Fine Precipitates Research Articles

SPINODAL DECOMPOSITION OF PRECIPITATION HARDENING Fe-17Cr-4Ni-4Cu STAINLESS STEEL AT 475 °C

Microstructure evolution and mechanical properties in an Fe-17Cr-4Ni-4Cu alloy aged at 475 °C after different aging times were studied. Conventional transmission electron microscopy (TEM) and high-resolution electron microscopy (HREM) studies revealed the formation of 9R-structure Cu-rich precipitates and Cr-rich α’ phase by spinodal decomposition in the samples aged at 475 °C after 100–1000 h. The fine Cu-rich precipitates and Cr-rich α’ phase by spinodal decomposition lead to a significant increase in the hardness, together in the early stages (100 h). Continued aging to 500 h leads to increased precipitation of the Cr-rich α’, which provides significant strengthening, reaching maximum hardening, despite the continued loss of hardening by weakening by the Ostwald ripening of the Cu-rich precipitates. Extending the aging time to 1000 h leads to substantial reversed austenite transformation and a large number of ripening -copper precipitates that causes softening. The results of the impact tests showed that the major fracture mode was cleavage and/or quasi-cleavage.

Effects of Si addition on properties and microstructure of CuCrZr alloy

In order to develop high-strength and high-conductivity copper alloys with better comprehensive properties, the effects of Si addition on the mechanical properties, electrical conductivity and microstructure of CuCrZr alloy were investigated in this paper. The addition of Si element can improve the hardness, strength and ductility of CuCrZr alloy. The optimal peak aging parameter for CuCrZrSi alloy is 430 °C for 3 h, the hardness 228.92 HV, the tensile strength 646.76 MPa, and the elongation after fracture 10.67%. Compared with the CuCrZr alloy under the same process, the mechanical properties are increased by 10.3%, 11.9% and 16.9%, respectively. The conductivity is 4 ± 1%IACS lower than that of the sample without Si addition. The addition of Si element refines the deformation substructure and produces more Σ3n (n = 1, 2, 3) grain boundaries, thus providing more nucleation sites and higher driving forces for the precipitation phases. In both CuCrZr and CuCrZrSi alloys, the fine precipitation phases (<10 nm) fcc-Cr phase and orthorhombic-Cu8Zr3 phase, together with coarse precipitation phases (>100 nm) bcc-Cr phase and hcp-Zr phase can be observed. The orientation relationship among the two fine precipitation phases and the matrix is (1¯1¯1)[11¯0]Cu∥(1¯1¯1)[11¯0]Cr∥(133)[01¯1]Cu8Zr3. The Si element tends to precipitate together with the Cr phase and Zr phase, and acts to refine and spheroidize the precipitation phases.

Precipitation of β-phase and twinning during deformation of Mg–Al alloy AZ91 at 150 °C after solution treatment

Abstract Precipitation of β-phase and deformation twinning have been studied in a solid solution-treated AZ91 alloy, which was subjected to compressive creep loading at 150 °C and 100 MPa for 163 h. It was found that nucleation of β-phase in solution-treated AZ91 is enhanced by concurrent deformation under creep conditions at 150 °C. The relatively fine precipitate structure leads to a reduction in minimum creep rate and, therefore, to an increase in creep life by a factor of 10 compared to the as-cast condition at the investigated conditions. In addition, twinning occurred in coarse grains of hard orientation. The twin boundaries lead to formation of incoherent precipitates and discontinuous precipitation within the former grains.

The Effect of Cooling Rate on the Microstructure Evolution and Mechanical Properties of Ti-Microalloyed Steel Plates.

