Types Of Precipitates Research Articles

Effect of mold temperature on microstructure and mechanical properties of rheo-squeeze casting Mg–3Nd–0.2Zn–0.4Zr alloy

Abstract

Microstructure and Wear Resistance of Laser-Clad (Co, Ni)61.2B26.2Si7.8Ta4.8 Coatings

It has been reported that a quaternary Co61.2B26.2Si7.8Ta4.8 alloy is a good glass former and can be laser-clad to an amorphous composite coating with superior hardness and wear resistance. In this paper, alloys with varying Ni contents to substitute for Co are coated on the surface of #45 carbon steel using a 5-kW CO2 laser source for the purpose of obtaining protective coatings. In contrast to the quaternary case, the clad layers are characterized by a matrix of α-(Fe, Co, Ni) solid solution plus CoB, Co3B, and Co3Ta types of precipitates. The cladding layer is divided into four regions: Near-surface dendrites, α-(Fe, Co, Ni) solid solution plus dispersed particles in the middle zone, columnar bonding zone, and heat-affected area that consists of martensite. The hardness gradually decreases with increasing Ni content, and the maximum hardness occurs in the middle zone. Both the friction coefficient and wear volume are minimized in the alloy containing 12.2% Ni. Compared with the previous cobalt-based quaternary alloy Co61.2B26.2Si7.8Ta4.8, the addition of the Ni element reduces the glass-forming ability and henceforth the hardness and wear resistance of the clad layers.

Microstructure evolution and fracture mechanism of a novel 9Cr tempered martensite ferritic steel during short-term creep

In this work, the microstructure evolution and fracture mechanism of a novel 9% chromium tempered martensite ferritic steel G115 were investigated over the temperature range of 625–675°C using uniaxial creep tests. The creep curves consist of a primary transient stage followed by an apparent secondary stage, and an accelerated tertiary creep regime. The relationship between the minimum creep rate and the applied stress followed Norton's power law. Based on the EBSD analysis, there were no obvious textural features formed after creep deformation, and with the increase in creep time, the number of subgrains slightly increased, and then sharply increased, indicating dynamic recrystallization (DRX) occurs after creep deformation. In addition, three types of precipitates can be observed after creep deformation: W-rich Laves phase, Nb-rich MX, and Cu-rich precipitates. The Nb-rich MX with a square shape and Cu-rich precipitates with an ellipsoidal shape remain very stable. However, the W-rich Laves phases distributed mainly on the grain boundaries have rod-like, chain-like, and bulky shape, which are coarsened significantly. Representative fractographs of the G115 steel after creep deformation exhibit significant necking with an elliptical shape. A dense array of deep and equiaxed dimples appear in the central region under the tested creep conditions. Ductile fracturing is the dominant fracture mechanism during short-term creep deformation.

Enhancing elevated temperature strength of copper containing aluminium alloys by forming L12 Al3Zr precipitates and nucleating \u03b8\u2033 precipitates on them

Strengthening by precipitation of second phase is the guiding principle for the development of a host of high strength structural alloys, in particular, aluminium alloys for transportation sector. Higher efficiency and lower emission demands use of alloys at higher operating temperatures (200 °C–250 °C) and stresses, especially in applications for engine parts. Unfortunately, most of the precipitation hardened aluminium alloys that are currently available can withstand maximum temperatures ranging from 150–200 °C. This limit is set by the onset of the rapid coarsening of the precipitates and consequent loss of mechanical properties. In this communication, we present a new approach in designing an Al-based alloy through solid state precipitation route that provides a synergistic coupling of two different types of precipitates that has enabled us to develop coarsening resistant high-temperature alloys that are stable in the temperature range of 250–300 °C with strength in excess of 260 MPa at 250 °C.

The Effect of Precipitate Evolution on Austenite Grain Growth in RAFM Steel.

