Microstructure and Phase Transformation of a Niobium-rich TiAl-based Alloy Containing Boron and Carbon

Microstructure and phase transformation of Ti46Al8Nb0.5B0.2C alloy have been investigated.X-ray diffraction (XRD),optical microscopy (OM),scanning electron microscopy (SEM) and transmission electron microscopy (TEM) results show that as-cast and hot isostatic pressing (HIP) alloy mainly composed of γ and α 2 phase have fully lamellar microstructure with point-like or ribbon-like TiB 2 distributing in lamellar colony or at grain boundary.The mean size of lamellar colony is about 150 and 450 μm for as-cast and HIP alloy,respectively.The lamellar spacing is about 550 and 600 nm for as-cast and HIP alloy,respectively.It has been found that cooling rates and quenching temperatures have significant effect on phase transformation of Ti46Al8Nb0.5B0.2C alloy.When the alloy is treated at 1380℃ for 1 h and cooled from α domain,water cooling leads to complete α→α 2 transformation,oil cooling leads to predominant α→α 2 and part α→γ m transformation,air cooling leads to α→α + γ p2 →L(α + γ) →L(α 2 + γ) transformation,and furnace cooling leads to α→α + γ p3 →L(α + γ) →L(α 2 + γ) transformation.However,when the alloy is treated at 1400℃ for 1 h and cooled from α domain,water cooling leads to predominant α→α 2 and part α→α + γ p4 →γ m transformation,oil cooling leads to α→α + γ p5 →γ m transformation,air cooling leads to α→α + γ p6 →L(α + γ) →L(α 2 + γ) transformation,and furnace cooling leads to α→α + γ p7 →L(α + γ) →L(α 2 + γ) transformation.Microstructural evolution of the alloy during various heat treatments has been examined and the phase transformation mechanisms have been elucidated.Based on the experimental observation,schematic CCT diagrams for the alloy have been given.     

Induced formation of polar phases in poly(vinylidene fluoride) by cetyl trimethyl ammonium bromide

Poly(vinylidene fluoride) (PVDF) has been widely used as transducers, actuators,and sensors due to its exceptional electroactive properties among polymer materials and the electroactive properties of PVDF heavily depend on its polymorphs. The effects of cetyl trimethyl ammonium bromide (CTAB) on the crystallization behavior of PVDF during isothermal crystallization and annealing process were studied, and the results showed that the polar phases (β, γ) were induced by CTAB when PVDF was isothermally crystallized at 145–160 °C. Increasing the isothermal crystallization temperature resulted in an increased γ-phase formation with a concomitant decrease of α and β phases. A 100 % γ-phase formation occurred when the sample isothermally crystallized at 155 and 160 °C. The crystalline phase transition of the sample annealed at 160 °C for various time revealed that CTAB induced the transformation from α to γ′ phase and a higher concentration of CTAB resulted in shorter transformation time, as well as higher transformation degree.     

Growth Behavior of α Phase in Ti-5.6Al-4.8Sn-2.0Zr-1.0Mo-0.35Si-0.7Nd Titanium Alloy

The thermal stability of the microstructure of a near-α titanium alloy after aging at 750℃ was investigated using optical microscope and transmission electron microscope as well as composition analysis. Aging treatment brings about significant coarsening of grain boundary α and α platelets within the colonies for martensitic microstructure. The observed changes are related to the growth steps or ledges of the interface and lamellar termination presented in the microstructure. The composition analysis of the coarsened α plate is consistent with the growth kinetics. The α Widmanstaetten plates were coarsened due to the movement of α /β interface for Widmanstaetten microstructure, and the phase boundaries of primary α（αp） phase directly moved into the transformed β for bimodal microstructure.     

Stereological characterization of γ′ phase precipitation in CMSX-6 monocrystalline nickel-base superalloy

The purpose of this investigation was to study in detail the means to quantitatively evaluate γ′ phase precipitation. Many of the mechanical properties of superalloys are directly influenced by the presence of the γ′ (gamma prime) precipitate phase dispersed in a γ matrix phase. The γ′ precipitates act as effective barriers to dislocation motion and restrict plastic deformation, particularly at high temperatures. Due to this, it is essential to accurately quantify the γ′ precipitate size, volume fraction and distribution. Investigations based on quantitative metallography and image analysis were performed on a monocrystalline nickel-base superalloy taking into consideration various γ′ precipitate sizes present in that alloy microstructure. The authors of the present paper propose a new method of quantifying the total volume fraction of the γ′ phase applying images of the microstructure with γ′ phase precipitates registered using light microscopy, scanning electron microscopy (at two different magnifications) and scanning transmission electron microscopy.     

