Accurate determination of the number density of G-phase precipitates in thermally aged duplex stainless steel

G-phase precipitation in the ferrite phase in thermally aged duplex stainless steel (DSS) was investigated. A needle-shaped sample of DSS aged at 673?K for 5000?h was observed by transmission electron microscopy (TEM), and subsequently by atom probe tomography (APT). The precipitates of the G-phase observed by TEM corresponded well to clustering atoms observed by APT. On the other hand, regarding the precipitates of the G-phase that formed in an earlier stage of aging, the present study suggests that not all the precipitates can be detected by TEM. A large area of DSS aged at 673?K for 5000?h containing both the ferrite and austenite phases was observed. The number density of precipitates of the G-phase in the ferrite phase was small in the vicinity of the phase boundary and increased with the distance between the phase boundary and each field of view. The number density reached an almost constant value at a distance of approximately 4?µm from the phase boundary. The suppression of G-phase precipitation in the vicinity of the phase boundary is discussed in terms of the depletion of alloying elements that comprise the G-phase.     

Al-Zn-Mg-Cu-Li合金时效过程微结构演化的小角x射线散射研究

应用小角x射线散射技术分析了Al-Zn-Mg-Cu-Li合金在130，150和160℃温度时效24 h析出粒子的微结构参数的变化情况. 粒子的半径随着时效温度的增高而增加，它的比内表面积和体积百分数随着时效温度的增高而减小. 对Porod曲线q³J(q)-q²的分析表明,析出粒子与基体之间有明显的界面. 关键词： 小角x射线散射 Al-Zn-Mg-Cu-Li合金 时效 析出粒子     

Direct observation of Cu interphase precipitation in continuous cooling transformation by atom probe tomography

Atom probe tomography (APT) combined with electron back scatter diffraction and transmission electronic microscopy (TEM) is utilized to characterize the nature of copper precipitation during austenite–ferrite transformation in a continuous cooling high-strength low-alloy steel. The copper precipitation manners in association with the austenite decomposition kinetics are studied. The prevailing microstructure of the continuous cooling steel consists of acicular ferrite (AF), which is formed at an intermediate cooling rate of 10?°C/s. Besides, a limited volume of polygonal ferrite (PF) because of fast cooling rate and a trace of retained austenite are detected. Numerous copper-rich phase is found by TEM observation both in highly dislocated AF and dislocation-free PF. Generally, the copper-rich precipitates have comparatively large sizes and are considered to be formed by interphase precipitation during austenite–ferrite transformation. A high number density of nanometre sized copper-rich clusters that are lack of diffraction contrast in conventional TEM observation are detected by APT. These smaller copper-rich clusters, which are usually located between the linear-arranged copper-rich precipitates, are considered to be formed from supersaturated solid solution after the cessation of austenite–ferrite transformation. That means an ageing reaction for Cu precipitation occurs during continuous cooling transformation. The copper-rich precipitates and clusters are both rich in nickel, manganese and iron.     

Effects of molybdenum on precipitation behaviours in aged cast stainless steels

The effects of Mo on precipitation behaviours in aged cast stainless steels have been investigated. Mo-free CF3 steel and Mo-bearing CF3M steel, both of which consisted of a duplex structure of ferrite and austenite phases, were prepared. Microstructural evolution in the ferrite phase during ageing at 723 K has been observed by transmission electron microscopy (TEM). Precipitates in CF3 steel were identified to belong to the G-phase and possess lattice coherency with the ferrite phase at interphase boundaries. On the other hand, precipitates in CF3M steel were found to be nanodomains of not only to the G-phase but also to another phase enriched in Mo. Some of the nanodomains containing Mo exhibited diffraction patterns having pseudo five-fold symmetry, but others exhibited regular periodicity in high-resolution TEM images. The atomic structure of the Mo-related nanodomains is proposed to be a distorted χ-phase that retains coherency with the matrix.     