Ti-bearing microalloyed steel plates with a thickness of 40 mm were subjected to ultra-fast cooling (UFC) and traditional accelerate cooling after hot-rolling, aiming to investigate the effect of cooling rate on the microstructure and mechanical properties homogeneity, and thus obtain thick plates with superior and homogeneous mechanical properties. Yield strength, tensile strength, and elongation were 642 MPa, 740 MPa, 19.2% and 592 MPa, 720 MPa and 16.7%, respectively, in the surface and mid-thickness of the steel with ultra-fast cooling, while in the steel with traditional accelerate cooling, 535 MPa, 645 MPa, 23.4% and 485 MPa, 608 MPa, 16.2% were obtained in the surface and mid-thickness of the plate. The yield strength has been greatly improved after UFC, for the refinement of grain and precipitates produced by UFC. In addition, the equivalent grain size and precipitates size in the thick plate with UFC are homogeneous in the thickness direction, leading to uniform mechanical properties. The crystallographic characteristics of different precipitates have been studied. The precipitates formed in the austenite deformation stage obey Kurdjumov–Sachs orientation relationship with the ferrite matrix, while the fine precipitates formed in the ferrite obey [112]MC//[110]α and // orientation relationship with the ferrite matrix.

Research on tribological behavior of graphene with in-situ MgO nanoparticles reinforced AZ91 alloy composite

Magnesium-coated graphene (GNPs) was successfully prepared by a novel organic chemical reduction method with significantly improved wettability and dispersion. The GNPs were uniformly distributed in the matrix, while a finer and more dispersed strengthening phase β-Mg17Al12 was precipitated around it. Moreover, MgO nanoparticles were formed by in-situ reaction on the surface of GNPs, forming nano-scale contact and tight interfacial bonding with the substrate, thereby significantly improving the wear resistance of the composites. The wear mechanism has also changed from severe delamination and abrasive wear to mixed wear of slight delamination, adhesion, and oxidation. The increase in wear resistance is attributed to the combined effect of the lubrication and load transfer of GNPs, the enhanced effect of fine precipitates on the mechanical properties of the matrix, and the synergistic effect of in situ generated MgO nanoparticles against deformation.

Fabrication and electrical sliding wear of graphitic Cu–Cr–Zr matrix composites

Abstract The present work was undertaken to prepare graphitic Cu–Cr –Zr matrix composites in order to develop new materials for application under sliding contact in an electrical field. The electrical sliding wear behavior of the graphite (Gr)/CuCrZr composite was investigated from a microstructural viewpoint, compared with the matrix alloy. The results indicate that by using the water atomized Cu–Cr–Zr powder and copper-coated graphite particles as starting materials, compact and homogeneous microstructure can be obtained in the composites. Fine and coherent particulate precipitations are formed in the Cu–Cr–Zr matrix during the aging treatment process, which help to improve the mechanical properties. Under the electrical sliding wear condition, incorporation of graphite in the Cu–Cr–Zr matrix not only decrease the wear loss of the sliding pair but also maintain steady-state friction coefficient. The main reason is that the graphite film that smears on the contacting surfaces can alleviate materials loss induced by electric arc discharge and adhesion wear. Proper aging treatment helps to make full use of the solid lubricant effect of the graphite particles.

Enhancing the strength-ductility trade-off in a NiCoCr-based medium-entropy alloy with the synergetic effect of ultra fine precipitates, stacking faults, dislocation locks and twins

In this work, multi strengthening/deformation mechanisms were introduced into a Ni-rich Ni40Co32Cr20Al3.5Ti3Nb1.5 (at%) medium-entropy alloy. After thermo-mechanical procedures, the microstructure is characterized by ultra fine γ' particles (2–18 nm) and short stacking faults. The alloy has superior mechanical properties with gigapascal yield strength, excellent ductility and strong work hardening ability. The initial high strength is caused by the blocking effect of ultra fine γ' particles, dislocation locks (Lomer-Cottrell locks), solute atoms and grain boundaries to dislocation movement. TEM observations at different strains indicate that the increasing number of stacking faults, Lomer-Cottrell locks and deformation twins are the main reasons for the improvement of work hardening ability during deformation. Twinning and dislocation slip, including perfect dislocations and partial dislocations, are the main deformation mechanisms. Our findings show that the coordination of multi strength/toughing mechanisms may be an effect way to enhance the strength-ductility trade-off.

