Needle-like Phase Research Articles

Effects of Iron, Manganese, and Cooling Rate on Microstructure and Dry Sliding Wear Behavior of LM13 Aluminum Alloy

ABSTRACTThe presence of Fe in eutectic Al-Si alloy can give rise to formation of a β-Al5FeSi needle-like phase because of low solubility of Fe in this alloy. The brittle nature of this phase leads to decreased mechanical properties of piston alloys, one of which is wear resistance. In this research, to modify β-Fe morphology, Sr and Mn elements were added to the samples prepared from LM13 containing different amounts of Fe. The effect of cooling rate on wear properties of this alloy was also investigated. Results show that the addition of Mn/Fe at aratio of 1/2 can modify the needle-like intermetallics present in the samples containing up to 1.2 wt% Fe. In addition, increasing the cooling rate from 3 to 15°C s−1 refined the eutectic structure and the intermetallics. Hence, it can be considered as another important factor to improve the wear properties of LM13 alloy. To study the wear behavior of this alloy, worn surface and subsurface areas were examined using scanning electron microscopy (SEM) equipped with an energy-dispersive spectrometry (EDS) analyzer.

Microstructure and mechanical properties of an Mg-4.0Sm-1.0Ca alloy during thermomechanical treatment

The microstructure and mechanical properties of an as-cast Mg-4.0Sm-1.0Ca alloy were investigated during thermomechanical treatments consisting of hot extrusion, rolling, and aging at 473 K. Mg41Sm5 phases containing Ca and needle-like Mg2Ca phases formed in the Mg matrix, and the average grain size and elongation were 4.2 μm and 27%, respectively, after hot extrusion, which implied an increase in ductility. In addition, after the rolling, the grain size was further refined, and the tensile strength increased to 293 MPa. A new precipitate Mg3Sm was found in the peak-aged Mg-4.0Sm-1.0Ca alloy and this alloy displayed the best mechanical properties, with a peak hardness of 83 HV and ultimate tensile strength of 313 MPa; these properties were attributed to grain refinement strengthening, solid solution strengthening, work hardening, and precipitation strengthening.

Evolution of microstructure, mechanical and corrosion properties of AlCoCrFeNi high-entropy alloy prepared by direct laser fabrication

High entropy alloy (HEA) is an emerging class of engineering materials that shows promising potential for high temperature applications. These multi-component alloys are mostly fabricated by arc melting. In this study, direct laser fabrication (DLF) is utilized to prepare AlCoCrFeNi high entropy alloy at optimized operation parameters. The phase, microstructure, mechanical and corrosion properties of as-deposited alloy as well as samples aged at temperatures of 600 °C, 800 °C, 1000 °C and 1200 °C for 168 h have been investigated. The results show that high cooling rate during deposition inhibits the formation of FCC phase, leading to a nearly single B2 solid solution structure for as-deposited sample. After ageing at 800 °C, 1000 °C and 1200 °C, the microstructures exhibit intergranular needle-like and plate-like FCC phase precipitates and wall shaped FCC phase precipitates along grain boundaries. As the FCC phase is softer than B2 phase, the formation of the FCC phase during ageing results in reduced compressive yield strength accompanied with enhanced ductility. The potential difference between Fe-Cr rich FCC phase and Al-Ni rich B2 matrix means the alloy is susceptible to galvanic corrosion, with the B2 matrix corroding preferentially.

The microstructure and mechanical properties of Zn-25Sn-XAl (X=0–0.09 wt%) high temperature lead free solder

The microstructure and mechanical properties of Zn-25Sn-XAl ( X=0, 0.01, 0.03, 0.05, 0.09wt% ) high temperature Pb-free solders were investigated. The addition of Al tends to refine the grain size and depress the undercooling behavior of the solder. The needle-like zinc-rich phase of the eutectic region was coarsened upon Al addition. The increasing addition of Al up to 0.09wt% enhanced the ultimate tensile strength (UTS) of the alloy from 67.28 to 78.61MPa (16.84% improved), and the yield strength from 42.52 to 52.81MPa (24.2% improved). The strain of the solder degraded from 39.02–32.83% when the addition of Al ranged from 0 to 0.09wt%.