To study the effects of various types of precipitates and precipitate evolution behavior on austenite (size and phase fraction) in reduced activation ferritic/martensitic (RAFM) steel, RAFM steel was heated to various austenitizing temperatures. The microstructures of specimens were observed using optical microscopy (OM) and transmission electron microscopy (TEM). The results indicate that the M23C6 and MX precipitates gradually coarsen and dissolve into the matrix as the austenitizing temperatures increase. The M23C6 precipitates dissolve completely at 1100 °C, while the MX precipitates dissolve completely at 1200 °C. The evolution of two types of precipitate has a significant effect on the size of austenite. Based on the Zener pinning model, the effect of precipitate evolution on austenite grain size is quantified. It was found that the coarsening and dissolution of M23C6 and MX precipitates leads to a decrease in pinning pressure on grain boundaries, facilitating the rapid growth of austenite grains. The austenite phase fraction is also affected by the coarsening and dissolution of precipitates.

A SANS and APT study of precipitate evolution and strengthening in a maraging steel

In this work a combination of the characterisation techniques small angle neutron scattering (SANS) and atom probe tomography (APT) are used to study the precipitation in a maraging steel. Three similar maraging steel alloys were aged at different temperatures and ageing times, and then characterised using SANS, APT and microhardness. The alloys consist of two types of precipitates, namely Laves phase and β-NiAl, the precipitates have different composition and hence precipitate ageing, which makes it complicated to model. The SANS experimental set-up was relatively simple and allowed the precipitate size and fraction of a large number of samples to be measured in a single experiment. The APT results were used for constraining the SANS modelling, particularly the composition, shape and distribution of phases. The characterisation led to the following description of precipitation: NiAl phase reaches coarsening at early stages of ageing and shifts its strength mechanisms from shearing to Orowan looping, which cause the characteristic peak strength; the Laves phase is in growth throughout and its strength contribution increases with ageing time. These observations were shown to be consistent with precipitate evolution and strengthening models, and the work of others. Although, there are some issues with the combination of SANS and APT approach, which are discussed, the methodology provides a valuable tool to understand complex precipitation behaviours.

Effect of prior-austenite grain refinement on microstructure, mechanical properties and thermal embrittlement of 9Cr-1Mo-0.1C steel

The effect of ‘conventional normalizing and tempering’ (CNT) and ‘double austenitization based normalizing and tempering’ (DNT) heat treatments on the microstructure, tensile, creep and impact toughness properties of 9Cr-1Mo steel has been studied. The tempered martensite microstructure obtained through DNT treatment exhibited smaller sizes of prior-austenite grains/martensite packets (28 μm/11 μm) compared to the CNT treatment (44 μm/14 μm). The tempered martensite morphology was largely retained after long-term thermal aging at 550 °C/5000 h, while the M23C6 and M2(C,N) type of precipitates were found to act as nucleation sites for precipitation of brittle Fe2Mo Laves phase. The grain refinement by DNT was found to be beneficial for minimizing the ductile-to-brittle transition characteristics (25 °C lower ductile-to-brittle transition temperature and 70 J higher upper shelf energy) over the CNT. Thermal embrittlement occurred in both heated treated steels, but the transition temperature of aged DNT steel remained below room temperature. Fractured Charpy specimens revealed ductile failure by void coalescence for high temperature tests, and a quasi-cleavage fracture at low temperatures with few isolated occurrence of intergranular crack in thermal embrittled steel. The DNT treated steel resulted in similar or better tensile and creep properties, when compared to the CNT treatment. The homogeneous fine grained tempered martensite microstructure obtained by DNT treatment resulted in improved embrittlement resistance and mechanical properties over the conventional treatment.

Effect of friction spot welding (FSpW) on the surface corrosion behavior of overlapping AA6181-T4/Ti-6Al-4V joints

