Characterization of NbC and (Nb, Ti)N nanoprecipitates in TRIP assisted multiphase steels

Multiphase steels utilising composite strengthening may be further strengthened via grain refinement or precipitation by the addition of microalloying elements. In this study a Nb microalloyed steel comprising martensite, bainite and retained austenite has been studied. By means of transmission electron microscopy (TEM) we have investigated the size distribution and the structural properties of (Nb, Ti)N and NbC precipitates, their occurrence in the various steel phases, and their relationship with the Fe matrix. (Nb, Ti)N precipitates were found in ferrite, martensite, and bainite, while NbC precipitates were found only in ferrite. All NbC precipitates were found to be small (5–20 nm in size) and to have a face centred cubic (fcc) crystal structure with lattice parameter a = 4.36 ± 0.05 Å. In contrast, the (Nb, Ti)N precipitates were found to have a broader size range (5–150 nm) and to have a fcc crystal structure with lattice parameter a = 8.09 ± 0.05 Å. While the NbC precipitates were found to be randomly oriented, the (Nb, Ti)N precipitates have a well-defined Nishiyama–Wasserman orientation relationship with the ferrite matrix. An analysis of the lattice mismatch suggests that the latter precipitates have a high potential for effective strengthening. Density functional theory calculations were performed for various stoichiometries of NbC_x and Nb_xTi_yN_z phases and the comparison with experimental data indicates that both the carbides and nitrides are deficient in C and N content.     

Neodymium-rich precipitate phases in a high-chromium ferritic/martensitic steel

Neodymium being considered as nitride forming element has been used in a design of advanced ferritic/martensitic (FM) steels for fossil fired power plants at service temperatures of 630 °C to 650 °C to effectively improve the creep strength of the steels. To fully understand the characteristics of neodymium precipitates in high-Cr FM steels, precipitate phases in an 11Cr FM steel with 0.03 wt% addition of Nd have been investigated by transmission electron microscopy. Three neodymium phases with a face-centered cubic crystal structure and different composition were observed in the steel. They consisted of neodymium carbonitride with an average lattice parameter of 1.0836 nm, Nd-rich carbonitride mainly containing Mn, and Nd-rich MN nitride mainly containing Mn and Co. Other three Nd-rich and Nd-containing phases, which appear to be Nd-Co-Cr/Nd-rich intermetallic compounds and Cr-Fe-rich nitride containing Nd, were also detected in the steel. Nd-relevant precipitates were found to be minor phases compared with M₂₃C₆ and Nb/V/Ta-rich MX phases in the steel. The content of Nd in other precipitate phases was very low. Most of added Nd is considered to be present as solid solution in the matrix of the steel.     

ELECTRON DIFFRACTION PATTERNS ANALYSIS OF PRECIPITATES IN LD_(10) ALLOY

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Study on the precipitation of M2C in high Co-Ni alloy steel

High CoNi ultrastrength alloy steel is a typical tempered martensite steel in which the secondary hardening is accomplished by the precipitation of fine-scale alloy carbides with a black-white contrast in a bright-field image until peak hardening. The crystal structure of precipitates is well determined as hexagonal M₂C by microbeam diffraction. From their lattice image in a high-resolution transmission electron microscope, M₂C carbides are shown to be completely coherent with the ferrite matrix completely and to have their own structure.     

Micro-Superplastic Behavior of Copper Oxide in AgCuO CompositesEI北大核心CSCD

In the investigation of AgCuO composites, we have found a type of copper oxide particles with a superplastic deformability behavior, which is similar to that of metals. To find the reason of the deformability of the copper oxide particles. SEM, STEM and TEM were used to analyze their crystal structures in AgCuO composites, The results show that the copper oxide particles with micro-superplasticity in the composites have cubic crystal structure, and their maximum elongation can be up to 300%, The copper oxide particles with no micro-superplasticity in the composites have monoclinic crystal structure. But the micro-superplastic behavior and mechanism of the copper oxides with the cubic crystal structure are not clear for the time being, the further investigation could be needed.     