Low Cycle Fatigue Behavior of Al-Mg-Si Alloys Extruded Parts

Over the past years, Al-Mg-Si alloys have been largely applied in automotive industry, which has required a deep knowledge of their mechanical properties and the influence of precipitates distribution on their mechanical behavior. This work evaluated the main mechanical properties of AA6005, AA6063, and AA6351 alloys by means of tensile and low cycle fatigue tests with 0.005 seg-1 deformation rate and 0.3% < εat <1.2% strain amplitudes. Besides, the hysteresis loop and internal stress analysis were investigated to analyze hardening and softening phenomena and to evaluate the friction and back stresses, respectively. Macro and microstructural were performed focusing in intermetallic distribution. Concerning the low cycle fatigue behavior, AA6351 presented shorter lives for strain amplitudes higher than 0.5%, and AA6005 showed the highest fatigue strength and fatigue ductility. AA6063 showed the lowest fatigue strength due to the presence of coarse particles (Fe,Mn)SiAl. During internal stress analysis, the highest value of friction stress for AA6351 indicates the effect of hardener precipitates are the most relevant role for cyclic loadings and the lowest back-to-friction stress ratio indicating that deformation is controlled by particles. In the other hand, AA6063 showed the lowest friction stress due to low amount of fine precipitates.

Synthesis and characterization of a ultrafine grained (CoCrFeNi)80Mn10Ti10 multi-principal element alloy nanocomposite

In this paper, a ultrafine-grained (CoCrFeNi)80Mn10Ti10 multi-principal element alloy (MPEA) nanocomposite reinforced by TiO nanoparticles was fabricated by an innovation alliance of mechanical alloying (MA) and spark plasma sintering (SPS). The microstructural evolution, mechanical properties, strengthening mechanism, and fracture mechanism of the (CoCrFeNi)80Mn10Ti10 MPEA were investigated systematically. The (CoCrFeNi)80Mn10Ti10 MPEA powder presented BCC and FCC phases. Following SPS, the phase transformation from BCC to FCC phases has occurred in the (CoCrFeNi)80Mn10Ti10 MPEA. And the (CoCrFeNi)80Mn10Ti10 MPEA after SPS (referred as SPS11 MPEA) presented a bimodal microstructure consisted of ultrafine grained FCC matrix phase (∼285 nm) and uniformly dispersed TiO nanoparticles (∼87 nm). And a large number of annealing twins were also observed in the SPS11 MPEA. The SPS11 MPEA exhibited an excellent combination of compressive yield strength of 1500 MPa, compressive strength of 2700 MPa and strain of 36%. The nanoindentation tests showed that TiO phase possessed the higher nano-hardness, 12.92 GPa, while it was just 6.56 GPa for FCC matrix phase. According to the nanoindentation results, it is estimated that the TiO phase had the better anti-wear ability, compared to FCC phase in the SPS11 MPEA. The quantitative contributions of different strengthening mechanisms suggests the fine grain strengthening and dispersed oxide precipitation strengthening make the SPS11 MPEA possess excellent compressive yield strength and fracture strength. The high compression stain of the SPS11 MPEA is thought to be attributed to the formation of dislocations cell structure and deformation twins during plastic deformation.

Optimizing the recovery process of ceramic grade calcium fluoride from hydrofluoric/hexafluorosilicic acid wastewater

Chemical precipitation using calcium salts has been widely applied in hydrofluoric acid wastewater treatment due to its simplicity, flexibility and economy. Coagulants/flocculants are used to improve solid-liquid separation in a fine precipitate of calcium fluoride (CaF2). As a result, a large amount of sludge is generated by CaF2 precipitation process. In order to move towards a circular economy, the current study developed an environment-friendly and cost-effective approach to treat concentrated hydrofluoric/hexafluorosilicic acid wastewater (∼117 g F−/L) obtained from an optoelectronic industry in Taiwan and to recover fluoride as the high purity CaF2, as well as to reduce the amount of final waste disposal. The influence of three independent variables, including calcium-to-fluoride molar ratio ([Ca2+/F−], silicon concentration ([Si4+]), and polymer dosage applied, on the purity of recovered CaF2 products, was investigated and optimized by combining response surface methodology (RSM) with central composite design (CCD). The ceramic grade CaF2 of 85.6–87.9% was achieved under the optimum conditions as [Ca2+/F−] of 0.618–0.645, [Si4+] of 1.48 g/L, and polymer concentration of 17.2–17.8 mg/L. The final turbidity, residual fluoride concentration in the supernatants and fluoride removal efficiency were 14.8–16.2 NTU, 62.2–67.6 mg/L and greater than 99%, respectively. The value of R2 = 0.9498 and adjusted R2 = 0.9319 indicated an adequate correlation between experimental results and predictions by the quadratic model. In addition, a new insight into the CaF2 purification method has been proposed based on the idea of pH control.