Phase decomposition and ordering in Ni-11.3 at.% Ti studied with atom probe tomography

The decomposition behavior of Ni-rich Ni–Ti was reassessed using Tomographic Atom Probe (TAP) and Laser Assisted Wide Angle Tomographic Atom Probe. Single crystalline specimens of Ni-11.3 at.% Ti were investigated, the states selected from the decomposition path were the metastable γ″ and γ′ states introduced on the basis of small-angle neutron scattering (SANS) and the two-phase model for evaluation. The composition values of the precipitates in these states could not be confirmed by APT data as the interface of the ordered precipitates may not be neglected. The present results rather suggest to apply a three-phase model for the interpretation of SANS measurements, in which the width of the interface remains nearly unchanged and the L1₂ structure close to 3:1 stoichiometry is maintained in the core of the precipitates from the γ″ to the γ′ state.     

Combined use of small-angle neutron scattering and atom probe tomography for the analysis of precipitates in a Fe-15 m% Co-25 m% Mo alloy

Atom probe tomography (APT) measurements were conducted to analyze nm-sized precipitates in a Fe-25 m% Co-15 m% Mo model alloy. Calculation of isoconcentration surfaces and application of cluster-search algorithms yielded the chemical compositions of matrix and precipitates. Small-angle neutron scattering (SANS) was performed in order to calculate the ratio of magnetic to nuclear scattering. This so-called A-ratio was compared with values calculated from the chemical information derived from the atom probe data. For this, the magnetism of the precipitates has to be taken into account. Ab-initio studies have been carried out to determine the magnetic moment of the precipitate phase. The results show that the extremely fine precipitates are ferromagnetic and exhibit near-equilibrium composition. In the very early stage of decomposition, however, the composition is different from near-equilibrium. The influences of the magnetism of the precipitates as well as of artifacts from atom probe measurements on the obtained results are discussed.     

Correlative atomic scale characterisation of secondary carbides in M50 bearing steel

Correlative atom probe tomography (APT) and transmission electron microscopy (TEM) are used to characterise the microstructure and chemistry of carbide precipitation in M50 bearing steel. This is a prerequisite in establishing relationships between the microstructure and hydrogen embrittlement (HE) resistance. Secondary carbides are the focus of this study, as they play a major role in improving HE-resistance. Secondary carbides are observed in APT, with compositions close to M₄C_3, M₂C and M₃C. Correlative TEM measured orientation relationships between the martensite matrix and carbides, enabling the confirmation of M₃C cementite precipitates in the corresponding APT reconstruction. Additionally, other precipitates observed in TEM were correlated to the M₂C carbides in APT data. The M₄C₃ carbides are found to have a significantly lower volume fraction than the M₂C carbides.     

Fourier transform infrared spectroscopy to detect thermal degradation of vegetable and chrome-tanned leather

Abstract

The present experimental study determined the thermal degradation stages for vegetable and chrome-tanned leathers (goat and sheep) at 90, 100, and 130?°C by using Fourier transform infrared spectroscopy (FTIR) and differential thermal analysis. Infrared spectra revealed that a temperature of 90?°C affected the adsorbed water band at 3400?cm^?1. Moreover, this temperature slightly reduced the vibrations of amide II and amid III (1340?cm^?1) confirming the preliminary decomposition of protein folds, but it is worth noting that the aliphatic side chains remained stable at this stage of aging. At 100?°C, there was a sharp rupturing in the phenolic-OH bond and side-by-side N–H vibrations decreased dramatically. The peak decomposition occurred at 130?°C, where the amide I and amide III intensities significantly increased, which can be considered indicative of protein unfolding. Those changes are substantiation of protein denaturation. Thermal analysis demonstrated that thermal aging significantly reduced the required temperature for the process of oxidation. The oxidation occurred at 217?°C in goat sample aged at 90?°C. Nevertheless, the reference sample suffered from oxidation at about 220?°C, while with increasing aging temperatures (at 100 and 130?°C), endothermic reactions were observed. Such reactions are usually associated with protein denaturation.     