一种[011]取向镍基单晶合金拉伸蠕变中的组织演化

在1040℃,137MPa下对[011]取向镍基单晶合金进行蠕变曲线测定,采用SEM观察热处理及蠕变后样品不同晶面的组织形貌,研究了[011]取向镍基单晶合金在蠕变期间的组织演化特征,并分析了组织演化的规律及影响因素。结果表明:取向差为4°的[011]取向镍基单晶合金经完全热处理后,组织是立方γ′相以共格方式嵌镶在γ基体相中,并沿<100>取向规则排列;在拉伸蠕变期间,合金中γ′相转变成与[001]取向平行的纤维状筏形组织。由于施加拉伸载荷,使立方γ′相中的(100)晶面及γ基体相承受挤压力,可排斥较大半径的Al、Ta原子,而在(001)晶面产生较大的晶格扩张应变,可诱捕较大半径的Al、Ta原子,因而促使γ′相沿[001]取向定向生长成为纤维状筏形组织。在外加应力作用下,不同晶面γ′/γ两相界面的应变能密度变化是促使发生元素扩散和γ′相定向粗化的驱动力。     

Micro-Superplastic Behavior of Copper Oxide in AgCuO Composites

采用扫描电镜、扫描透射电镜、透射电镜等分析手段,对银氧化铜复合材料中氧化铜颗粒的结构进行了分析。结果表明,可变形氧化铜的晶体结构为立方结构,其变形量最大可达300%;不变形的氧化铜颗粒的晶体结构为单斜结构。但是对于这种立方氧化铜的变形机理尚不明确,需要进一步的研究     

Coherency loss of Al3(Sc,Zr) precipitates by deformation of an Al–Zn–Mg alloy

Al alloys with additions of Sc and/or Zr exhibit a reasonably stable grain structure due to a uniform distribution of coherent Al₃(Sc,Zr) precipitates that forms at temperatures >300 °C. These precipitates are stable up to the solution treatment temperature and are able to pin subgrain and grain boundaries, inhibiting grain coarsening. The crystallographic structure of these precipitates presents a L1₂ superstructure coherent with the face-centred cubic Al matrix. Changes in the orientation relation between precipitates and the matrix are described in deformed, recovered and partially recrystallized samples of extrusions of AW7010 (AlZn6Mg2Cu2). The coherency of the intracrystalline Al₃(Sc,Zr) precipitates present in the extrusions is lost by severe deformation performed by an equal channel angular pressing process, which produced a fine-grained microstructure. The deformed sample recovers, forming a subgrain structure with restored coherency of the Al₃(Sc,Zr) precipitates. Rapid heating to 470 °C causes partial secondary recrystallization, which transforms the precipitates within the recrystallized grains into incoherent groups of particles that maintain their original orientation with each other.     

Phase transformation of Cu precipitates from bcc to fcc in Fe–3Si–2Cu alloy

Phase transition of Cu precipitates during aging of an Fe–3Si–2Cu alloy was studied by transmission electron microscopy. The precipitation of 3–5-nm-sized body-centered cubic (bcc) Cu in ferrite matrix was confirmed by high-angle annular dark-field scanning transmission electron microscopy imaging. The bcc Cu precipitates transformed to 9R Cu as they grew. Many 9R Cu precipitates were twinned, but untwinned 9R Cu particles were also observed. The 9R Cu transformed to twinned face-centered cubic (fcc) Cu by the glide of ±a/3 [1 0 0]_9R Shockley-type partial dislocations. Formation of the 3R structure previously reported could not be confirmed in this study. Finally, twins in fcc Cu precipitates disappeared to form stable fcc Cu particles. The importance of electron beam-orientation-dependent moiré fringes in the correct identification of Cu structure is discussed in detail.     

Control of microstructures and properties of a phosphorus-containing Cu-0.6 Wt.% Cr alloy through precipitation treatment

A phosphorus-containing Cu-0.6 wt.% Cr alloy was solution treated and then aged using various combinations of time and temperature. The influence of aging time and temperature on microstructures and properties of this alloy was investigated by means of an analytical transmission electronic microscope (TEM) and measurements of hardness and electrical conductivity. It was found that neither underaging nor overaging could harden the alloy significantly. The microstructure corresponding to peak hardness was characterized by very fine and coherent precipitates. Increasing aging time and temperature caused the precipitates to grow into rodlike incoherent Cr particles having body-centered cubic (bcc) crystal structure, but aging temperature influenced the microstructures and properties more intensively than did aging time. Undissolved body-centered tetragonal (bct) Cr₃P particles, which were found in both assolution-treated and as-aged structures, were not harmful to electrical conductivity and might act as obstacles impeding dislocation motion. As compared to a Cu-0.65 wt.% Cr alloy not containing phosphorus, the studied alloy needs aging at a higher temperature to reach peak hardness.     

