Magnetic hysteresis properties of internally oxidised Ni-Si alloys

Results are presented of magnetic hysteresis measurement made on some internally oxidised Ni-Si alloys. The alloys investigated were originally of nominal composition 0.058, 0.48, 0.77 and 0.96 wt % Si in Ni. Hysteresis loops have been obtained for all the alloys in both unoxidised and several internally oxidised states using a standard technique. The effect of the internal oxide precipitation on the coercivity and remanence of the specimens is made clear by these measurements. The dependence of coercivity on precipitate morphology and alloy compositions is considered and results of the variation of coercivity with the temperature of measurement in the range 20 to 265° C are discussed.     

Electron microscopy of internally reduced (Mg,Ni)O

(Mg, Ni)O single crystals were internally reduced at 1273 and 1673 K in graphite ampules. As a result, discrete nickel precipitates developed inside the MgO matrix. The precipitates, as well as the dislocations that surround them, and the pores have been observed by means of electron microscopy. A model is proposed, whereby the morphology of the metal precipitates is controlled by the kinetics and the mechanism of internal reduction. The interactions between point defects and dislocations or pores are also commented upon.     

Interaction of plutonium with iron- and chromium-containing precipitates under the conditions of reservoir bed for liquid radioactive waste

The behavior of Pu under the conditions of liquid radioactive waste (LRW) disposal in a reservoir bed was studied. The pH dependence of the Pu sorption is the same in the cases when the radionuclide is introduced together with the simulated LRW solution or after the precipitate formation. The composition and morphology of Fe,Cr-containing precipitates formed under the conditions simulating the radioactive waste disposal were determined. The solid phase was characterized by X-ray diffraction analysis and by transmission and scanning electron microscopy. The phase composition of the precipitates was determined in relation to the presence of acetate ions, Fe concentration in the solution, and storage time and temperature. The main phases formed are ferrihydrite, goethite, hematite, and grimaldiite.     

Kinetics of two-phase layer growth during reactive diffusion

A quantitative model is proposed which describes the growth kinetics of a two-phase layer in a reactive diffusion experiment. The model is based on the concept of diffusion through the matrix phase being gradually blocked by the precipitates of a second phase with a low diffusivity of the components. The effect of precipitate morphology on the reaction zone growth is accounted for by introducing the dependence of diffusivity on the second phase fraction. Calculations according to this approach yielded a good match with experimental kinetics in the Ti–AlN system.     

Technical note Fabrication of alumina dispersion strengthened copper strips by internal oxidation and hot roll bonding

Abstract

Alumina dispersion strengthened copper strips were fabricated by internal oxidation and hot roll bonding of Cu–Al alloy strips. Cu–Al alloy strips were internally oxidised without using any oxidant powders by a surface oxidation method. Several of the internally oxidised alloy strips were stacked and bonded by rolling at high temperatures. The bonded strip was cold rolled to achieve tensile strengths of 484–539 MPa and yield strengths of 472–522 MPa with thermally stable mechanical properties.     

Precipitate engineering in GaAs studied by laser scattering tomography

Annealing experiments were carried out with semi-insulating LEC GaAs. The volumes of the precipitates were determined by an advanced technique of laser scattering tomography (LST) qualified for quantitative volume measurements and their statistical relevant evaluation. The dependence of the volume distributions of precipitates on annealing conditions is presented. The dependence of the total volume of the precipitate ensemble on hold temperature and time is shown. The experimental results cannot be explained completely in the framework of Ostwald ripening of the As precipitates. A behaviour not described earlier has been observed: at cooling inside the assumed homogeneity range of GaAs the growth of As precipitates followed by their dissolution.     

Morphology characteristics of α precipitates related to the crystal defects and the strain accommodation of variant selection in a metastable β titanium alloy

Profound and comprehensive investigations on the morphology characteristics of α precipitates are essential for the microstructural control of metastable β titanium alloys.At the very beginning of aging treatment,intragranular α precipitates with a dot-like morphology begin to generate nearby the dislo-cations,then those dot-like α precipitates with the same crystallographic orientation tend to connect with each other to develop a lath-like morphology.With the progress of aging treatment,the orientated lath-like α precipitates gradually combine with each other to form the V-shaped clusters or the triangular ones.The dislocations of{1(-1)0}β < 11 (-1)>β edge type are evidenced within the β grains,and it is found that variant selection ofα precipitates induced by the transformation strain and the interplay between α vari-ants and the dislocations are confirmed as the key factors for the formation of the V-shaped or triangular clusters.The results of this work could provide underlying knowledge on the morphology characteristics of intraguranular α precipitates related to the crystal defects and the strain accommodation of α variants in metastable β titanium alloys.     