Negative magnetoresistance in Cr-containing diamond-like carbon-based heterostructures

We have characterized the local structure around the Cr atom, as a function of Cr content, in films of chromium-doped hydrogenated amorphous diamond-like carbon (Cr-DLC) synthesized by plasma-enhanced chemical vapor deposition (PECVD). The composition appears to be related to the structure and forward bias magnetoresistance in heterojunction devices. Chromium in diamond-like carbon (DLC) has a chemical state much like chromium carbide and, at low Cr content, the Cr is dissolved in an amorphous DLC matrix forming an atomic-scale composite. At higher Cr content, Cr is present as nano-composite and chromium carbide precipitates preferentially form at the surface of the film. In these films of higher chromium concentration, a large coefficient of negative magnetoresistance is observed in heterojunction devices with n-type silicon.     

A Subnanoscale Investigation of Sb Segregation at MnO/Ag Ceramic/Metal Interfaces

We have studied Sb segregation at MnO/Ag(Sb) ceramic/metal heterophase interfaces employing three-dimensional atom-probe (3DAP) microscopy. Specimens are prepared by the internal oxidation of Ag(Mn) alloys, leading to the formation of nanometer-size MnO precipitates within a Ag(Mn) matrix. Sb is introduced into the internally oxidized specimens with a vapor diffusion treatment. Appreciable Sb segregation is observed only after a subsequent segregation anneal is performed, and the measured interfacial excess of Sb at the MnO/Ag(Sb) interfaces, _Sb ^MnO/Ag, is determined directly. The temporal evolution of the MnO precipitates is followed for the different processing steps employed. It is shown that the concentration of silver within the MnO precipitates decreases from an initial value of 45–50 at.% Ag to less than 5 at.% Ag with increasing annealing time at the different processing temperatures. Thus the MnO precipitates form under paraequilibrium conditions and the precipitates inherit Ag from the matrix. With increasing aging time orthoequilibrium conditions prevail and the MnO precipitates reject the silver atoms they inherited from the matrix.     

The feasibility of Al-based oxide precipitation in Fe–10%Cr alloy by ion implantation

This paper reports the feasibility of nano-oxide precipitate formation in Fe–Cr alloy by ion implantation synthesis. High contents of Al⁺ and O⁺ ions were implanted into thin films of high purity Fe–10%Cr alloy at room temperature and were studied by transmission electron microscopy (TEM) and atom probe tomography (APT). In contrast, to the common two-stage implantation/annealing scheme of precipitate ensemble synthesis by ion beams, cluster formation took place at the implantation stage in our study, requiring no subsequent high-temperature annealing. The post-implantation microstructural examination revealed in the as-implanted thin foil an array of precipitates with diameters in the range of 3–30?nm. The precipitate number density distribution was found to depend on the foil thickness. The precipitate enrichment with both Al and O was confirmed by the energy-filtered TEM analysis. Judging from the electron diffraction pattern and high-resolution TEM analysis, the crystal lattice of precipitates corresponds to some cubic modification of aluminium-rich oxide or pure aluminium oxide. The precipitate lattice alignment with the host matrix was revealed for at least a part of precipitates. The analysis of APT data using cluster detection algorithm indicates the presence of local zones enriched in Al and O, even in those areas of as-implanted samples where no clusters were visible by TEM.     

Magnetic and mechanical properties of Cu-strengthened aged HSLA-100 steel

Magnetic and mechanical properties were evaluated for various heat-treated Cu-strengthened HSLA-100 steels. After austenitizing at 1183?K for one hour and then water quenching, the material was aged at different temperatures ranging between 623?K and 973?K for one hour. Scanning and transmission electron microscopes with an energy dispersive X-ray attachment were used for microstructural analysis. Nanosize coherent copper precipitates caused an increase in hardness with increase in ageing temperature. The steel exhibited maximum hardness at 773?K followed by decrease in hardness with over-ageing. Copper precipitates coarsened and lost coherency during over-ageing. Ageing behaviour again exhibited an increase in hardness at 973?K due to the formation of new martensite islands. Magnetic hysteresis loop and magnetic Barkhausen emissions techniques were used to characterize the aged materials. No correlation was observed between the magnetic parameters and hardness. The results were explained by the fact that nanosize Cu precipitates, the size of which is much smaller than the domain wall width, did not influence the magnetic domain dynamics.     