Modulation of Multiple Precipitates for High Strength and Ductility in Al-Cu-Mn Alloy.

The category and morphology of precipitates are essential factors in determining the mechanical behaviors of aluminum alloys. It is a great challenge to synthetically modulate multiple precipitates to simultaneously improve strength and ductility. In the present work, by optimizing the precipitations of the GP zone, θ’-approximant and θ’ phase for an Al-Cu-Mn alloy, a high tensile strength of 585 MPa with large elongation of 12.35% was achieved through pre-deformation and aging. The microstructure evolution pattern was revealed by detailed characterizations of scanning electron microscopy and transmission electron microscopy. It was found that such high tensile strength of the samples was due to a combination of strengthening by the high density of dispersive fine precipitates and dislocations, and the high elongation to failure was primarily attributed to the multimodal precipitates and elimination of precipitation-free zones along the grain boundaries. The strategy proposed here is a promising way of preparing ultra-strong Al-Cu-Mn alloys.

A review of progress on high nitrogen austenitic stainless-steel research

High nitrogen austenitic stainless steels are commonly used in wide range of applications because of their excellent properties, attracting super attention over the past decades. Compared with other metal materials, high nitrogen austenitic stainless steel increases the nitrogen content under the premise that the structure is austenite, giving it excellent mechanical properties and corrosion resistance. Based on relevant documents from the past ten years, this article summarizes and compares three preparation methods for high nitrogen austenitic stainless steels, namely: powder nitriding, melt nitriding and bulk nitriding. They can be divided into six categories according to other differences as explained by the latest research progress on strengthening and toughening mechanism for high nitrogen austenitic stainless steels: composite structure strengthening, fine grain strengthening, precipitation strengthening and strain strengthening. This article also reviews the research progress on excellent properties of high nitrogen stainless steel, including strength, hardness and corrosion resistance. It further describes the emerging nickel-free high nitrogen austenitic stainless steels and its biocompatibility. Welding applications of high nitrogen austenitic stainless steels are also described from three aspects: friction stir welding, arc welding and brass solder. Finally, this article puts forward the development direction of high nitrogen austenitic stainless steels in the future.

Crystallography of the precipitates formed on {10-11} twin boundary in Mg Al alloys

{101¯1} contraction twinning is an important deformation mode in Mg alloy, which can be easily activated during sheet rolling process. The introduction of twin boundaries in Mg alloys can promote the nucleation of fine precipitates, leading to better mechanical properties. However, the crystallographic characteristic of the precipitates formed on {101¯1} twin boundaries has rarely been reported before. In this work, the crystallography of the Mg17Al12 precipitates formed on {101¯1} twin boundary (TB) in a MgAl alloy was systematically studied by transmission electron microscopy. It was for the first time revealed that the precipitates formed at either broad TB or twin tip hold an identical orientation relationship, i.e., [21¯1¯0]α//[111¯]β and (011¯1)α//(011)β with both the matrix and the twin. Such preferred crystallographic orientation should be driven by the minimization of the total energy between the matrix-precipitate interface and the twin-precipitate interface.