The microstructural, mechanical and corrosion behavior of the surface of an AA6181-T4 aluminum alloy overlap welded onto Ti-6Al-4V titanium alloy by friction spot welding (FSpW) was analyzed based on optical microscopy (OM), scanning electron microscopy (SEM), microhardness and potentiodynamic polarization techniques. The results indicated that the FSpW process modified the microstructure of the AA6181-T4 aluminum alloy, as indicated mainly by the size, type and quantity of precipitates in various welding regions. The microhardness tests showed similar hardness values in the stir zone (SZ) and base metal (BM), which was ascribed to the breakdown and homogenization of precipitates in the SZ. On the other hand, the heat affected zone (HAZ) showed the lowest hardness, which was attributed to the coalescence of Mg2Si precipitates caused by the thermal cycle of the welding process. The potentiodynamic polarization tests indicated that the SZ showed the highest pitting potential due to the refined microstructure in this zone. SEM images recorded after potentiodynamic polarization testing indicated that preferential sites for pitting nucleation were regions adjacent to the Al (Fe, Si, Mn, Mg) precipitates, and the mildest corrosive attack was found in the SZ.

Influence of Tool Rotational Speed and Post-Weld Heat Treatments on Friction Stir Welded Reduced Activation Ferritic-Martensitic Steel

The effects of tool rotational speed (200 and 700 rpm) on evolving microstructure during friction stir welding (FSW) of a reduced activation ferritic-martensitic steel (RAFMS) in the stir zone (SZ), thermo-mechanically affected zone (TMAZ), and heat-affected zone (HAZ) have been explored in detail. The influence of post-weld direct tempering (PWDT: 1033 K (760 °C)/ 90 minutes + air cooling) and post-weld normalizing and tempering (PWNT: 1253 K (980 °C)/30 minutes + air cooling + tempering 1033 K (760 °C)/90 minutes + air cooling) treatments on microstructure and mechanical properties has also been assessed. The base metal (BM) microstructure was tempered martensite comprising Cr-rich M23C6 on prior austenite grain and lath boundaries with intra-lath precipitation of V- and Ta-rich MC precipitates. The tool rotational speed exerted profound influence on evolving microstructure in SZ, TMAZ, and HAZ in the as-welded and post-weld heat-treated states. Very high proportion of prior austenitic grains and martensite lath boundaries in SZ and TMAZ in the as-welded state showed lack of strengthening precipitates, though very high hardness was recorded in SZ irrespective of the tool speed. Very fine-needle-like Fe3C precipitates were found at both the rotational speeds in SZ. The Fe3C was dissolved and fresh precipitation of strengthening precipitates occurred on both prior austenite grain and sub-grain boundaries in SZ during PWNT and PWDT. The post-weld direct tempering caused coarsening and coalescence of strengthening precipitates, in both matrix and grain boundary regions of TMAZ and HAZ, which led to inhomogeneous distribution of hardness across the weld joint. The PWNT heat treatment has shown fresh precipitation of M23C6 on lath and grain boundaries and very fine V-rich MC precipitates in the intragranular regions, which is very much similar to that prevailed in BM prior to FSW. Both the PWDT and PWNT treatments caused considerable reduction in the hardness of SZ. In the as-welded state, the 200 rpm joints have shown room temperature impact toughness close to that of BM, whereas 700 rpm joints exhibited very poor impact toughness. The best combination of microstructure and mechanical properties could be obtained by employing low rotational speed of 200 rpm followed by PWNT cycle. The type and size of various precipitates, grain size, and evolving dislocation substructure have been presented and comprehensively discussed.

Isochronal phase transformation of Nb–V–Ti microalloyed ultra-high strength steel upon cooling

The transformation behavior of Nb–V–Ti microalloyed ultra-high strength steel during continuous cooling was investigated by Gleeble-1500 thermal mechanical simulator. Thermal dilation curves were measured at different cooling rates, based on which continuous cooling transformation (CCT) curve was established. The microstructure at slow cooling rate, primarily consists of polygonal ferrite, pearlite and lower bainite, while at intermediate cooling rate besides dominated lower bainite, small amount of polygonal ferrite can also be observed. At higher cooling rate, martensite lath is obtained. The nano-sized precipitates at slow cooling rate include M23C6, M2C, M3C, rich-Nb MC and rich-Ti MC and at intermediate cooling rate and higher cooling rate, the type of precipitates were similar, M3C and two types of MC. At cooling rates ranging from 0.2 to 1°C/s, Vickers hardness of the steel decreases significantly. While at intermediate and higher cooling rates, the Vickers hardness increases gradually with increasing cooling rate. In addition, the polygonal ferrite transformation start temperatures (Ar3) decreases with increasing cooling rate, which is related to the carbide precipitation.