Characterization of Na precipitates in electron irradiated NaCl crystals by wide angle X-ray scattering (WAXS)

Samples of synthetic NaCl crystals have been exposed to different doses of electron irradiation up to 1500 MGy (150 Grad) at elevated temperatures, and studied subsequently by X-ray diffraction. Our experimental results clearly show that there is a close correspondence between the geometrical properties (such as lattice distances and the crystal orientation) of the host crystal and the radiation-induced Na precipitates, which is referred to as the Kurdjumov-Sachs orientation relationship (K-S OR). The size of the precipitates has been estimated.     

A study on the early-stage decomposition in the Al–Mg–Si–Cu alloy AA6111 by electrical resistivity and three-dimensional atom probe

Electrical resistivity measurements and three-dimensional atom probe (3DAP) analysis were employed to investigate early-stage decomposition of the Al alloy AA6111 in the temperature range 60–180°C where electrical resistivity initially increased with ageing time. 3DAP measurements provided information on the shape, number density and solute content of the precipitates, as well as the solute concentration of the matrix, for the ageing conditions corresponding to the resistivity maxima. Using the 3DAP results, the precipitate size distributions for these ageing conditions were determined in terms of the measured number of solute atoms per precipitate. The number density and the Cu content of the precipitates decreased with increasing temperature, whereas the Mg/Si ratio increased. The size distribution of precipitates at the higher ageing temperatures showed the addition of larger size precipitates to the precipitate population. A modification to Matthiessen's law was employed to describe the anomalous resistivity increase by considering the effect of solutes and precipitates on the resistivity evolution. Using the 3DAP results in analysing the resistivity anomaly, it was found that the decrease in the resistivity maxima with increasing temperature was associated with the decrease in the number density of precipitates and not the scattering power of precipitates. The 3DAP results were further used to provide information on the mechanisms of early-stage decomposition and the temperature dependence of the nucleation rate. From this, the nucleation rate appeared to be controlled by the migration of solute atoms, which was assisted by quenched-in vacancies.     

Nucleation and growth of α-Ti on TiB precipitates in Ti–15Mo–2.6Nb–3Al–0.2Si–0.12B

The microstructure was investigated of a β-stabilized Ti–15Mo–2.6Nb–3Al–0.2Si–0.12B alloy at two different aging temperatures, 540°C/8?h and 660°C/8?h. In particular, the heterogeneous nucleation of α-Ti from TiB particles was studied at these aging temperatures. At the lower aging temperature, α-Ti precipitated as needle-like shapes on the TiB phase. In contrast, the higher aged sample exhibited globular α-Ti morphology around the TiB phase. This difference was rationalized in terms of the coarsening behavior of α-Ti around the TiB phase. Various orientation relationships were observed between these two samples. This difference is because of the precipitation of α-Ti on two different TiB planes. In addition, atom probe analysis confirmed the segregation of alpha and beta stabilizing elements to the respective phases. At the lower aging temperature, it was noted that silicon enriched the α-Ti/β-Ti interface when the α-Ti/β-Ti/TiB were all in contact. Upon α-Ti coarsening, silicon enrichment was observed at the α-Ti/TiB interface at the higher aging temperature.     

Thermal relaxation behavior of residual stress in laser hardened 17-4PH steel after shot peening treatment

In order to investigate the residual stress relaxations of shot peened layer, isothermal annealing treatments were carried out on tempered and laser hardened 17-4PH steel after shot peening with different temperatures from 300 °C to 600 °C. The results showed that the residual stresses were relaxed in the whole deformation layer especially under higher temperature. The maximum rates of stress relaxation took place at the initial stage of annealing process in all conditions. The relaxation process during isothermal annealing could be described by Zener-Wert-Avrami function. The thermal stability of residual stress in tempered 17-4PH was higher than that in laser hardened 17-4PH as well as that in α-iron, which was due to the pinning effects of ?-Cu precipitates on the dislocation movement. As massive ?-Cu precipitates formed in the temperature about 480 °C, the activation enthalpies for stress relaxation in laser hardened 17-4PH were the same as that in tempered 17-4PH in the conditions of isothermal annealing temperatures of 500 °C and 600 °C.     

