Primary Mg2Si Particles Research Articles

Dry sliding wear behaviour of Mg–Si alloys

This study aims to investigate the dry sliding wear behaviour of pure Mg, different hypo (Mg–0.5, 0.7, 1.15wt% Si) and hyper (Mg–2, 4, 6, 8 and 10wt% Si) eutectic Mg–Si alloys prepared using the gravity casting method. Various parameters such as alloy composition, normal loads with constant sliding speed and sliding distance have been used to study the wear behaviour of these alloys using pin on disc configuration against a hardened chromium steel counter face. Detailed microstructural morphology studied for the pure Mg and the Mg–Si alloys are correlated with the wear properties obtained. The hypereutectic alloys yield better wear resistance as compared to the hypoeutectic Mg–Si alloys because of the presence of coarse and hard primary Mg2Si particles. The SEM analysis carried out on the worn surfaces and wear debris to understand the wear mechanisms. The size and volume of Mg2Si phase play an important role in deciding the wear behaviour of the Mg–Si alloys.

The influence of Cu–15P master alloy on the microstructure and tensile properties of Al–25 wt% Mg2Si composite before and after hot-extrusion

This study was undertaken to investigate the effect of CuP master alloy on the microstructure and tensile properties of hot extruded Al–25wt% Mg2Si in-situ metal matrix composite. Cu–15wt% P master alloy was added to produce different levels of P (0–3wt%) in the composite. Then, these small billets were hot extruded by using a hydraulic press at a ram speed of 1mm/s with the extrusion ratio of 6:1 at 500°C. Microstructural examinations were carried out by using optical and scanning electron microscopy (SEM). The results showed that P addition changes the morphology of primary Mg2Si from large dendritic to polyhedral shape and its average particle size decreases from 220μm to 22μm in as-cast condition. Microstructural studies also showed that extrusion process not only refines the primary Mg2Si particles, but also fragments intermetallic phases to fine particles. Tensile testing results also revealed that the optimum P level for the improvement of both ultimate tensile strength and elongation values of hot-extruded specimens is 1wt%. The examination of fracture surfaces via scanning electron microscopy (SEM) revealed a transition from brittle fracture in as-cast to ductile fracture in hot-extruded and P-modified specimens. This can be attributed to the changes in the size and morphology of intermetallics and reduction in pore size after hot-extrusion.

The influence of Ni addition and hot-extrusion on the microstructure and tensile properties of Al–15%Mg2Si composite

The effects of nickel addition and hot-extrusion on the microstructure and tensile properties of in situ Al–15%Mg2Si composite specimens have been investigated. Al–15%Mg2Si composite ingots were prepared by an in situ process and different amounts of nickel (0.1, 0.3, 0.5, 1.0, 3.0 and 5.0wt% Ni) were added to the remelted composite. Optical microscopy (OM) and scanning electron microscopy (SEM) indicated that Ni addition changes the morphology of both primary and eutectic Mg2Si phases and decreases the size of primary Mg2Si particles from 42μm to 17μm. Hot-extrusion was found to be powerful in breaking the eutectic network and changing the size and morphology of pseudo-eutectic Mg2Si phase. The results obtained from tensile testing revealed that both Ni addition and hot-extrusion process improve ultimate tensile strength (UTS) and elongation values. Fracture surface examinations revealed a transition from brittle fracture mode in as-cast composite to ductile fracture in hot-extruded composite after Ni addition. This can be attributed to the changes in size and morphology of primary and eutectic Mg2Si phases and also the formation of more and finer α-Al phase.

Microstructure and tensile properties of cast Al–15%Mg2Si composite: Effects of phosphorous addition and heat treatment

The effects of solution heat treatment and phosphorous addition on the microstructure and tensile properties of in situ Al–15%Mg2Si composite specimens have been investigated. The Al–15%Mg2Si composite ingot was made by in-situ process and different amounts of phosphorous (0.1, 0.3, 0.5, 0.7 and 1wt% P) were added to the remelted composite. Then, the specimens were subjected to solutionizing at 500°C for holding time of 4h followed by quenching. Optical microscopy (OM) and scanning electron microscopy (SEM) indicated that phosphorous addition not only changes the morphology of primary Mg2Si particles from dendritic to a regular shape, but also it reduces Mg2Si particle size. Solutionizing led to the dissolution of the Mg2Si particles and changed their morphology to round shape. The results obtained from tensile testing revealed that both phosphorous addition and solution heat treatment improve ultimate tensile strength (UTS) and elongation (El.%) values. According to the results, the optimum tensile property was achieved by adding 0.5wt% P to the Al–Mg2Si composite after solution heat treatment. Fractographic analysis revealed a cellular nature for the fracture surface of the MMC. As a result of P addition the potential sites for stress concentration and crack initiation areas were reduced due to microstructural modification, while increase in the number of fine dimples rendered the nature of fracture from brittle to ductile and also improved tensile properties.