Structural analysis of a new precipitate phase in high-temperature TiNiPt shape memory alloys

Aging of the high-temperature shape memory alloy Ti₅₀Ni₃₀Pt₂₀ (at.%) results in precipitation of a previously unidentified phase, which plays a key role in achieving desirable shape memory properties. The precipitate phase has been analyzed with electron diffraction, high-resolution scanning transmission electron microscopy and three-dimensional atom probe tomography. The experimental observations show that the precipitates have unique crystallography due to their non-periodic character along one of the primary crystallographic directions. It will be shown that the structure can be explained in terms of crystal intergrowth of three variants of a monoclinic crystal. The monoclinic crystal structure is closely related to the high-temperature cubic B2 phase; the departure of the structure from the B2 phase can be attributed to ordering of Pt atoms on the Ni sublattice and relaxation of the atoms (shuffle displacements) from the B2 sites. The shuffle displacements and the overall structural refinement were deduced from ab initio calculations.     

Review on ω Phase in Body-Centered Cubic Metals and Alloys

An to phase with a primitive hexagonal crystal structure has been found to be a common metastable phase in body-centered cubic(bcc) metals and alloys.In general,to phase precipitates out as a high density of nanoscale particles and can obviously strengthen the alloys;however,coarsening of the co particles significantly reduces the alloy ductility.The co phase has coherent interfacial structure with its bcc matrix phase,and its lattice parameters are a_ω =2~(1/2)×a_(bcc) and C_ω= 3~(1/2)/2 ×a_(bcc).The common {112}(11 l)-type twinning in bcc metals and alloys can be treated as the product of theω→ bcc phase transition,also known as the ω-lattice mechanism.The ω phase's behavior in metastable β-type Ti alloys will be briefly reviewed first since the ω phase was first found in the alloy system,and then the existence of the ω phase in carbon steels will be discussed.Carbon plays a crucial role in promoting the ω formation in steel,and the ω phase can form a solid solution with various carbon contents.Hence,the martensitic substructure can be treated as an α-Fe matrix embedded with a high density of nanoscale ω-Fe particles enriched with carbon.The recognition of the ω phase in steel is expected to advance the understanding of the relationship between the microstructure and mechanical properties in bcc steels,as well as the behavior of martensitic transformations,twinning formation,and martensitic substructure.     

Study of Nb-N alloys after quenching and aging

Conclusion Maximum strengthening for alloys of the Nb-N system is observed after quenching from 1600°C and subsequent aging at 1300°C, which is due to formation of highly dispersed precipitates of stable NbN nitride phase uniformly distributed in the matrix. Generation of nitride phase particles during aging is accompanied by occurrence of elastic fields due to distortion of the crystal lattice of the matrix.N. É. Bauman MGTU. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 7, pp. 15–19, July, 1990.     

Crystal structure of ultrathin lamellar precipitates

The crystal structure of lamellar precipitates formed in alloys 1213 (Al-Cu-Ag), V-1461 (Al-Cu-Li) and V-1469 (Al-Cu-Li-Ag) has been studied during age hardening. The experimental studies have been performed using transmission electron microscopy. The precipitates have {111} habits and thicknesses of several atomic planes. In diffraction patterns, these plates give a system of diffuse streaks. These ultrathin plates scatter electrons similar to two-dimensional crystal lattices. It is shown that thin plates of precipitates in alloys 1213, V-1461, and V-1469 give identical systems of diffuse streaks. The two-dimensional crystal lattices that give the system of these streaks have a hexagonal symmetry with the following orientation relationship: {ie481-1} and the lattice parameter a _f = 0.495 nm (there is no lattice parameter c _f for two-dimensional lattices). A sequence of the steps of reconstruction of the spatial structure of plane precipitates is proposed in terms of their thickness, structures of two-dimensional lattices, and type of precipitates (extrinsic or intrinsic). The influence of Ag on the structure of lamellar precipitates in the V-1469 alloy is discussed.     

Stability of (Ti, M)C (M = Nb, V, Mo and W) carbide in steels using first-principles calculations

The lattice parameters, formation energies and bulk moduli of (Ti, M)C and M(C, Va) with the B1 crystal structure have been investigated using first-principles calculations, where M = Nb, V, Mo and W. The replacement at 0 K of Ti by Mo or W in the TiC lattice is found to be energetically unfavorable with respect to the formation energy. However, it decreases the misfit strain between the carbide and ferrite matrix, a factor which is of critical importance during the early stages of precipitation, thus favoring the substitution of Ti by Mo, as is observed in practice. The effect of Mo in enhancing the coarsening resistance of (Ti, Mo)C precipitates is discussed in terms of its role in the nucleation process, but followed by a more passive contribution during coarsening itself. The role of tungsten has been predicted to have a similar effect to molybdenum on the nucleation and coarsening process. Analysis of precipitates in Ti-, Ti-Mo- and Ti-W-bearing steels shows results consistent with the calculations.     