Failure Analyses of Six Cylinder Aircraft Engine Crankshafts

Results of failure analyses of two aircraft crankshafts are described. These crankshafts were forged from AMS 6414 (similar composition to AISI 4340) vacuum arc remelted steels with sulfur contents of 0.003% (low sulfur) and 0.0005% (ultra-low sulfur). A grain boundary sulfide precipitate was caused by overheat of the low sulfur steel, and an incipient melting of grain boundary junctions was caused by overheat of the ultra-low sulfur steel. The precipitates and incipient melting in these two failed crankshafts were observed during the examination. As expected, impact fractures from the low sulfur steel crankshaft contained planar dimpled facets (PDF) along separated grain boundaries with a small spherical manganese sulfide precipitates within each dimple. In contrast, PDFs along separated grain boundaries of impact fractures from the ultra-low sulfur crankshaft steel contained a majority of small spherical particles consisting of nitrogen, boron, iron, carbon, and a small amount of oxygen. Some other dimples contained manganese sulfide precipitates. Fatigue samples machined from the ultra-low sulfur steel crankshaft failed internally at planar grain boundary facets. Some of the facets were covered with nitrogen, boron, iron, and carbon film, while other facets were relatively free of such coverage. Results of experimental forging studies defined the times and temperatures required to produce incipient melting overheat and facets at grain boundary junctions of ultra-low sulfur AMS 6414 steels.     

Microstructure and Fracture Morphology in the Welding Zone of Cr18Mo2 Ferritic Stainless Steel

Microstructure, precipitates and fracture morphology in the weld metal and the heat-affected zone (HAZ) of Cr18Mo2 ferritic stainless steel have been studied by means of metalloscope, SEM,TEM and X-ray diffractometer. Experimental results indicate that crystalline grain coarsening in HAZ is one of the reason resulting in the embrittlement fracture in the welding zone of the ferritic stainless steel. Some precipitates (TiC, TiN and Cr2N) in the steel promote production and development of the brittle cracks. In practical applications. the welding heat input should be as small as possible to prevent embrittlement caused by HAZ grain coarsening.     

Influence of the microstructural inhomogeneities on the martensite-to-austenite phase transformation temperatures in TiNiCu-based shape-memory alloys

The morphology and distribution of Ti₂Ni precipitates in two batches of Ti–40.8Ni–9Cu (at.%) shape-memory alloys were studied to understand the influence of microstructural inhomogeneities on the martensite-to-austenite phase transformation. Although both batches were produced using nominally identical processing conditions at the same facility, the transformation temperatures were remarkably different. Such behaviour was linked to stereological differences in the precipitates morphology and size distributions, as well as slight differences in the chemical composition.     

Oxide particle densities in internally oxidised Ag-Cd alloys: Part 2 Alloys with ternary additions

Abstract

The densities of oxide particles are studied as a function of depth below the surface of internally oxidised Ag–9·5 at.-%Cd alloys with additions of ~0·2 or 2 at.-%Al, Mn, Ni, Sn, or Zn. Because of its low diffusivity and low free energy of oxide formation, Ni has no influence on particle density. Tin forms needlelike oxides and reduces the particle density as a result of geometric effects. Zinc has a marked effect on particle shape with platelike precipitates which coalesce during subsequent precipitation of CdO, but does not greatly affect particle density. Both Al and Mn increase particle density up to two orders of magnitude. Planar size distributions are presented which indicate that two types of nucleation site are present in the Mn containing alloys. In the alloy containing 0·15 at.-%Al the Al₂O₃ particles are too fine to act as nuclei for CdO precipitation; two types of band both with a narrow size distribution of CdO particles, but with slightly different size and largely different density are formed. Heterogeneous nucleation and depletion of the centre region of sample sheets, both of which result from the counterdiffusion of the oxidising elements, are shown to have a great influence on particle density, resulting in a more homogeneous distribution of particle densities over the sample cross-section than that predicted by available theories.

