Microstructural and wear behavior of hypoeutectic Al-Si alloy (LM25) grain refined and modified with Al-Ti-C-Sr master alloy

Present article reports the microstructure and wear behavior of grain refined and modified Al-7Si-0.3Mg (LM25) alloy. Combined grain refinement and modification is achieved by inoculating LM25 melt with various inoculation levels (0.2, 0.5 and 1.0 wt%) of a novel Al base master alloy containing Ti, C and Sr (synthesized in the authors’ laboratory) at 720 °C. The wear resistance of LM25 alloy improves with the addition of this master alloy up to 0.5 wt%. It is observed that α-Al cell size and morphology of the eutectic silicon of LM25 alloy have significant effect on the wear resistance of the alloy.     

Microstructure and tribological behavior of Ti–Si eutectic alloys with Al addition

The effect of Al element alloying on the microstructure and tribological behavior of Ti–Si eutectic alloys has been studied. The experimental results show that Al element changes the microstructure from large eutectic cells that consist of layered tablet phases Ti₅Si₃ and α-Ti (for the Al-free alloy) to near-equiaxed or rod-like Ti₅Si₃ particle reinforced continuous α-Ti (Al, Si) solid solutions. This microstructural change greatly improves the ductility and reduces brittle fracture of massive superficial materials during wear process. Microplough and local delamination are the main wear mechanisms of Al-added Ti–Si alloys. Therefore, the wear resistance and friction stability are simultaneously improved.     

Modification of Mo-Si alloy microstructure by small additions of Zr

Molybdenum and its alloys are potential materials for high-temperature applications. However, molybdenum is susceptible to embrittlement because of oxygen segregation at the grain boundaries. In order to alleviate the embrittlement small amounts of zirconium were alloyed to a solid solution of Mo-1.5Si alloy. Two Mo-based alloys, namely Mo-1.5Si and Mo-1.5Si-1Zr, were investigated by the complementary high-resolution methods transmission electron microscopy and atom probe tomography. The Mo-1.5Si alloy shows a polycrystalline structure with two silicon-rich intermetallic phases Mo₅Si₃ and Mo₃Si located at the grain boundaries and within the grains. In addition, small clusters with up to 10 at% Si were found within the molybdenum solid solution. Addition of a small amount of zirconium to Mo-1.5Si leads to the formation of two intermetallic phases Mo₂Zr and MoZr₂, which are located at the grain boundaries as well as within the interior of the grain. Transmission electron microscopy shows that small spherical Mo-Zr-rich precipitates (<10 nm) decorate the grain boundaries. The stoichiometry of the small precipitates was identified as Mo₂Zr by atom probe tomography. No Si-enriched small precipitates were detected in the Mo-1.5Si-1Zr alloy. It is concluded that the presence of zirconium hinders their formation.     

Influence of structural parameters on magnetoresistive properties of CuFeNi melt spun ribbons

The microstructure of Cu₈₀Fe₁₀Ni₁₀ (at%) granular ribbon was investigated by means of atom probe tomography (APT). A granular system is composed of magnetic precipitates embedded in a non-magnetic matrix. In this ribbon, the magnetic precipitates have a diameter smaller than 5 nm in the as-spun state, and their crystallographic structure is very similar to the one of the matrix, which makes it difficult to characterize them using conventional techniques. Those data are of great importance to understand the magnetic and the transport behaviour of these ribbons. Using atom probe tomography, a 3D reconstruction of the microstructure of the as-spun and annealed ribbons was achieved and a precise characterization of the compositions of the two phases and of the composition profile at interfaces was carried out. In the as-spun state the composition of the matrix is Cu₈₉Fe₃Ni₈, the one of the precipitates is Cu₃₀Fe₄₀Ni₃₀. Upon annealing, the precipitates get enriched in iron. After annealing at 600 °C for 24 h, the measured compositions are close to the one predicted by Thermocalc, with Cu₉₄Fe₁Ni₅ for the matrix and Cu₅Fe₆₄Ni₃₁ for the precipitates.     

Fabrication of high-aspect ratio Si pillars for atom probe ‘lift-out’ and field ionization tips

A process for fabricating high-aspect ratio (∼1:20), micron-sized Si [0 0 1] pillars using mechanical and chemical size reduction is presented. A dicing saw was used for mechanically patterning an array of square pillars with side lengths of >20 μm. These pillars were then reduced in size using an aqueous NaOH and KOH solution heated to 100 °C. The chemical etch reduces the pillar size within the time range amenable for focus ion beam milling and/or attachment for atom probe ‘lift-out’ specimens. The pillars can be formed with either a flat top surface or into <100 nm tip points for direct field ionization.     