Effects of grain refinement and precipitate strengthening on mechanical properties of double-extruded Mg-12Gd-2Er-0.4Zr alloy

The Mg-12Gd-2Er-0.4Zr (GE122K) is fabricated by double extrusion, and the effects of grain refinement and precipitate evolution on the mechanical performance of the extruded alloys are deeply analyzed. Scanning electron microscope results spectacle that some bulky precipitates in the single extruded alloy are fragmented to fine precipitates during double extrusion and distribute linearly along the extrusion direction (ED). The fine precipitates exhibit the strongest dispersion strengthening effect, whereas the bulky precipitates act as crack resources and decrease the elongation. After double extrusion, a refined grain structure with an average grain size of 2.7 µm is achieved, which is attributed to dynamic recrystallization (DRX). Moreover, double extrusion boosts the content of rare-earth elements into the matrix by dissolving the fine precipitates. During aging, extensive β′ precipitates form in the double extruded alloy than in the single extrude alloy. As a result, refined grains and strong precipitation strengthening make a significant contribution to a high yield strength (YS) of 422 ± 1.59 MPa with an elongation (EL) of 6.5 ± 0.89% in the double extruded+peak-aged alloy (ACX2). The possible strengthening mechanisms are discussed, and it is found that grain refinement and precipitate strengthening are the main contributors to the high strength of the GE122K alloy.

Effects of Cu-alloying in minute quantities on the corrosion-induced mechanical degradation behaviors of medium C-based ultrahigh-strength steel in aqueous environments

This study examined the applicability of Cu-alloying in minute quantities to medium C-based ultrahigh-strength automotive steel with a tensile strength of more than 2000 MPa to improve the resistance to mechanical degradation caused by aqueous corrosion. Cu-alloying of this steel favored the precipitation of fine-(Ti,Mo)C particles, leading to a smaller prior-γ grain size (PAGS) and inter-lath size, resulting in higher tensile strain. In addition, Cu-alloyed steel showed the surface inhibiting characteristics in a neutral aqueous condition based on higher corrosion resistance (i.e., larger polarization resistance and less weight loss). The smaller particle size of corrosion scale (CuxFe3-xO4), more uniform surface roughness, and smaller PAGS can be the major mechanistic reasons for suppressing the reduction in the mechanical properties of Cu-alloyed ultrahigh-strength steel in the corrosive environment.

Solubility product of a Nb–N bearing austenitic stainless-steel biomaterial

This research investigates the Z-Phase solubility product of austenitic stainless steel (ASS) ASTM F-1586 used as orthopedic implants under the solubilization procedure. Precipitate extraction by an electrolytic separation technique and analytical thermodynamic calculations as a function of the solubilization temperature and chemical composition were employed. In the first procedure of preferential electrolytic phase dissolution, samples were submitted to a solubilization treatment, followed by quenching. The specimens were subjected to a surface treatment then dissolved in solution. Following the dissolution, the electrolyte was vacuum filtered on a 0.20 μm polyvinylidene difluoride (PVDF) membrane and vacuum dried for 24 h. The microstructures were characterized via scanning electron microscopy (SEM/EDS) and transmission electron microscopy (TEM/EDX) techniques. The results revealed by the Z-phase (Cr2Nb2N2) precipitation potential (Ks) that the precipitates acted as a softening inhibitor agent and retarded the grain growth at lower solubilization temperature conditions (Ts < 1100 °C). The considerable amount of fine precipitates were mainly formed due to the nucleation and coalescence of Cr/NbN in the matrix. On the other hand, larger precipitates nucleated and coalesced above 1200 °C, and had their volume fraction reduced at this condition.

Effect of increasing Ti content on the phase, interface, dynamic mechanical properties and ballistic performance of W–Ti–Zr alloys

W2Zr intermetallic is an important reason for the brittleness of W–Zr binary system. When W–Zr alloys are used as the energetic fragment, the integrity of a projectile cannot be maintained under the explosion even propellant loading process, resulting in severe fracture of the projectile. Moreover, due to the strong atomic bonding in W2Zr intermetallic, the oxidation reactivity of the Zr atoms with oxygen in the air is significantly suppressed, limiting the effective energy release of W–Zr fragment under high-speed impacting process. In this study, based on the thermodynamic analysis, 30W-xTi-(70-x)Zr (wt.%) alloys were designed and prepared by powder metallurgy. It was found that the formation and stabilization temperature of W2Zr phase can be reduced by increasing the Ti content. The hard W-rich phase has changed from the coarse W2Zr particle to fine W2Zr precipitate and finally to the WTix solid solution with the increase of Ti content. Meanwhile, a semi-coherent interface between fine W2Zr precipitate and α matrix and even coherent interface between WTix and α matrix were formed, by which the intergranular fracture ratio between W-rich phases and α matrix was significantly decreased in the W–Ti–Zr alloys. Consequently, higher dynamic compressive strength and ductility of the W–Ti–Zr alloys were obtained by increasing the Ti content. The selected W–Ti–Zr alloys with high strength and proper failure strain exhibited good penetration and energy-releasing behaviors when they were used as reactive bullet launching at a velocity of about 1250 m/s. The relationship among the dynamic compressive strength, ductility and ballistic performance of the W–Ti–Zr reactive structural alloys was also discussed in detail.