Microstructure and mechanical properties of AlSi10Cu3 alloy with (La+Yb) addition processed by heat treatment

The optical microscopy, scanning electron microscopy (SEM) and energy-dispersive spectrometry (EDS) were used to assess the influence of micro-addition of (La+Yb) on the microstructure and mechanical performance of the AlSi10Cu3 alloy in heat treatment conditions. It was shown that the appropriate (La+Yb) addition (0.3 wt.% or 0.6 wt.%) transformed the needle-like β-Al5FeSi phase into Chinese script or spherical α-Al8Fe2Si phase. Eutectic silicon refined the long needle-like particles into granular or round particles at 0.6 wt.% (La+Yb) content. Moreover, the La3Al11 and YbAl3 phases acted as strengthening phases during the heat treatment processing in the alloy with the addition of (La+Yb). Consequently, the alloy with 0.6 wt.% (La+Yb) exhibited an enhanced mechanical properties response with ultimate tensile strength, elongation, and hardness at 69.35%, 113.26% and 23.61% higher than those of the unmodified alloy, respectively. Further addition (0.9 wt.%) of (La+Yb) resulted in the increasing of the black acicular RE-rich intermetallics during heat treatment, which could aggravate the situation of stress concentration leading to deterioration of the mechanical properties of alloy.

Effect of pre-annealing on microstructure and compactibility of gas-atomized Al-Si alloy powders

Effect of pre-annealing treatment temperature on compactibility of gas-atomized Al-27%Si alloy powders was investigated. Microstructure and hardness of the annealed powders were characterized. Pre-annealing results in decreasing Al matrix hardness, dissolving of needle-like eutectic Si phase, precipitation and growth of supersaturated Si atoms, and spheroidisation of primary Si phase. Compactibility of the alloy powders is gradually improved with increasing the annealing temperature to 400 °C. However, it decreases when the temperature is above 400 °C owing to the existence of Si-Si phase clusters and the densely distributed Si particles. A maximum relative density of 96.1% is obtained after annealing at 400 °C for 4 h. In addition, the deviation of compactibility among the pre-annealed powders reaches a maximum at a pressure of 175 MPa. Therefore, a proper pre-annealing treatment can significantly enhance the cold compactibility of gas-atomized Al-Si alloy powders.

Improved age-hardening response and altered precipitation behavior of Al-5.2Mg-0.45Cu-2.0Zn (wt%) alloy with pre-aging treatment

In this research, pre-aging treatment has been performed on our newly developed Al-5.2Mg-0.45Cu-2.0Zn alloy. Results show that the age-hardening response of the alloy has been significantly improved with pre-aging treatment due to a modified precipitation microstructure. Without pre-aging treatment, the microstructure in peak aged condition consists of coarse lath-like T- Mg32(Al, Zn)49 phase and needle-like S-Al2MgCu phase. However, the microstructure of the alloy in peak aged condition with pre-aging treatment only consists of high density of finer and equiaxed T phase with the S phase disappeared. Though both kinds of T phases have the same body-centred cubic (BCC) Mg32(Al, Zn)49-type structure (Im3, a = 1.422 nm), they have big difference in composition. Schematic illustration of the precipitation behaviors in the alloy during aging at 180 °C with or without pre-aging treatment is made according to the analysis results.

Microstructure Investigation on Metal Hydride Alloys by Electron Backscatter Diffraction Technique