Three-dimensional model of precipitation of ordered intermetallics

The development of the two-phase (f.c.c.+L1₂) coherent microstructure in the prototype Ni–Al superalloy is studied by using the three-dimensional computer simulation technique. The dynamics and morphology of the microstructure evolution are described by our three-dimensional version of the stochastic time-dependent kinetic equation which explicitly includes the coherency strain, elastic anisotropy and L1₂ ordering of the preciptate phase. The input parameters, the crystal lattice misfit, elastic moduli, interfacial energy and equilibrium compositions of the coexisting phases are taken from the published independent measurements. The simulation results demonstrate that the strain accommodation in the microstructure evolution results in the cuboidal-like precipitates faceted by the {100} planes. The size of the precipitates obtained in the simulation is of the order of 50 nm. The important conclusion is that the precipitates are always single-domain particles with no antiphase boundaries. This effect is associated with the ordered structure of precipitates. It causes the slowing down of the coarsening kinetics since it excludes the agglomeration of the out-of-phase precipitates in one particle. As has been shown previously, the latter is a very important coarsening mode in an absence of ordering.     

Prediction of the morphology of Mg32(Al,Zn)49 precipitates in a Mg–Zn–Al alloy

A geometrical model has been applied to predict the morphology of faceted Mg₃₂(Al, Zn)₄₉ precipitates in a Mg–Zn–Al alloy using the observed orientation relationship (OR) and the lattice parameters of the precipitates and the matrix as inputs. Planes in rational or in irrational orientations with higher densities of good matching sites are more likely to be preferred, which agrees well with experimental observations.     

Geometrical Model of Polymorphous Transformations in Titanium and Zirconium

A geometrical model of transformation of a body-centered cubic lattice of α-phase into a hexagonal close-packed lattice of α-phase is developed with the aim of explaining the special features of the crystal geometry of formation of martensite phases in titanium and zirconium and in alloys based on them. The transformation is described as mutual reconstruction of coordination polyhedra of the cubic and hexagonal lattices through an intermediate configuration of the crystal structure of ω-phase. In the language of algebraic geometry the transition is implemented as reconstruction of an 11-atom cluster that represents a union of three octahedra around a common edge into an 11-atom cluster composed of 11 tetrahedra united over faces. Experimentally observed orientation relations and habit planes at α → ω and β → α transformations are describable by elements of the structure of the mentioned clusters.     

Hydrogen redistribution in the solid solutions ZrV2Dx, 2.2≤x≤2.7.: II. Structure of the intermediate phase: ‘Lattice liquid crystal’. A neutron-diffraction study

We have studied the crystal structure of the uncommon phase with k=0 in ZrV₂D_x, 2.2<x<2.5, which is an intermediate between the hydrogen-disordered phase and two hydrogen superstructures, ZrV₂D_<2 with k=(1/2 1/2 1/2) and ZrV₂D_>2.7 with k=(001). This phase is a primary superstructure combining the features of the disordered phase and, depending on the hydrogen concentration, one or another superstructure with k≠0. Its lattice (translational symmetry) is the same as in the disordered phase, which is k=0. Simultaneously, the lattice sites (the hydrogen arrangement in them) are prototypes of the sites of the subsequent superstructure with k≠0. Specifically, each site of the primary superstructure with k=0 is a mix of the sites with different spatial orientation of the superstructure with k≠0. In this sense the primary superstructure can be considered as a ‘lattice liquid crystal’ whereas usual superstructure with k≠0 is a ‘lattice crystal’. In addition, we have determined the crystal structure of the ‘ordered’ phase with k=(001) in ZrV₂D_2.73. It is a transitional state between the primary superstructure and the regular superstructure with the same k.     

Microstructure Evolution of a Single Crystal Nickel-Base Superalloy During Heat Treatment and Creep

Microstructure evolution of a single crystal nickel-base superalloy during heat treatment and tensile creep at 1010℃ and 248 MPa for 30h was observed and analyzed. Internal stresses because of lattice mismatch between γ and γ‘ phase provided the driving force for γ‘ shape evolution during heat treatment. More than 65 vol. % distorted cubic γ‘ phase keeping coherency with the γ matrix precipitated after solution at 1295℃ for 32h. The shape of γ‘ phase was perfectly cubic with increasing precipitate size during the two-step aging treatment. Due to the applied stress and intemal stress field the continuous γ-γ‘ lamellar structure perpendicutar to the apptied stress was fonmed after 30h tensile creep.