Mechanical properties and precipitation behavior of high strength hot-rolled ferritic steel containing Nb and V

The mechanical properties and precipitation behavior of hot-rolled microalloyed steels with varied Nb and V additions were investigated. The steels had a predominantly ferritic microstructure with a very good combination of strength ( σ U T S = 800–1000 MPa), ductility (total elongation = 16–19%) and hole expansion ratio (32–34%). The strength increased with increasing Nb and V content. The stretch-flangeability of the steels did not deteriorate with increasing strength. The favorable hole expansion behavior of the steels is attributed to the fine ferrite grain size (1.6–1.8 μm) and uniformity of the microstructure, as well as the fine precipitates of Nb and V carbonitrides in the matrix. Nb carbonitrides formed during hot rolling and contributed to the grain refinement through their interaction with recrystallization. The co-precipitation of Nb and V carbonitrides during coiling was confirmed and characterized by 3D atom probe tomography. The formation of these fine precipitates contributed to further increase in the strength of the V-containing steel allowing the steel to achieve strength >1000 MPa.

Quantitative analysis of Nb in solid solution in low carbon steels by atom probe tomography and inductively coupled plasma mass spectroscopy

The influence of solute drag effects on austenite grain growth in microalloyed steels has been overshadowed by the well-established phenomenon of Zener pinning associated with a dispersion of fine precipitates. Enhanced toughness in the weld heat-affected zone (HAZ) of high Nb steels suggests that microalloying effects are still operative at peak temperatures even beyond known precipitate stability. As the cornerstone for unveiling the solute drag effect, the solute concentration in the matrix should be accurately measured. In this work, we compared inductively coupled plasma mass spectroscopy (ICP-MS) and atom probe tomography (APT) in determining the extent of Nb dissolution as a function of increasing temperature in two commercial American Petroleum Institute steel grades with different levels of Nb and marked variations in weld HAZ toughness. Both techniques demonstrated that the majority of Nb exists in solid solution at the peak temperatures. APT provided a more consistent determination of soluble Nb, in a better agreement with thermodynamic calculations, whilst ICP-MS underestimated the concentration of Nb in the matrix. This work provides valuable insights in determining soluble Nb concentration in steels.

The development of a588 modified laterite steel using thermomechanical and low-temperature tempering process for weather resistant steel

Laterite Steel A-588 has the potential to be a high strength low alloy for Corten steel application. Laterite steel A-588 is developed through a thermomechanical process followed by a tempering process to obtain high strength and corrosion resistance. This study aims to determine the correlation between the addition of nickel content, the variation of the cooling rate during heat treatment to the mechanical properties, and the corrosion resistance of A-588 laterite steel. The Cu, Cr, Ni, P, and Si elements significantly impact microstructure transformation. Laterite Steel A-588 with nickel and thermo-mechanical process variation has been focused on in this research. Laterite steel with 0,42%, 1%, 2%, and 3% nickel varied was homogenized, hot rolled, and heat treated with three cooling variations by water, oil, and air. They are processed with 150 C tempering. Low tempering temperature caused fine carbide precipitation and phase transition of martensite to bainite. This resulted in bainite as the final microstructure, lath tempered martensite, carbide, and ferrite. 3% Ni with a fast cooling rate increased the tempered martensite and bainite phase formation. It allowed the strength and hardness to increase relatively, followed by decreased elongation and corrosion resistance caused by the galvanic reaction. Most optimal of mechanical properties determined at a sample with 2% nickel in a water medium (strength 1203 MPa, elongation 10%, hardness 404 BHN, corrosion rate 1,306 mpy).