Characterization of Gd-rich precipitates in a fully lamellar TiAl alloy

We report microstructural characterization of a Gd-modified TiAl alloy with a fully lamellar structure by utilizing Cs-corrected transmission electron microscopy. Two types of Gd-contained precipitates, Gd2O3 and Al2Gd, with sizes ranging from several nanometers to micrometers were detected at grain boundaries and lamellar interfaces. Atomic-scale images reveal a well-defined crystallographic relationship between the nano-scale Al2Gd precipitates and γ-TiAl while Gd2O3, formed from TiAl melt during solidification and precipitated from α phase, is incoherent with TiAl matrix. The microstructural characterization provides direct evidence that the microstructure refinement of the Gd-modified TiAl is mainly from both oxide and intermetallic precipitates.

Growth of carbon and nitrogen containing precipitates in crystalline solar silicon and their influence on solar cells

magnified imageThe growth mechanisms, structural, mechanical, and electrical properties of silicon carbide and silicon nitride precipitates in solar silicon are reviewed and some new aspects about the distribution of trace elements in these precipitates are reported in this review. SiC and Si3N4 precipitates may have detrimental impact on the quality of solar silicon material. Therefore intensive research has been done at these precipitates in the past, which is summarized first. The properties of the different types of precipitates reported in literature are then described in detail. Especially SiC precipitates may have detrimental impact on the electrical behavior of solar cells by causing severe ohmic shunting. Furthermore SiC precipitates are very hard and may harm the wafering process. An outlook of the impact of such precipitates on new cell concepts and new solar silicon materials, such as n‐type, is given.

The Influence of Composition on the Clustering and Precipitation Behavior of Al-Mg-Si-Cu Alloys

The natural aging (NA) and artificial aging (AA) behavior of Al-Mg-Si-Cu alloys with different Mg/Si ratios and Cu additions were systematically investigated by means of hardness test, atom probe tomography, transmission electron microscopy, and Monte Carlo simulation. The Si-rich low-Cu alloys displayed higher hardness compared to the Mg-rich equivalents because Si atoms play a dominant role in clustering of solute atoms during both natural and artificial aging. In the high-Cu alloys, Cu did not obviously change the cluster distribution during NA, but significantly refines the clusters and precipitates due to the strong interaction of Cu atoms with Mg atoms during AA. In contrast to the low-Cu alloys, the Mg-rich high-Cu alloys exhibit higher hardness in the early and over-aged stages of artificial aging, with similar or slightly higher hardness in the peak aging condition compared to their Si-rich counterparts. Three types of precipitates (β″, Q′, and L) are favored in the high-Cu alloys. The Mg-rich high-Cu alloy has more L phase, while the Si-rich variant is abundant in Q′ phase. The negative effect of NA on subsequent AA behavior is less dependent on Mg/Si ratio in the high-Cu alloys due to a synergistic action of the residual Si and Cu atoms, but is closely related to Mg/Si ratio in low-Cu alloys.

Micro Structural Changes in Welded AlCuLi-Alloys by Positron Annihilation Spectroscopy, SAXS and DSC

We follow changes in the micro structure at several distances from the weld nugget of friction stir welded AlCuLi-alloy (AA2198) plates occurring due to the tool movement and the created heat by employing different methods: Small Angle X-ray Scattering (SAXS), giving information on type, size and density of precipitates, Differential Scanning Calorimetry (DSC), giving information on formed precipitates by their dissolution signal, and positron annihilation lifetime spectroscopy (PALS), being sensitive to vacancies and dislocations as well as to the formation and growth of precipitates. We start by characterizing the base material as a reference and proceed via the heat-affected zone to the weld nugget. By the use of complementary methods, we obtain information on structure, kind and distribution of precipitates and correlate this with hardness measurements.