Bulk interfaces in a Ni-rich Ni-Au alloy investigated by high-resolution Z-contrast imaging

Interfaces between Au-rich precipitates and the Ni-rich matrix in a decomposed Ni-10 at.% Au alloy were investigated by low-magnification and high-resolution Z-contrast imaging. During aging at 923 K, the originally single crystalline sample decomposed and recrystallized resulting in a microstructure consisting of subgrains separated by small-angle grain boundaries. These small-angle grain boundaries are decorated by Au-rich precipitates. The interfaces between the Au-rich precipitates and the Ni-rich matrix were characterized with respect to the orientation relationship between precipitates and matrix, misfit dislocations and concentration gradients. Two transformation modes were identified that are involved in the decomposition of bulk Ni-rich Ni–Au alloys. While in the first mode the interface is semi-coherent, in the second mode the interface corresponds to an incoherent twin boundary. It is further shown that strain fields around misfit dislocations can result in systematic errors in the determination of the concentration gradients across interfaces between precipitates and matrix.     

Observations of glide and decomposition of a⟨101⟩ dislocations at high temperatures in Ni-Al single crystals deformed along the hard orientation

Ni-44 at.% Al and Ni-50 at.% Al single crystals were tested in compression in the hard d001 ¢orientation. The dislocation processes and deformation behaviour were studied as a function of temperature, strain and strain rate. A slip transition in NiAl occurs from a?111? slip to non-a?111? slip at intermediate temperatures. In Ni-50 at.% Al single crystals, only a?010? dislocations are observed above the slip transition temperature. In contrast, a a?101?{101} glide has been observed to control deformation beyond the slip transition temperature in Ni-44 at.% Al. a?101? dislocations are observed primarily along both ?111? directions in the glide plane. High-resolution transmission electron microscopy observations show that the core of the a?101? dislocations along these directions is decomposed into two a?010? dislocations, separated by a distance of approximately 2 nm. The temperature window of stability for these a?101? dislocations depends upon the strain rate. At a strain rate of 1.4 210^?4 s^?1, a?101? dislocations are observed between 800 and 1000 K. Complete decomposition of a?101? dislocations into a?010? dislocations occurs beyond 1000 K, leading to a?010? climb as the deformation mode at higher temperatures. At lower strain rates, decomposition of a?101? dislocations has been observed to occur along the edge orientation at temperatures below 1000 K. Embedded-atom method calculations and experimental results indicate that a?101? dislocations have a large Peierls stress at low temperatures. Based on the present microstructural observations and a survey of the literature with respect to vacancy content and diffusion in NiAl, a model is proposed for a?101?{101} glide in Ni-44 at.% Al, and for the observed yield strength versus temperature behaviour of Ni-Al alloys at intermediate and high temperatures.     

Size distribution and volume fraction of T1 phase precipitates from TEM images: Direct measurements and related correction

Transmission Electron Microscopy (TEM) can be used to measure the size distribution and volume fraction of fine scale precipitates in metallic systems. However, such measurements suffer from a number of artefacts that need to be accounted for, related to the finite thickness of the TEM foil and to the projected observation in two dimensions of the microstructure. We present a correction procedure to describe the 3D distribution of disc-like particles and apply this method to the plate-like T₁ precipitates in an Al–Li–Cu alloy in two ageing conditions showing different particle morphologies. The precipitates were imaged in a High-Angular Annular Dark Field Microscope (HAADF-STEM). The corrected size distribution is further used to determine the precipitate volume fraction. Atom probe tomography (APT) is finally utilised as an alternative way to measure the precipitate volume fraction and test the validity of the electron microscopy results.     

Study of the dielectric loss of heavily doped NaCl:CaCl2

The dielectric loss of NaCl doped with varying CaCl₂ concentrations is investigated at different frequencies and temperatures. An analysis of the results reveals that the observed relaxation loss is caused by three superimposed loss maxima, which are attributed to two kinds of impurity vacancy complexes and to precipitates.