MST/900b     

Hydrogen permeation through internally oxidized Pd alloys

Hydrogen diffusivity and solubility were determined by electrochemical hydrogen permeation tests in samples of Pd_0.97 Al_0.03 and Pd_0.97 Ce_0.03 in the as received and internally oxidized conditions. Internal oxidization caused the appearance of nanosized oxide precipitates in the Pd matrix. The shape and size of precipitates and also the coherence between the precipitates and the matrix were observed by transmission electron microscopy (TEM). Electrochemical hydrogen permeation tests revealed that the presence of oxides increases the apparent hydrogen solubility, S _app, but decreases the hydrogen diffusivity, D _app. The values of D _app were 2.0 × 10⁻¹¹ m² s⁻¹ for vacuum heat treated Pd_0.97 Al_0.03 alloy and 4.0 × 10⁻¹² m² s⁻¹ for internally oxidized Pd_0.97 Al_0.03 alloy. However, the concentration of trapped hydrogen was 49 mol H.m⁻³ for the Pd_0.97 Al_0.03 alloy in the vacuum heat treated condition and 403 mol H.m⁻³ for the Pd_0.97 Al_0.03 alloy in the internally oxidized condition. Both heat treatments were performed at 1073 K for 24 h. The influence of the nature, size and distribution of the precipitates on the hydrogen permeation parameters are discussed in this paper.     

Al-Cu合金析出相在等径角挤压中的演变

为了研究Al-Cu合金中两种不同析出相(θ′和θ相)在ECAP变形过程中的变化.采用透射电镜(TEM)和硬度测试方法研究了析出相形貌变化以及对合金性能的影响.结果表明:在本实验中,θ′和θ相其破碎、回溶速度明显不同,两者的破碎方式也不同.θ′相先是与基体失去取向关系,随后从其内部产生位错使其破碎,而θ相是被外部基体位错所切割、破碎.θ′相与位错的相互作用方式类似于绕过机制,θ相与位错的作用方式类似于切割机制.两种状态样品的硬度在变形过程中的变化趋势相同,但在第1道次后θ相状态样品的硬度增加值高于θ′相状态.     

Microstructural transformations of heat affected zones in duplex steel welded joints

The influence of the welding thermal conditions exemplified by heat input and heat treatment after welding on the structure of the heat affected zone (HAZ) UNS S31803 has been analysed. The post weld treatment was used to create the precisely defined thermal conditions for the decomposition of primary phases in the HAZ, by a multi-layer welding thermal cycle stimulation. Detailed analyses of the microstructure and chemical composition of the phases in the different post welded conditions were performed by scanning electron microscopy (SEM) combined with energy dispersive spectrometry (EDS) and transmission electron microscopy (TEM). Three types of secondary precipitates have been observed: secondary austenite (γ₂), carbides: M₂₃C₆ and M₇C₃. The dependence of the secondary austenite volume fraction and morphology in the HAZ on thermal cycle have been interpreted. The eutectoid decomposition of the primary phases in the analysed thermal conditions was confirmed.     

Quantitative pressure and strain field analysis of helium precipitates in silicon

The structural properties of overpressurised helium precipitates formed by low dose ion implantation and subsequent annealing of silicon are investigated by quantitative transmission electron microscopy techniques. These precipitates, which show pronounced platelet geometry, are analysed with respect to their geometry, crystallographic orientation and their particular gas pressure values. The dependence of the measured platelet pressure versus the radius is discussed in terms of a Griffith crack. Experimental results on the shape and the crystallographic orientation of the platelets are discussed in the framework of anisotropic elastic properties and surface energies of silicon. The ability of the precipitates to punch-out dislocation loops is discussed in terms of associated threshold shear stress values and evaluated with regard to the defect size dependency.     