Surface segregation of Au–Pd alloys in UHV and reactive environments: Quantification by a catalytic atom probe

The surface composition of an Au-62 at% Pd alloy has been studied by means of a catalytic atom probe (CAP) before and after exposures to nitric oxide (NO) at temperatures ranging from 300 to 573 K for 20 min. Subsequent CAP analysis at 100 K revealed a considerable surface enrichment in Pd (to ∼80 at%) after exposure at 573 K. This is correlated with the occurrence of NO dissociation, and the formation of strong Pd–O bonds at the surface. Blank experiments in ultra-high vacuum reflect the surface composition of the bulk material, in excellent agreement with electron microprobe analysis. At 573 K, no detectable surface segregation occurs in the absence of NO adsorption for the times and temperatures studied. However, classical Metropolis Monte-Carlo simulations performed with a semi-empirical potential on the Au₄₀Pd₆₀ (1 1 1), (1 1 0) and (1 0 0) systems show surface enrichment of gold at equilibrium. This suggests that the temperatures of the clean surface segregation experiments are too low to reach equilibrium within times of the order of hours.     

Investigations of the nitrided subsurface layers of an Fe–Cr-model alloy

An Fe–5 wt%Cr alloy was nitrided in gaseous atmosphere at 590 °C for 12 h. In the resulting diffusion layer, nitrides precipitate on a nanometre scale. The microstructure in the diffusion layer was characterised by optical microscopy and hardness measurements. The morphology, volume fraction and chemical composition of the nitrides were determined by means of atom probe tomography. The orientation of the nitrides with respect to the matrix was investigated using three-dimensional field ion tomography. The evolution of the nitrides was studied at different depths from the surface and their nanoscopic features were correlated with the obtained hardness profile. At a depth of 270 μm from the surface, the first stages of nitride formation could be analysed.     

APT analyses of deuterium-loaded Fe/V multi-layered films

Interaction of hydrogen with metallic multi-layered thin films remains as a hot topic in recent days. Detailed knowledge on such chemically modulated systems is required if they are desired for application in hydrogen energy system as storage media. In this study, the deuterium concentration profile of Fe/V multi-layer was investigated by atom probe tomography (APT) at 60 and 30 K. It is firstly shown that deuterium-loaded sample can easily react with oxygen at the Pd capping layer on Fe/V and therefore, it is highly desired to avoid any oxygen exposure after D₂ loading before APT analysis. The analysis temperature also has an impact on D concentration profile. The result taken at 60 K shows clear traces of surface segregation of D atoms towards analysis surface. The observed diffusion profile of D allows us to estimate an apparent diffusion coefficient D. The calculated D at 60 K is in the order of 10⁻¹⁷ cm²/s, deviating 6 orders of magnitude from an extrapolated value. This was interpreted with alloying, D-trapping at defects and effects of the large extension to which the extrapolation was done. A D concentration profile taken at 30 K shows no segregation anymore and a homogeneous distribution at c_D=0.05(2) D/Me, which is in good accordance with that measured in the corresponding pressure–composition isotherm.     

The influence of Cu addition on precipitation in Fe–Cr–Ni–Al–(Cu) model alloys

Precipitation in Fe–Cr–Ni–Al–(Cu) model alloys was investigated after ageing for 0.25, 3, 10 and 100 h at 798 K. Characterization of nanoscale precipitates was performed using three-dimensional atom probe microscopy and transmission electron microscopy. The precipitates are found to be enriched in Ni and Al (Cu) and depleted in Fe and Cr. After 0.25 h of ageing the number density of precipitates is ∼8×10²⁴ m⁻³, their volume fraction is about 15.5% and they are near-spherical with an average diameter of about 2–3 nm. During further ageing the precipitates in the both alloys grow, but the coarsening behaviour is different for both alloys. The precipitates of the Cu-free alloy grow much faster compared with the Cu-containing alloy and their density decreases. Precipitates in Cu-free alloy change to plate shaped even after 10 h of ageing, whereas those of Cu-containing alloy remain spherical up to 10 h of ageing. The influence of Cu addition on precipitation in these model alloys is discussed with respect to the different coarsening mechanisms.     

Anomalous distribution in atom map of solute carbon in steel

The distribution of carbon in atom probe tomography maps was investigated in various phases of steel. Carbon atoms in 3D atom maps of martensite and cementite phases showed an almost uniform distribution. On the other hand, carbon atoms in ferrite were consistently enriched along the zone line joining the (0 0 2) and the (2 2 2) poles, and in the depth direction of analysis, which was different from the actual distribution. The width and concentration of the enriched regions remained unchanged at a specimen temperature ranging from 90to 30 K. Moreover, the ratio of molecular carbon ions to total carbon ions decreased with decreasing temperature, but did not change between the enriched and diluted regions. Based on the results, the reason for the anomalous distribution of solute carbon atoms in atom maps is discussed.     