Ni-25Cr-20Co合金时效组织结构及性能研究

在Thermo-Calc热力学软件模拟计算基础上,采用光学显微镜、扫描电子显微镜、能谱检测和透射电子显微镜研究了Ni-25Cr-20Co合金在长期时效过程中析出相的变化情况及对性能的影响,理论分析了γ′相颗粒粗化对合金拉伸变形过程中第二相与位错交互作用机制的影响。结果表明:经750℃时效后合金中析出MC、M_(23)C_6和γ′相,γ′相的体积分数约为16%。长期时效后,γ′相颗粒的平均尺寸与时间t符合LSW理论,受溶质原子扩散及γ/γ′界面能的影响。时效后合金的拉伸强度明显增加,随时效时间的延长,拉伸强度逐渐降低。随γ′相的粗化,拉伸变形过程中第二相与位错交互作用的机制由位错热攀移机制→位错切割机制→Orowan绕越机制转变为位错热攀移机制→Orowan绕越机制→位错切割机制。     

Investigation on the Microstructure Homogenization in a TiAl Based Alloy Prepared by Elemental Powder Metallurgy

1.IntroductionTiAlbasedalloyhasdrawnintensiveattentioninhightemperaturestructuralmaterialarea,be-causeofitslowdensity,excellellthightemperaturepropertiesf1~2].However,itspoorworkabilityisanobstacleforitsapplication.Inordertoovercomethisdifficulty,extensiveworkhasbeenconductedtoim-proveitsworkabi1ityandnearnetshaping.Elementalpowdermetallurgy(EPM)hasbeenusedforfabricatingTiAl-basedalloys,becauseofitstwoapparentadvantages'lowcostandconvenientad-ditionofalloyelemellts[3].Thistechniqueincludesr…     

The role of microstructural morphology on creep properties of a [0 0 1] oriented nickel‐base single crystal superalloy

The experimentally observed microstructure of nickel‐base single crystal alloy consists of a large volume of cuboidal γ′ precipitates coherently embedded in the γ matrix. In calculation, a representative volume element is usually used to represent the whole structures due to the regular γ/γ′ topological structures. Here, three experimentally found microstructures have been extracted to generate the representative volume elements. One is constituted by one cuboidal γ′ phase surrounded by γ phase. The other two consisted of two cuboidal γ′ phases and one rectangle γ′ phase with different arrangement of the two γ′ phases. The misfit stress is taken into consideration by different thermal expansion coefficients of the two phases. The influences of different microstructures on the macro‐creep strain evolution, rafting and stress distributions are discussed.     

Stoichiometric NbTiAl_3 (γ1 Phase) Alloy in Ti-Al-Nb Ternary System

The microstructure and phase constitution in stoichiometric NbTiAl3 (γ1 phase) alloy treated at 1000℃ were examined by metallography and X-ray diffraction. The alloy microstructure is mainly γ1 phase containing η second phase [(Ti,Nb)Al3] less than 1%. DTA analysis shows no phase transformation from room temperature to 1200℃. In the diffusion couple of NbTiAl3 with 7-TiAl compound, clear phase boundary and composition jump exist between γ1 and γ phase.These results further confirm the existence of γ1 single phase at 1000℃ in Ti-Al-Nb ternary system.     

A high-resolution-electron-microscopy study of γ/γ′-α directionally solidified eutectic alloy

A master alloy with eutectic compositions of Ni-30.26Mo-6.08Al-1.43 V (wt%) has been directionally solidified (DS) into γ/γ′-α alloy. The microstructural as-ageing treatment was studied by means of high resolution electron microscopy (HREM). A majority of α fibres still display the Bain orientation relationship with the γ′/γ matrix. In a few cases, however, the so-called Nishiyama-Wasserman (NW) orientation relationship is found in specimens aged at 850 °C for 2000 h. Different microdomain structures of the γ phase, corresponding to different ageing temperatures, were revealed. Orthorhombic Ni₃Mo phase, with a size of tens of nanometres, was found to precipitate inside α fibres after ageing at both 850 and 650 °C. Occasionally, an γ′-Ni₃Al phase with lamellar twin structure was also found to coexist with Ni₃Mo precipitate inside the α fibres. The orientation relationships between the precipitates and the α fibres were determined. Energy dispersive X-ray analysis (EDX) showed that the precipitation of Ni₃Mo and Ni₃Al is due to solid solution of Ni and Al in the α fibres.     