Influence of Hot Isostatic Pressing on Microstructuresand Properties of NiAl/Cr(Mo)-TiC in-situMultiphase Composites

A novel in-situ multiphase NiAl-based composite was investigated on its microstructures and properties influenced by HIP treatment. In the as- processed composites, a fine array of NiAl precipitates was found to be distributed in Cr(Mo) phase. TiC particles tended to segregate at grain or phase boundaries. In addition, a number of dislocations were observed to exist in NiAl grains meanwhile fewer in Cr(Mo) grains. Little change in the morphology was found except that nearly fully densification was achieved. Some changes in microstructures were notable including coarsening of NiAl precipitates in Cr(Mo) phase and increase in dislocation density in NiAl phase.After HIP, the strength of the composites increased significantly compared with the as-processed composites. The reason for the enhanced Strength of the composites is also discussed.     

Roles of precipitates on corrosion fatigue crack growth of high-strength steels in corrosive environments

The effect of microstructures on resistance to corrosion fatigue cracking and fracture surface morphology for age-hardened steels were investigated in a 3.5% NaCl aqueous solution under a cathodic potential of –0.85 V (Ag/AgCl). The free corrosion was about –0.63 V (Ag/AgCl). The resistance to corrosion fatigue cracking of materials containing coherent precipitates in the matrix (underaged conditions) was less than that of materials containing incoherent precipitates (reheated conditions) at equal strength levels. Accelerated fatigue crack growth rates of the underaged material in the aqueous solution were followed by cracking along prior-austenite grain boundaries, due to hydrogen embrittlement, while the overaged material did not show accelerated fatigue crack growth rates and had fracture surfaces similar to those in air. The difference in the fracture surfaces of both materials in air and in the aqueous solution was considered to depend on the ease of diffusion of hydrogen to the prior-austenite grain boundaries. It is concluded that incoherent precipitates in the matrix made hydrogen accumulation at prior-austenite grain boundaries much slower than for coherent precipitates.     

Insight from in situ microscopy into which precipitate morphology can enable high strength in magnesium alloys

Magnesium alloys, while boasting light weight, suffer from a major drawback in their relatively low strength. Identifying the microstructural features that are most effective in strengthening is therefore a pressing challenge. Deformation twinning often mediates plastic yielding in magnesium alloys. Unfortunately, due to the complexity involved in the twinning mechanism and twin-precipitate interactions, the optimal precipitate morphology that can best impede twinning has yet to be singled out. Based on the understanding of twinning mechanism in magnesium alloys, here we propose that the lamellar precipitates or the network of plate-shaped precipitates are most effective in suppressing deformation twinning. This has been verified through quantitative in situ tests inside a transmission electron microscope on a series of magnesium alloys containing precipitates with different morphology. The insight gained is expected to have general implications for strengthening strategies and alloy design.     

X70针状铁素体管线钢中的析出行为研究

利用透射电镜和X射线能谱研究了Nb-V-Ti微合金X70针状铁素体管线钢中析出物的形貌、结构和化学成分.观测结果表明所研究钢中的析出物可分为两大类:一类为高温析出的尺寸较大的复合析出物,尺寸较大的方形TiN粒子为复合析出物的核心,而含Nb的尺寸较小的析出物附着在其一侧或多个侧面生长.在较低温度下,Nb的碳化物和(或)碳氮化物在位错和亚界面上非均匀形核析出,尺寸较小,外貌呈球形.根据钢的化学成分、热力学和X射线能谱分析认为,NbC粒子是本工艺条件下所研究的Nb-V-Ti微合金X70针状铁素体管线钢中的主要析出相.     

An experimental investigation of the structure of some internally oxidised ferromagnetic alloys

Results are presented of optical, scanning electron and transmission electron microscope observations on some internally oxidised ferromagnetic alloys. The alloys investigated were of nominal composition 0.058 wt % Si in Ni, 0.48 wt % Si in Ni and 0.17 wt % Si in Co 66.2%-Ni 33.7%. The alloys were in polycrystalline form and in addition single crystals of 0.058 wt % Si in Ni were examined. The diffusion and oxidation rate constants are calculated and the state of the oxide product as a function of (i) depth in the alloy, (ii) oxidising temperature and (iii) alloy composition is considered. The possibility of using such systems as device material and as test material for theories of coercivity and approach to magnetic saturation is considered.