保温时间对ATN陶瓷刀具材料力学性能及显微结构的影响

为了开发一种陶瓷刀具材料,以Al2O3/TiN为主要成分,利用热压烧结工艺,在烧结温度1700℃,烧结压力32MPa条件下制备了复合陶瓷材料.FiN含量取30%、40%、50%、60%,保温时间取5min、10min、15min.研究了四种配比的材料在不同保温时间下抗弯强度、硬度、断裂韧性等力学性能与材料显微结构的关系...     

Shaping the lens of the atom probe: fabrication of site specific, oriented specimens and application to grain boundary analysis

The random sampling provided by classical atom probe sample preparation methods is one of the major factors limiting the types of problems that can be addressed using this powerful technique. A focused ion beam enables not only site-specific preparation, but can also be used to give the specimen, which acts as the lens in an atom probe experiment, a specific shape. In this paper we present a technique that uses low accelerating voltages (10 and 5 kV) in the focused ion beam (FIB) to reproducibly produce specimens with selected grain boundaries <100 nm from the tip at any desired orientation. These tips have a high rate of successfully running in the atom probe and no Ga contamination within the region of interest.This technique is applied to the analysis of grain boundaries in a high purity iron wire and a strip-cast steel. Lattice resolution is achieved around the boundary in certain areas. Reconstruction of these datasets reveals the distribution of light and heavy elements around the boundary. Issues surrounding the uneven distribution of certain solute elements as a result of field-induced diffusion are discussed.     

Positive effect of natural pre-ageing on precipitation hardening in Al–0.44 at% Mg–0.38 at% Si alloy

Age hardening in a purely ternary Al-Mg0.4–Si0.4 (0.44 at% Mg, 0.38at%Si) alloy that is similar to AA6060 was investigated by hardness measurement, TEM and three-dimensional atom probe (3D-AP). In particular, the effect of natural pre-ageing before artificial ageing, which is known to have a positive effect in this alloy, was studied by comparing three different conditions: natural ageing only, artificial ageing for 1.5 h at 180 °C only and combined natural pre-ageing and subsequent artificial ageing for 1.5 h at 180 °C. Natural ageing influences the mechanical properties significantly. Naturally aged alloys exhibit a hardening response that is governed by the presence of small clusters. Subsequent artificial ageing of naturally aged specimens increases the value of peak hardness, which is attributed to the increase of the number density of needle-shaped precipitates as compared to the samples without natural ageing. It is assumed that besides these precipitates, the small Si clusters formed at room-temperature storage remain stable during artificial ageing.     

Thermal stability of TiAlN/CrN multilayer coatings studied by atom probe tomography

This study is about the microstructural evolution of TiAlN/CrN multilayers (with a Ti:Al ratio of 0.75:0.25 and average bilayer period of 9 nm) upon thermal treatment. Pulsed laser atom probe analyses were performed in conjunction with transmission electron microscopy and X-ray diffraction. The layers are found to be thermally stable up to 600 °C. At 700 °C TiAlN layers begin to decompose into Ti- and Al-rich nitride layers in the out-of-plane direction. Further increase in temperature to 1000 °C leads to a strong decomposition of the multilayer structure as well as grain coarsening. Layer dissolution and grain coarsening appear to begin at the surface. Domains of AlN and TiCrN larger than 100 nm are found, together with smaller nano-sized AlN precipitates within the TiCrN matrix. Fe and V impurities are detected in the multilayers as well, which diffuse from the steel substrate into the coating along columnar grain boundaries.     

Carbide characterization in low-temperature tempered steels

The nature of the initial carbides formed during the early stages of the tempering of steels is still a matter of debate. Conventionally, the main transition carbide is described as epsilon carbide, with a composition of approximately Fe_2.4C. However, earlier one-dimensional atom probe (1DAP) results indicated the existence of carbon-rich regions having much lower carbon contents, with maxima of around 10 at%. There was some uncertainty about the interpretation of the 1DAP results, because of possible problems with alignment of the aperture and with trajectory aberration effects. We have therefore re-visited this topic, using the three-dimensional (3D) atom probe, and studying both a model Fe–Ni–C alloy and a well-known engineering steel (AISI4340). We demonstrate that, for both materials, low-temperature (20–150 °C) aging produces carbon-rich regions with average peak carbon contents of up to 10%. We show for the first time the three-dimensional structure of these carbon-rich regions, and demonstrate that fine-scale faulting exists within them.     