Comments on “Microstructural and mechanical behavior of Mg/Mg2Si composite fabricated by a directional solidification system” by Mirshahi et al. [Mater. Sci. Eng. A 528 (2011) 8319–8323

Abstract This paper comments on a series of papers published in materials journals. These papers explain the effects of cooling rate and chemical modification on the microstructure and mechanical behavior of an in situ Mg/Mg2Si composite. The issues discussed here include the size and morphology of primary Mg2Si particles in the composite. The purpose of the present comment is to show that the sizes reported for primary Mg2Si particles are not reliable largely underestimating the correct values. The paper also includes criticisms of other mistakes in these papers.

Microstructural evolution and tensile properties of the in situ Al–15%Mg2Si composite with extra Si contents

In the present work, the effect of extra Si addition on the microstructure and tensile properties of Al–15%Mg2Si composite has been investigated. The Al–15%Mg2Si composite ingot was made by in situ process and different amounts of extra Si (0.5, 1, 1.5, 2, 5 and 7wt.% Si) were added to the remelted composite. Optical microscopy and scanning electron microscopy (SEM) indicated that the addition of extra Si up to 2wt.%, reduces the average Mg2Si particle size from 39μm to 26μm and increases the volume fraction of α-Al phase from 6% to 22%. Addition of extra Si content up to 7wt.% leads to the formation of primary Mg2Si particles with larger size (38μm). The results of tensile test revealed that the addition of extra Si improves ultimate tensile strength (UTS) and elongation values of the composite from 176MPa and 1.7% to 222MPa and 3.0% respectively. Fractographic analysis of specimens exposed a cellular nature for the fracture surface. On the cellular fracture surface, the features of both brittle and ductile fracture were present simultaneously. Raising the amount of extra Si up to 7% has increased the number and decreased the size of dimples. These microstructural findings led to a change in the mode of fracture from brittle to ductile and increased elongation values.

Preliminary study of the characteristics of a high Mg containing Al-Mg-Si alloy

An Al-20Mg-4Si high Mg containing alloy has been produced and its characteristics investigated. The as-cast alloy revealed primary Mg2Si particles evenly distributed throughout an α-Al matrix with a β-Al3Mg2 fully divorced eutectic phase observed in interdendritic regions. The Mg2Si particles displayed octahedral, truncated octahedral, and hopper morphologies. Additions of Sb, Ti and Zr had a refining influence reducing the size of the Mg2Si from 52 ± 4 μm to 25 ± 0.1 μm, 35 ± 1 μm and 34 ± 1 μm respectively. HPDC tensile test samples could be produced with a 0.6 wt.% Mn addition which prevented die soldering. Solution heating for 1 hr was found to dissolve the majority of the Al3Mg2 eutectic phase with no evidence of any effect on the primary Mg2Si. Preliminary results indicate that the heat treatment has a beneficial effect on the elongation and the UTS.

Study on fracture behaviour of Al–15%Mg2Si metal matrix composite with and without beryllium additions

In this study, the influence of Beryllium (Be) content on the fracture behaviour of Al–15%Mg2Si composite was investigated. The results showed an increase in mechanical properties with increasing of Be content. The stress–strain curves of samples showed a same category of serrations reflecting non-uniform deformation. Scanning electron microscopy was employed to examine the crack nucleation and fracture model. The results indicate that Al–15%Mg2Si composite shows different behaviours of crack initiation and fracture for samples with and without Be. Differences observed in the fracture behaviour were attributed to microstructural changes as well as morphological aspects of primary Mg2Si particles.

Microstructure and Mechanical Behavior of in Situ Primary Si/Mg2Si Locally Reinforced Aluminum Matrix Composites Piston by Centrifugal Casting

Al-Si pistons are frequently damaged by burning piston top surface due to elevated combustion temperature, and by rubbing the first ring groove against the engine cylinder liner. To prevent piston from these damages, some technologies were invented, such as mounting high Ni cast iron ring around the first ring groove in Al alloy piston body and thermal resistant steel on piston top surface, and fabricating Al composite pistons by squeeze casting for enhancing the whole or local piston performance. In this paper, composite pistons locally reinforced with in situ primary Si and primary Mg2Si particles are fabricated by centrifugal casting. The microstructure characteristics, hardness and wear resistance of the composite piston are investigated and the motion characteristic of the in situ particles in centrifugal field is analyzed. The results of the experiments show that primary Si and Mg2Si particles mix up with each other in melt and segregate at the regions of piston top and piston ring grooves under the effect of centrifugal force. Particulate reinforced regions have a higher hardness and better wear resistance compared with the unreinforced regions and this performance increases after heat treatment. The analysis result of particle movement shows that, primary Si and primary Mg2Si particles move at approximately the same velocity in the centrifugal field, because of the growth of primary Si and fusion after colliding between primary Si particles, which compromised the velocity difference of primary Si and primary Mg2Si particles caused by the difference of their densities. Research results have some theory significance and applicative value of project in development of new aluminum matrix composites piston products.