The microstructures of two metal hydride (MH) alloys, a Zr7Ni10 based Ti15Zr26Ni59 and a C14 Laves phase based Ti12Zr21.5V10Ni36.2Cr4.5Mn13.6Sn0.3Co2.0Al0.4, were studied using the electron backscatter diffraction (EBSD) technique. The first alloy was found to be composed of completely aligned Zr7Ni10 grains with a ZrO2 secondary phase randomly scattered throughout and a C15 secondary phase precipitated along the grain boundary. Two sets of orientation alignments were found between the Zr7Ni10 grains and the C15 phase: (001)Zr7Ni10A//(110)C15 and [100]Zr7Ni10A//[0 1 ¯ 1]C15, and (01 1 ¯ )Zr7Ni10B//( 1 ¯ 00)C15 and [100]Zr7Ni10B//[313]C15. The grain growth direction is close to [313]Zr7Ni10B//[ 1 ¯ 11]C15. The second alloy is predominated by a C14 phase, as observed from X-ray diffraction analysis. Both the matrix and dendrite seen through a scanning electron microscope arise from the same C14 structure with a similar chemical composition, but different orientations, as the matrix with the secondary phases in the form of intervening Zr7Ni10/Zr9Ni11/(Zr,Ni)Ti needle-like phase coated with a thin layer of C15 phase. The crystallographic orientation of the C15 phase is in alignment with the neighboring C14 phase, with the following relationships: (111)C15//(0001)C14 and [1 1 ¯ 0]C15//[11 2 ¯ 0]C14. The alignments in crystallographic orientations among the phases in these two multi-phase MH alloys confirm the cleanliness of the interface (free of amorphous region), which is necessary for the hydrogen-storage synergetic effects in both gaseous phase reaction and electrochemistry.

Correlation between Zn-Rich Phase and Corrosion/Oxidation Behavior of Sn–8Zn–3Bi Alloy

The microstructure of Sn–8Zn–3Bi alloy was refined by increasing the solidification rate and the correlation between Zn-rich phase and the corrosion/oxidation behavior of the alloy was investigated. The Zn-rich phase transforms from coarse flakes to fine needles dispersed in the β-Sn matrix with the increase of the cooling rate. The transformation of Zn-rich precipitates enhances the anticorrosive ability of Sn–8Zn–3Bi alloy in 3.5 wt.% NaCl solution. On the contrary, Sn–8Zn–3Bi alloy with a fine needle-like Zn-rich phase shows poor oxidation resistance under air atmosphere, due to the fast diffusion of Zn atoms in Sn matrix.

Microstructural evolution of Li-Ti ferrite/composite braze/Li-Ti ferrite joints reinforced by Bi5(Ti3Fe)O15 whiskers

The composite brazes consisting of (i) Bi2O3-B2O3-SiO2 glass and (ii) Li-Ti ferrite powder were used to join the Li-Ti ferrite. The microstructural evolution and resulting mechanical strength of the joint were systematically investigated. Both needle-like and plate-like Bi5(Ti3Fe)O15 phases were observed in the joint. With a sufficiently pronounced Ti concentration gradient, the Bi5(Ti3Fe)O15 crystal nucleuses evolve into whiskers along the c-axis orientation. Contrary to that, the Bi5(Ti3Fe)O15 plates were observed when the Ti concentration gradient was small enough to make the driving force of the preferential growth markedly weaker in comparison to the attraction between the (Bi2O2)2+ and (Am−1BmO3m+1)2− domains in a bismuth layer-perovskite structure. The Bi5(Ti3Fe)O15 whiskers having a large size could potentially absorb the fracture force by deflecting or stopping its propagation. The highest shear strength of 81.0MPa was obtained for the joint with the composite braze containing 10.0wt% of Li-Ti ferrite powder.

Comparative study regarding the effect of Al, Zn, and Gd on the microstructure and mechanical properties of Mg alloy Mg-Sn-Li

The effect of Al, Zn, and Gd respectively on the microstructure and mechanical properties of Mg alloy Mg-Sn-Li (the base alloy) is performed. Our results reveal that with Al addition (Al-modified alloy) or Zn addition (Zn-modified alloy), the secondary dendrite arm spacing of the primary α-Mg phase is refined and no new phase is formed. Gd addition (Gd-modified alloy) results in the formation of needle-like or particle-like GdMgSn phase. The extruded base alloy has a low tension-compression yield asymmetry (about 1.00). Addition of Al or Gd yields a slight increase in value of tension-compression yield asymmetry, while Zn a notable reduce. Under tension along the extrusion direction, the yield strength and ultimate tensile strength of Al-modified and Zn-modified alloy other than Gd-modified alloy are enhanced when compared with the base alloy. This is attributed to solute strengthening, the reduced volume fraction of tin-containing metallic compounds and the existence of many needle-like GdMgSn phases in α-Mg matrix. The yield strength of the Gd-modified alloy under tension is the lowest whereas not under compression. This associates with the fact that GdMgSn phases distributed mainly in the interior of grains would offer resistance to twin propagation and growth.