Quantitative analysis of the microstructure of transitional region under multi-heat isothermal local loading forming of TA15 titanium alloy

In this paper, an analogue experiment was carried out to study the effect of processing parameters including deformation temperature, deformation degree, cooling mode and loading pass on the microstructure of transitional region under isothermal local loading forming of TA15 titanium alloy. The volume fraction, grain size and aspect ratio of primary α phase of transitional region were quantitatively characterized. It is found that deformation temperature and deformation degree also have interaction on the microstructure evolution of transitional region under isothermal local loading forming. At a certain deformation degree, primary α grain size increases first and then decreases with increasing temperature. However, primary α grain size varies little with deformation degree at higher temperature (in upper two phase region) but increases firstly and then decreases with deformation degree at lower temperature (in lower two phase region). Primary α aspect ratio increases with deformation degree at lower temperature but varies little at higher temperature. The morphology of transformed structure in β matrix is greatly influenced by deformation temperature and less influenced by deformation degree under air-cooling. The precipitated Widmanstatten α phase in β matrix is in lamellar form and arranges in colonies under air-cooling, but it is in thinner acicular form and distributes disorderly under water quenching. Loading pass has little influence on the morphology of microstructure.     

Interface conditions during mixed-mode phase transformations in metals

A fast three-dimensional phase transformation model is formulated for the transformation from ferrite to austenite in low-carbon steel. The model addresses the parent microstructure, the nucleation behaviour of the new phase and the growth of the new phase. During the growth, the interface velocity of the ferrite grains is calculated using a mixed-mode growth model. The simulated transformation kinetics is compared with experimental kinetics for an Fe–C–Mn steel for four different cooling rates. In general, the model predicts the kinetics quite well. In addition, the mixed-mode character of the transformation is shown for the different cooling rates.     

Modifications structurales du nitrure de silicium en fonction de la temperature

The structural evolution of α and β - Si₃N₄ has been studied up to 900°C by means of the time of flight neutron diffraction method.It has been shown that the relative evolution of lattice parameters is the same for both phases; the modifications versus temperature are isotropic along the cristallographic axas a and c.However, the atomic positions, bond angles and bond lengths at different temperatures, as well as the volume evolution of SiN4 tetrahedra compared to that of the unit cell volume, suggest a less stability of α Si₃N₄ with regard to the β phase which explains the α → β transformation at higher temperatures.     

Equilibrium Phase Constituents and Discontinuous Precipitation Behavior of Al-Zn-Cu Alloy with Symmetrical Composition

Through microstructure observation and X-ray diffraction analysis, the equilibrium phase constituents of Al-Zn alloy that contains 2 at. pct Cu at room temperature have been determined as AI-based solid solution (α), Zn-based solid solution and AlCu3Zn phase (T′-phase), which are different from α phase, Zn phase and CuZn4 phase originally believed. It is determined that the products of discontinuous precipitation transformation below 277℃ are not the equilibrium phase constituents, but the metastable phases made up of α phase, Zn phase and CuZn4 phase. The phase constituents after discontinuous precipitation of AlZn-2Cu alloy would transform to the ones in equilibrium status: Al-based solid solution (α) in fcc structure, Zn-based solid solution in hcp structure and Al4Cu3Zn phase (T′-phase) ultimately through plastic deformation at room temperature and re-heating treatment below 277℃.     

Creep behaviour of single crystal nickel base superalloy

Abstract

The creep activation energy and structure constant at the different creep stages have been calculated, and the microstructures have been observed by SEM and TEM. The results showed that the internal stress σo decreased with an increase in temperature. Over the stress and temperature range, there are different activation energies, time exponents, and structure constants at different creep stages. The change in microstructure has an influence on creep resistance in this superalloy (Ni-6.0AI-7.0Ta-8.5Mo, wt-%). It is shown that the dislocation climb is the major deformation mechanism during tensile creep stages I and II, but during the tertiary stage, the creep resistance decreased as a result of dislocations shearing into the γ′ rafts. Creep fracture occurs mainly by the cavities and microcracks produced at the γ′/γ phase interface due to the interaction of multislips.     

Electron diffraction study of the low temperature phase in lead orthovanadate

Using electron diffraction and transmission electron microscopy it is shown that lead orthovanadate can follow two different transformation paths on cooling. The high temperature γ-phase transforms according to the scheme γ → β → α. The metastable γ′ phase, which results from frozen-in γ, transforms into a different low temperature phase, the α′-phase. In the literature, two different structures, both called α, were proposed for the low temperature phase. It turns out that both structures (here called α and α′) occur, but under different conditions.     

