying with hydrogen and the main parameters of the deformation process are determined. Hardening and softening of the alloy in the single-phase - and -ranges and the double-phase ( + )-range are investigated. The temperature range of maximum ductility of hydrogen-charged alloy Ti – 6% Al is determined.     

Phase stability among the α(A1), β(A2), and γ(D83) phases in the Cu-Al-X system

The phase equilibria among the α(A1), β(A2), and γ(D8₃) phases in the Cu-Al-X systems (X: Ti, V, Mn, Fe, Co, Ni, Zn, Sn, and Sb) at 700 and 800 °C were investigated, and the effects of these alloying elements on the phase boundaries, homogeneous solid solution ranges, and tie-lines among the α, β, and γ phases were accurately determined by the diffusion couple method. The phase stabilities among α, β, and γ phases are discussed in terms of the partition coefficient. In addition, a modified Guillet’s method, which is used to estimate the effect of alloying elements on microstructure, is proposed by taking into account the change of equivalent coefficient with the phase fraction, and applied to predict the effect of alloying elements on the microstructure of the Cu-Al base alloy.     

The phase evolution and microstructural stability of an orthorhombic Ti-23Al-27Nb alloy

The phase evolution and microstructural stability were studied for an orthorhombic Ti-23Al-27Nb alloy. Monolithic sheet materials were produced through conventional thermomechanical processing techniques comprising nonisothermal forging and pack rolling. Phase evolution studies showed that, depending on the heat treatment schedule, this alloy may contain several constituent phases including: α₂ (ordered close-packed hexagonal D0₁₉ structure), B2 (ordered body-centered cubic, bcc), β (disordered bcc), and O (ordered orthorhombic based on Ti₂AlNb). Differential thermal analysis studies indicated that the B2 transus temperature was 1070 °C. Heat treatment and transmission electron microscopy studies showed that the α₂+B2 phase field extended between 1010 and 1070 °C. From 875 to 975 °C, a two-phase O+B2 field existed. Sandwiched between these two-phase regimes was a narrow three-phase α₂+B2+O field. Below 875°C, an O+β field existed. All heat treatments at or above 875°C, followed by quenching, resulted in equiaxed microstructures. However, below 875 °C, the B2 phase transformed into a mixture of O and bcc phases with lenticular morphologies. Cellular precipitation of O+β platelets at O/B2 and α₂/B2 grain boundaries occurred depending on solutionizing and aging temperatures, which is explained by the compositional gradient between the bcc phases.     

The phase evolution and microstructural stability of an orthorhombic Ti-23Al-27Nb alloy

The phase evolution and microstructural stability were studied for an orthorhombic Ti-23Al-27Nb alloy. Monolithic sheet materials were produced through conventional thermomechanical processing techniques comprising nonisothermal forging and pack rolling. Phase evolution studies showed that, depending on the heat treatment schedule, this alloy may contain several constituent phases including: α₂ (ordered close-packed hexagonal D0₁₉ structure), B2 (ordered body-centered cubic, bcc), β (disordered bcc), and O (ordered orthorhombic based on Ti₂AlNb). Differential thermal analysis studies indicated that the B2 transus temperature was 1070 °C. Heat treatment and transmission electron microscopy studies showed that the α₂+B2 phase field extended between 1010 and 1070 °C. From 875 to 975 °C, a two-phase O+B2 field existed. Sandwiched between these two-phase regimes was a narrow three-phase α₂+B2+O field. Below 875°C, an O+β field existed. All heat treatments at or above 875°C, followed by quenching, resulted in equiaxed microstructures. However, below 875 °C, the B2 phase transformed into a mixture of O and bcc phases with lenticular morphologies. Cellular precipitation of O+β platelets at O/B2 and α₂/B2 grain boundaries occurred depending on solutionizing and aging temperatures, which is explained by the compositional gradient between the bcc phases.     

Phase stability among the α(A1), β(A2), and γ(D83) phases in the Cu-Al-X system

The phase equilibria among the α(A1), β(A2), and γ(D8₃) phases in the Cu-Al-X systems (X: Ti, V, Mn, Fe, Co, Ni, Zn, Sn, and Sb) at 700 and 800 °C were investigated, and the effects of these alloying elements on the phase boundaries, homogeneous solid solution ranges, and tie-lines among the α, β, and γ phases were accurately determined by the diffusion couple method. The phase stabilities among α, β, and γ phases are discussed in terms of the partition coefficient. In addition, a modified Guillet’s method, which is used to estimate the effect of alloying elements on microstructure, is proposed by taking into account the change of equivalent coefficient with the phase fraction, and applied to predict the effect of alloying elements on the microstructure of the Cu-Al base alloy.     

Investigation of fatigue crack initiation in Ti-6Al-4V during tensile-tensile fatigue

Fatigue crack initiation in Ti-6Al-4V has been investigated in high cycle fatigue (HCF) and low cycle fatigue (LCF) regimes at stress ratio R=0.1 using the replication technique. In all four tested α/β microstructures, the crack was initiated by fracture of equiaxed alpha grain. Fractured alpha grains are seen on the fracture surface as flat facets with features characteristics of cleavage fracture. In the regime of low stress amplitudes and in the absence of reverse loading, cleavage fracture contributes to crack initiation and early stages of crack growth in Ti-6Al-4V. This mechanism is discussed in relation to the anomalous mean stress fatigue behavior exhibited by this alloy.