Microstructure evolution of an EB-PVD NiAl coating and its underlying single crystal superalloy substrate

A NiAl coating was deposited onto a Ni-based single crystal superalloy with (001) crystal orientation by electron beam physical vapor deposition (EB-PVD). The as-deposited NiAl coating showed a columnar microstructure with (110) preferred orientation. The microstructure evolution behavior near interface between the NiAl coating and superalloy substrate at 1100 °C was investigated. Kirkendall voids were formed in the NiAl coating, indicating the different elements diffusion coefficients in the coating and substrate. Interdiffusion zone (IDZ) with rod-like and granular topological close-packed (TCP) phases and substrate diffusion zone (SDZ) with needle-like TCP phases were formed during diffusion annealing at elevated temperature. The equi-axed β-NiAl grains were developed in the IDZ after diffusion annealing at 1100 °C for 10 h, which showed different orientations from the coating and substrate. However, after 50 h diffusion annealing, the equi-axed β-NiAl phases in the IDZ were transformed into γ′-Ni3Al phases which had the same orientation as the substrate. Furthermore, oriented rafting of the substrate occurred during diffusion annealing and the rafts were parallel to the coating/substrate interface.

Enhanced Thermoelectric Properties of Sn0.8Pb0.2Te Alloy by Mn Substitution

A series of (Sn0.8Pb0.2)1−x Mn x Te alloys with x = 0, 0.03, 0.06, 0.09, 0.12 and 0.15 were prepared by melting, quenching and spark plasma sintering (SPS) techniques to investigate their phases and thermoelectric properties. Mn was used as doped element in Sn0.8Pb0.2Te solid solution to reduce the carrier concentration, enhance the Seebeck coefficient and reduce the thermal conductivity of the material. Experimental results show that the SnTe-based solid solution single phase was formed in the alloys with x = 0 and 0.03. The minor irregular-shaped MnTe2 phase presents in the alloys with x ≥ 0.06, while the minor needle-like MnTe phase appears in the alloys with x ≥ 0.12, together with the SnTe-based solid solution matrix. The lattice parameter a of SnTe-based solid solution decreases nearly linearly as Mn content x increases up to 0.12, but keeps constant as x further increases. All the samples show p-type conduction. Mn doping in Sn0.8Pb0.2Te decreases its carrier concentration and thus increases its Seebeck coefficient. The solute Mn and Pb atoms in the SnTe-based solid solution, and the minor phases MnTe2 and MnTe, enhance the phonon scattering and thus reduce the thermal conductivity. As a result, the figure-of-merit ZT of the (Sn0.8Pb0.2)1−x Mn x Te composites can be enhanced with proper Mn substitution. The maximum ZT of 0.65 was obtained in the sample (Sn0.8Pb0.2)0.88Mn0.12Te at 723 K, which is higher than the 0.29 of its parent alloy Sn0.8Pb0.2Te.

Influence of Curing Humidity on the Compressive Strength of Gypsum-Cemented Similar Materials

The analogous simulation experiment is widely used in geotechnical and mining engineering. However, systematic errors derived from unified standard curing procedure have been underestimated to some extent. In this study, 140 gypsum-cemented similar material specimens were chosen to study their curing procedure with different relative humidity, which is 10%–15%, 40%, 60%, and 80%, respectively. SEM microstructures and XRD spectra were adopted to detect the correlation between microstructures and macroscopic mechanical strength during curing. Our results indicated that the needle-like phases of similar materials began to develop in the early stage of the hydration process through intersecting with each other and eventually transformed into mat-like phases. Increase of humidity may inhibit the development of needle-like phases; thus the compressive strength changes more smoothly, and the time required for the material strength to reach the peak value will be prolonged. The peak strength decreases along with the increase of humidity while the humidity is higher than 40%; however, the reverse tendency was observed if the humidity was lower than 40%. Finally, we noticed that the material strength usually reaches the peak value when the water content continuously reduces and tends towards stability. Based on the above observation, a curing method determination model and experimental strength predication method for gypsum-cemented similar materials were proposed.