Formation mechanism of lamellae during β→α transformation in polycrystalline dual-phase Ti alloys

Phase field simulations incorporating contributions from chemical free energy and anisotropic interfacial energy are presented for the β→α transformation in Ti-6Al-4 V alloy to investigate the growth mechanism ofα lamellae of various morphologies from undercooled β matrix.The α colony close to realistic microstructure was generated by coupling the Thermo-Calc thermodynamic parameters of α and β phases with the phase field governing equations.The simulations show that α lamellar side branches with feathery morphology can form under a certain combination of interfacial energy anisotropy and temperature.α lamellae tend to grow slowly at high heat treatment temperature and become wider and thicker as temperature increase from 800 to 900 ℃ provided that the interfacial energy anisotropy ratio kx: ky was set as 0.1: 0.6.Besides,higher interfacial energy anisotropy can accelerate the formation of α lamellae,and the equilibrium shape of α lamellae changes from rod to plate as the interface energy anisotropy ratio kx∶ ky vary from 0.1∶ 0A to 0.1∶ 0.8 under 820 ℃.Experiments were conducted to study the α lamellar side branches in Ti-6Al-4 V (Ti-6.01Al-3.98 V,wt.％) and Ti-4211 (Ti-4.02Al-2.52V-1.54Mo-1.03Fe,wt.％) alloys with lamellar microstructure.Electron backscatter diffraction (EBSD) results show that α lamellar side branches and their related lamellae share the same orientation.The predicted temperature range for α lamellar side branches formation under various interfacial energy anisotropy is consistent with experimental results.     

Influence of α morphology and volume fraction on the stress-induced martensitic transformation in Ti-10V-2Fe-3Al

This paper reports an investigation into the influence of α morphology and volume fraction on the critical stress levels for the stress-induced martensitic transformation in Ti-10V-2Fe-3Al. Control of the stress-induced martensite transformation offers a new route to the optimization of the load bearing capabilities of titanium alloys. Various heat treatments in α + β and β + (α + β) conditions were imposed to study their effect on both microstructure and mechanical properties. The results show that in samples with globular α, the β matrix stability is the main factor controlling the stress-induced martensitic transformation. For samples with acicular α, the “grain size” (i.e. the size of the retained β domain), which governs the mean length and the width of martensitic plates, also plays an important role in such transformations. The linear combination of both effects provides a good prediction of the stress-induced martensite formation stress as a function of the volume fraction and morphology of the α phase.     

Modelling of Phase Transformation of Plain Carbon Steels during Continuous Cooling

A model based on Avrami equation and Scheil‘s additivity rule was proposed to simulate the phase transformation in plain carbon steels during continuous cooling in hot strip mill. In this model, a wide range of composition, cooling rate, primary austenite grain size and retained strain has been taken into account. It can be used to calculate the phase fraction transformed at different temperatures during continuous cooling. The phase equilibrium and transformation starting temperature can be determined by using Thermo-Calc and DICTRA. The simulated results containing the transformation at starting and finishing temperatures, Ae1, Ae3 and the maximum volume fraction for Q235B, were obtained. The calculated phase volume fractions are in good agreement with the experimental results.     

Mechanism and Kinetics of Phase Transformation in Two-phase TiAl-based Alloys

The aged and quenched microstructures of both alloys, Ti-42at-%Al and Ti-45at -%Al,homogenized in the disordered single phase field. were investigated And the results show that the quinched microstructure is a supersaturated single phase of ordered 22. When the supersaturated phase is aged in the two phase range at 1273 and 1373 K, it will transform to a lamellar microstructure of γ+α2. with a discontinuous decomposition mechanism in Ti-42at-%Al alloy and a semicontinuous decomposition mechanism in T1-45at-%Al alloy. With the methods of quantitative metallograph examination and X-ray diffraction analysis. the relationship between the amount of γ, phase precipitation and the time of isothermal transformation is agreed     

Behavior and modeling of flow softening and ductile damage evolution in hot forming of TA15 alloy sheets

Flow softening and ductile damage behavior of TA15 titanium alloy with initial bimodal microstructure were studied using uniaxial hot tensile tests at different temperatures (750–850 °C) and strain rates (0.001–0.1 s^− 1). SEM examination of deformed specimens shows that deformation mainly occurs in β and secondary α phase (α_s). Globularization of α_s is also observed. According to the SEM observations of the cracked specimens, the mechanism of ductile damage is attributed to the breakdown of the compatibility requirements at the α/β interface. Based on the experimental results, a set of mechanism-based unified elastic-viscoplastic constitutive equations have been formulated to model the flow behavior and the damage evolution of TA15 alloy in hot forming conditions. Dislocation density, ductile damage evolution, deformation heat, phase transformation and globularization of the α_s have been modeled. The model constants have been determined by using a Genetic Algorithm (GA)-based optimization method. Furthermore, the proposed constitutive equations were evaluated in terms of correlation coefficient (R), average absolute relative error (AARE), and root mean square error (RMSE). The results indicate that the calibrated predictions, including flow stress, volume fraction of each phase, and fracture strain, are in good agreement with experimental results.