Features of Titanium Carbide Precipitation in Low-Carbon High-Strength Steels Microalloyed with Titanium and Molybdenum

Theoretical and experimental study is carried out to establish features of all types of excessive phase precipitation in low-carbon steels microalloyed with titanium and molybdenum. It is established that the main type of precipitates controlling grain boundary and precipitation hardening mechanisms is FCCcarbide based on TiC. The optimum thermal deformation treatment regime is established providing simultaneously effective grain refi nement of steel structure and the formation of a system of nanosize carbide precipitates that control strengthening by a precipitation hardening mechanism. On the basis of the results obtained, chemical composition and heat-treatment regime are proposed for producing low-carbon TiM–Mo microalloyed steels with a ferritic microstructure and set of mechanical (tensile strength more than 850 MPa) and other service properties.

The Significance of Thermo-Mechanical Fabrication on Long Term Creep Life of Type 316H Austenitic Stainless Steel Components

The UK’s Advanced Gas Cooled Reactor (AGR) nuclear power generating plant operates at temperatures up to 550 °C, where creep life is important. We consider Type 316H austenitic stainless steel headers and tubes with thermo-mechanical fabrication histories that result in significantly different initial microstructures. The heat affected zone of weldments, in these thick section headers and thin walled boiler tubes, have been found to be susceptible to creep damage leading to cracking during service. In this work we explore these differences in the long term service aged microstructure and the link to overall creep life of these components. To achieve this, samples containing weldments have been removed from plant after extended periods of service. Specifically parent and HAZ regions have been examined to determine the types of precipitates arising from the long term ageing. In particular, thin foils have been examined in a JEOL ARM transmission electron microscope operating at 200KeV fitted with an Oxford Instruments energy dispersive spectrometer to allow comparison between high resolution images and chemical composition. The thin foils were removed from predetermined locations using gallium ion milling and finally thinned using a low ion current to minimise ion damage. Differences between the distributions and types of precipitates are considered in the context of the initial microstructure arising from the thermo-mechanical history on the loss of creep strength and initiation of creep cavities at grain boundaries. The impact on overall service life is addressed.

Effect of Nickel addition and solution treatment time on microstructure and hardness of Al-Si-Cu aged alloys

The effects of the addition <2wt.% of Nickel and solid solution treatment times on microstructure and hardness of Al-Si-Cu alloy (A319) are reported. The results obtained show that Ni additions and heat treatments influence changes in microstructure, hardness and precipitation kinetics. The dendrites size and number density of Al-Cu-Ni and Al-Ni intermetallic compounds in as-cast condition are affected with Ni additions; however, occurs a reduction of Al-Cu-Ni phases at longer solution times (7h). The hardness values in alloyed samples with Ni are higher than those of reference samples, the maximum hardness peak are reached at shorter aging times; nonetheless, the decrease was slower during overaging step. The hardness values increase with Ni content ~4% HV where an increase of maximum hardness ~6% HV is reached with aging. After 10h of aging treatment at 170°C, the alloyed samples and not alloyed present different type of precipitates. In reference sample θ′-Al2Cu precipitates were identified, and in the alloyed samples, two types of coherent precipitates with different thickness and size were observed, in addition those precipitates present smaller size and higher number density than those observed in the reference alloy.

Potential formation damage and mitigation methods using seawater-mixed acid to stimulate sandstone reservoirs