Effect of Precipitated Phases on Corrosion of Mg95.8Gd3Zn1Zr0.2 Alloy with Long-Period Stacking Ordered Structure

The microstructure of the precipitated phases of Mg95.8Gd3Zn1Zr0.2 alloys with long-period stacking ordered structure before and after heat treatment is discussed. The corrosion properties of the as-cast (F), solid-solution (T4) and aging-treated (T6) alloys in 1% NaCl solution are studied. The hydrogen evolution and electrochemical measurements display that the as-cast Mg95.8Gd3Zn1Zr0.2 alloy with the continuous network eutectic phase exhibits the greatest corrosion resistance, while T6 sample with some needle-like phases and the particle phases is the worst among the three alloys. It is proposed to be mainly related to the amount, composition, microstructure and distribution of the precipitated phases.

Microstructure characteristics of burning products of Ti-V-Cr fireproof titanium alloy by frictional ignition

Titanium fire in the aero-engine is a typical accident caused by ignition and burning of titanium alloy, which leads to a huge damage. Some articles wrote it as to turn pale at the mention of titanium. Fireproof titanium alloy, a new type of structural titanium alloy with fireproof function, has been developed to prevent titanium from fire hazard and to ensure safe and reliable service of aero-engine. In view of the lack of clear understanding of the microscopic mechanisms found currently for the structural functionality of fireproof titanium alloys, in this work, using frictional ignition technology in oxygen-rich environment (friction oxygen concentration method), associated with in-situ observation, SEM, EDS and XRD analyses, the microstructure characteristics of burning products of Ti-V-Cr fireproof titanium alloys are investigated and the element distribution law associated with microscopic mechanism during combustion reaction process is disclosed. Results show that Ti-V-Cr fireproof titanium alloys produce dazzling white light during combustion, with the typical flame characteristics of metal combustion. The generated products after burning are mainly TiO2, V2O5 and Cr2O3 oxides, in the form of dispersive particles and dense continuous body. The dispersive particles are in regular spheric shape, with a size of 10-50 m; the dense continuous products after burning presents divisional feature. After the combustion lasts 18 s, four distinct zones form from the alloy matrix to the combustion surface and they are in the sequence of transitional zone, heat-affected zone fusion zone, and combustion zone, with sizes of 40-50, 200-210, 60-70, and 18-21 m respectively. Further, some small granular shaped bulges exist in the transitional zone, in some fixed directions; in the heat-affected zone, a large number of V-based solid solution and some Ti-based solid solution form, and the titanium containing V-based solid solution is much higher than the needle-like precipitation phase in the matrix. In the fusion zone, there are some V-based solid solutions in most of Ti-based solid solution; while, the combustion zone mainly contains the mixed oxides of Ti, V, and Cr. The V-based solid solution in the heat-affected zone reduces the diffusion rate of titanium to the fusion zone, slowing the preferential reaction between titanium and oxygen in the combustion zone; while the generated mixed oxides of TiO2, V2O5, Cr2O3, etc. in the combustion zone and the solution of oxygen in titanium in the fusion zone jointly prevent the diffusion of oxygen to the alloy matrix, thus the Ti-V-Cr fireproof titanium alloys can have excellent fireproof functions.

Effect of configuration and composition of interlayer on TLP joints of FSX-414 superalloy

Different interlayers including commercial foils of MBF-80, MBF-60 and MBF-100 with the thickness of 50μm and pure Ni-coating with the thickness of 10μm were used for TLP bonding of FSX-414 superalloy. TLP bonding was conducted at the optimum condition (1150°C/5min). The joints microstructures were studied using SEM and WDS/SEM was used for phase analysis. For MBF-80 interlayer, no eutectic phases were seen at the joint but some granular and needle-like phases were visible at the diffusion affected zone. Considerable amount of eutectic phases were visible at the joint made using MBF-60 interlayer which were identified as NiP2 and CrP. These phases were detected for MBF-100 interlayer as Co and Cr-rich borides. Isolated eutectic phases, analysed as Ni, Co, Cr and W-rich ones, were seen at the joints made using pure Ni-coating as the interlayer. Mechanical tests including microhardness and shear strength were conducted for all the samples. The highest shear strength and the lowest hardness of eutectic compounds were observed for the joints made using MBF-80 interlayer. This was in agreement with the lowest concentration of alloying elements at these compounds, obtained from SEM/WDS analysis.