Methods of quantitative matrix analysis of Zircaloy-2

The zirconium-based alloy Zircaloy-2 contains small amounts of iron, chromium and nickel dissolved in the matrix. Several attempts to measure these amounts have been made in the past, but the results are conflicting and inconclusive. The advent of wide angle, laser pulsed atom probe tomography motivates a new attempt to analyze the matrix. Large datasets are now easily obtained using laser pulsing but quantification is not straightforward due to rather complex mass spectra. Zircaloy-2 contains about 1 wt% tin, 0.1 wt% oxygen and trace amounts of Si, C and Al. Severe overlaps make quantification of any Fe⁺, Cr⁺ and Ni⁺ ions impossible. Quantification of Fe, Cr and Ni therefore requires that they appear as doubly charged ions only, and consequently the field must be kept high enough. In addition, adsorbed CO⁺ may appear at the main peak of Fe²⁺. In the paper a method is reported, which gives what we believe an accurate quantitative analysis of at least iron and chromium in the matrix.     

Three-dimensional atom mapping of boron in implanted silicon

The redistribution of boron in highly implanted 〈1 0 0〉 silicon (10 keV; 5×10¹⁵ at/cm²) annealed at 600 °C for 1 h was studied using both laser-assisted wide-angle atom probe (LaWaTAP) and secondary ion mass spectrometry (SIMS). As expected, the concentration was found to increase steeply to 10²¹ boron atoms/cm³ at a distance close to 35 nm and to decrease slowly to 10¹⁹/cm³, a value close to the boron level of the silicon substrate. For depth under 75 nm, the implantation profile of boron as given by LaWaTAP was found very close to that given by SIMS investigations without any calibration of the LaWaTAP data. For larger depth, the LaWaTAP profile is observed above that of SIMS. Detection limits of LaWaTAP for low dopant concentrations are discussed. The contribution of the background noise in the spectrum and sampling errors are considered. Fine-scale fluctuations not detected in SIMS profile and related to clustering were evidenced in LaWaTAP maps and profiles. Numerous boron clusters lying on {0 0 1} planes parallel to the implanted surface, a few nanometer in size, were identified and interpreted as boron interstitial clusters (BICs), in agreement with Cristiano et al. observations. They contained between 50 and 300 atoms (Si and B). This is much higher than that generally assumed in particular in ab-initio modelling where a few atoms BICs are considered. These clusters contained 7 at% of boron in average.     

Effects of Sb,Sr, and Bi on the material properties of cast Al-Si-Cu alloys produced through heated mold continuous casting

Rare earth metals can create a fine eutectic Si structure in cast Al-Si_10.6-Cu_2.5 (ADC12) alloys produced through heated mold continuous casting. Fine and spherical eutectic Si phases are created in the ADC12 alloys through the addition of Sr0.04, and fine lamellar eutectic Si phases are created through Sb and Bi addition. Crystal orientation on the face perpendicular to the casting direction is formed by [110]; however, this uniform formation is collapsed in the ADC12 alloy with an increasing amount of Sr addition, such as Sr > 0.04%. The shape of the eutectic Si is statically analyzed, and the effects of the eutectic Si characteristics on the mechanical properties are examined experimentally. On the one hand, the mechanical properties of the ADC12-Sr alloy increase with increasing Sr content because of the fine eutectic Si, the randomly orientated crystal formation, and so on. On the other hand, the material ductility increases in the ADC12 alloy with increasing addition of Sb and Bi elements. A high fracture strain of approximately 14% is obtained for the ADC12-Bi1.5 alloy.     

Sr元素添加对铝硅合金电化学腐蚀行为的影响

利用电化学测试、扫描电镜和能谱分析等方法,对不同 Sr 元素添加量合金在1 M HCl 溶液中电化学腐蚀表面形貌的变化以及开路电位和动电位极化曲线进行了研究。结果表明,由于 Cl-的侵蚀作用,铝硅合金在1 M HCl中发生点蚀破坏,随着腐蚀时间的延长,未添加 Sr 元素合金的点蚀扩展并形成腐蚀缝隙,而经0.04%Sr变质的合金则保持点蚀的纵向加深与扩展。微量 Sr 元素的加入,一方面,促使铝硅合金中片状初生硅转变为纤维状共晶硅,使开路电位负移;另一方面,促进铝氧化,形成钝化膜,减缓Cl-侵蚀作用,增强合金耐点蚀性。在0.1 M HCl溶液中,Sr元素的添加量为0.1%时,ZL102合金的耐腐蚀性最佳。     

Characterisation of the early stages of solute clustering in 1Ni–1.3Mn welds containing Cu

Microstructural characterisation of neutron irradiated low alloy steels is important for developing mechanistic understanding of irradiation embrittlement. This work is focused on the early stages of irradiation-induced clustering in a low Cu (0.03 wt%), high Ni (∼1 wt%) weld. The weld was irradiated at a very high dose rate and then examined by atom probe (energy-compensated position-sensitive atom probe (ECOPoSAP) and local electrode atom probe (LEAP)) with supporting microstructural information obtained by small angle neutron scattering (SANS) and positron annihilation (PALA).