It has been a practice to use seawater to prepare hydrochloric acid for carbonate reservoir stimulation in the offshore operations where fresh water is relatively expensive or logistically impossible to use. However, compare with carbonate matrix stimulation, F− and secondary SiF62− are released into the solution using hydrofluoric acid to treat sandstone reservoirs. The released components will combine with Mg2+, Ca2+, Na+, and K+ ions in seawater. Large quantities and various types of precipitates are formed when critical scaling tendency was exceeded, which results in serious formation damage.In this research, artificial simulated seawater (salinity about 35‰) was mixed with mud acid (almost hydrofluoric acid) and multi-hydrogen acid (an organophosphonic acid complex). Chemical effects of current seawater-mixed acid were investigated using spectrophotometry methods. The chemical effects include acid composition and concentration, HCl volume, seawater dilution ratio, and inorganic salt on damage quantities during acid preparation and reaction. Effective mitigation methods favorable to the inhibition of precipitation were provided.Results show that fluoride precipitates including CaF2 and MgF2 can be effectively mitigated by decreasing the free F− concentration and the pH value, increasing the seawater dilution and adding the inorganic electrolytes to elevate the ionic strength of solution. Because the organic phosphonic acid forms complexes with the metal ions, precipitation mitigation capacity of the multi-hydrogen acid is stronger than that of the mud acid. A special and abnormal common-ion effect was discovered based on the multi-hydrogen acid system. The addition of calcium ions in the seawater-mixed multi-hydrogen acid solution produces a super-clear liquid system with the turbidity <30 FTU. Static simulation shows that fluorosilicate precipitates as products of the secondary reaction in acidizing are mainly determined by the dissolution ratio. The precipitates can be mitigated by adding proper amount of hydrochloric acid, increasing the concentration of ammonium chloride in acid, and using appropriate inhibitors such as organophosphonic acid HA and DA. According to a parametric analysis, a seawater-mixed acid system based on the synergistic reactions of acid effect, salt effect and complex effect was proposed to mitigate formation damage for the sandstone reservoir stimulation using seawater-mixed acid. Reactions between the acid and different minerals indicate that this acid system could remain clear and eliminate secondary deposition during acidizing treatment.

Failure of a nickel aluminium bronze (NAB) canned motor pump impeller working under polluted sea water – Influence of material selection, section thickness dependent microstructure and temper annealing heat treatment

Abstract In the present study, a damaged pump impeller working under highly polluted sea water was taken up for investigation on probable cause(s) for its failure. Major steps in the investigation were visual examination, thorough microstructural observation of the affected (surface and sub-surface) and unaffected portions of the base material at different locations on the impeller having varying section thickness. An effort has been made for correlating unaffected microstructural attributes to corroded features with respect to various section thicknesses. Failure of the impeller is found to be due to combined effect of a corrosion susceptible microstructure and highly polluted corroding medium that resulted in selective phase corrosion (SPC) of certain phases in the microstructure leading to surface roughening and perforation, predominantly through pitting mechanism governed by dealuminification initiated at phase interfaces. Networked alpha plus kappaIII eutectoids, Cu-O type of precipitates, dendritic Cu-Al phase and retained beta have been found to be corrosion susceptible microstructure. Principle c orroding features in thick sections were found to be kappaII and alpha plus kappaIII eutectoids, while that in thin sections include severe cracking involving Cu-S-O rich coarse precipitate (of 15 μm approx size), corrosion along interfaces of dendritic Cu-Al phase and retained beta with alpha matrix. Presence of alpha plus kappaIII eutectoids and retained beta has been the result of combined effect of improper selection of material (aluminium) composition, complex cast shape of the impeller and ineffective temper annealing treatment, while dendritic Cu-Al phase and high level of porosity are casting defects.

Generating Mechanism of Large Heterogeneous Carbonitrides with Multiple Layers in H13 + Nb Bar

The characteristics and generating mechanism of heterogeneous carbonitride with micron size in H13 + Nb bar are studied. Three types of precipitates are observed according to the number of layers, that is, two-layer ones, three-layer ones, and four-layer ones. Two-layer precipitate contains the oxide core and the outer Ti–V-rich carbonitride, while three-layer precipitate has one more Nb-rich carbonitride at the outside. Four-layer precipitate has a V-rich carbonitride at the outmost layer besides the inner three types of phases similar to that in the three-layer precipitate. The size of these large precipitates can be even up to tens of microns. The oxide cores are mainly stoichiometric spinel MgAl2O4 and generated in the liquid steel before solidification. The compositional characteristics and precipitating sequence of carbonitride can be well explained through calculation by Scheil–Gulliver model in Thermo-Calc. Ti–V-rich phase precipitates first during solidification, after which Nb-rich phase and V-rich phase precipitates successively. Some dissolution occurs between the different layers during the following temperature reducing and heating treatment process.