Microstructure and mechanical properties of Gd-modified A356 aluminum alloys

The effect of Gd modification on the microstructure and mechanical properties of A356 aluminum alloys was investigated using the metallurgical microscopy, scanning electronic microscopy, X-ray diffraction and mechanical testing. The addition of 0.2 wt.%–0.4 wt.% Gd had an excellent refining effect on primary α-Al grains and a modification effect on Si phases in its as-cast state. The needle-like Si phases were adjusted into fine particles and uniformly distributed in the matrix by the T6 treatment, especially for the 0.2 wt.% Gd-modified alloy. The Gd additions introduced the Fe-containing GdAl2Si2 compounds, which precipitated in the forms of flakes and bulks. The GdAl2Si2 and β-Al5FeSi phases were also refined by the T6 treatment. The mechanical properties of the Gd-modified alloys were very poor in as-cast state; however, the highest strength and elongation were obtained for the 0.2 wt.% Gd-modified alloy by the T6 treatment.

Microstructure evolution during heat treatment and mechanical properties of Mg–2.49Nd–1.82Gd–0.19Zn–0.4Zr cast alloy

Microstructure evolution during heat treatment and mechanical properties of Mg–2.49Nd–1.82Gd–0.19Zn–0.4Zr cast alloy were investigated experimentally in different stages of heat treatment process. As-cast alloy was found to be consisting of dendritic α(Mg) grains, interdendritic α(Mg)+Mg12Nd eutectic. Within α(Mg) grains, needle-like Mg12Nd phase, quadrate-like Mg5Gd phase and α(Zr) particles were also observed. After a solution treatment at 515°C for 18h, α(Mg)+Mg12Nd eutectic was almost dissolved into α(Mg) grain, but a new Zn2Zr3 phase was observed within α(Mg) grains and along grain boundaries. After an aged treatment at 205°C for 18h, more dispersed interdendritic Mg12Nd precipitates, coarsened Zn2Zr3 and rod-shaped metastable precipitates of β″(Mg3Nd) and β′(Mg7Nd) were observed within α(Mg) grains. The ultimate tensile strength and the yield strength of the experimental alloy after the aged treatment were measured to be 287MPa and 206MPa at room temperature, 240MPa and 175MPa at 200°C, and 220MPa and 165MPa at 250°C, respectively. The improved mechanical properties were also discussed based on the observed microstructure evolution.

Effect of Cr content and heat-treatment on the high temperature strength of eutectic Al–Si alloys

In this work, the effects of Cr content (0–0.67 wt%) and heat treatment on the microstructure evolution and high temperature (HT) tensile strength (350 °C) of the Fe-containing eutectic Al–Si alloy were studied. The results show the main intermetallics are similar in tested specimens at both different heat treatment states, including γ-Al3CuNi, δ-Al7Cu4Ni, Al4Cu2Mg8Si7, and AlFeSi. The addition of Cr leads to the transformation from needle-like β-AlFeSi phases to fishbone-like α-Al(Fe,Cr)Si phases. For the HT strengths (350 °C), the UTSs of Al–Si alloys at T5 state are higher than T6 state correspondingly, both of which are progressively decreased with the Cr contents. For the elongation, the Al–Si alloys at T6 state have better plasticity than T5 state correspondingly. Comparably, the elongations remain stable at T5 state, and keep increasing at T6 state with the addition of Cr. The variations of HT strengths and elongations dependent on the Cr content and heat treatment are accounted on the microstructural evolutions, including AlFeSi phase transformation and the Si spheroidization.