Microstructure modification of Al–17%Si alloy by addition of Mg

The calculated phase diagrams of Al–17%Si alloy with additions of Mg up to 10 wt% Mg have indicated two critical compositions at 4.6 and 6.8% Mg where the liquidus, the binary reaction temperatures as well as the temperature range of the formation of Mg2Si particles are changed. The eutectic formation temperature is decreased with the addition of Mg up to 4.6% Mg, followed by constant value due to the change of the eutectic formation reaction from the binary (Liq. → Al ± Si) to the ternary (Liq. → Al + Si + Mg2Si) reaction. This change contributes to the transformation of the eutectic silicon in matrix from long platelet, to fine and compact particles resulting in overall increased hardness of the high Mg alloys even though the hardness of the primary Mg2Si particles is much smaller than that of the primary silicon particles. A pronounced decrease of both primary and eutectic silicon particles size was also observed for 0.4% Mg alloy when compared to the base alloy showing significant microstructural modification.

EFFECTS OF Sr ON MICROSTRUCTURE AND MECHANICAL PROPERTIES OF Mg–9Al–1Si–0.3Zn ALLOY

Mg-Al-Si alloys (AS series alloys) with a high content of silicon performed low ductility and strengths because of large amount of coarse Chinese scripts Mg2Si particles distributed in the matrix. Trace elements, such as Ca, P, Sr and RE were selected to modify the morphology of Mg2Si particles. Among these, Sr showed remarkable modification effect, however, the modification mechanism was not clarified yet. In this work, the effects of Sr on the morphology of Mg2Si in Mg-9Al-1Si-0.3Zn (mass fraction, %) alloy and the mechanical properties were investigated, and the modification mechanism was explained by comparing Mg2Si morphologies. The Mg-9Al-1Si-0.3Zn alloy was composed of primary hopper Mg2Si particles of 40 µm in size, coarse Chinese scripts Mg2Si of 50—80 µm, island-like Mg17Al12 and the �-Mg matrix. 3D hopper crystals and Chinese scripts Mg2Si particles were extracted by the electrochemical method. Both morphologies reveal the corner effect which leads to the preferential growth orientation of edges and corners of the octahedron Mg2Si particle, respectively. When Sr was added into the alloy, the growth rates of both edges and corners of the Mg2Si particles were depressed significantly due to the adsorption of Sr on the surface of Mg2Si particles. When Sr addition went up to 0.32%, Mg2Si phase was fully modified to small polygonal particles of 5—20 µm. Moreover, the grain size of the alloy decreased from 210 µm to 160 µm with Sr increasing, which is in accord with the GRF (growth restriction factor) mechanism. The increasing of mechanical properties is mainly attributed to the refinement of Mg2Si phase.

Influences of Si and Mg contents on microstructures of Al–xSi–yMg functionally gradient composites reinforced with in situ primary Si and Mg2Si particles by centrifugal casting

Hypereutectic Al–xSi–yMg functionally gradient composites reinforced with primary Si and Mg2Si particles were fabricated by centrifugal casting. The influence of Si and Mg contents on microstructures of the Al–xSi–yMg functionally gradient composites was investigated. Calculations of the volume fractions and the sizes of primary Si and Mg2Si particles in the cross section of each tube along the radial direction from the inner-to-outer surface revealed that this type of gradient composite tube can be fabricated by centrifugal casting when the contents of Si and Mg are more than or equal to 19 and 4%, respectively. The tubes consist of an inner layer, the middle layer, and the outer layer measured in the radial direction on the cross section. The inner layer segregates blocky primary Si and Mg2Si particles, the middle layer contains no primary Si and Mg2Si particles, and the outer layer contains few primary Si and Mg2Si particles. We compared resulting mean volume fractions and average sizes of produced primary Si and Mg2Si particles with initial Si and Mg contents to determine their relationship in this process. The morphology of the Mg2Si phase is another key factor in the formation of the composites. It was found that the blocky primary Mg2Si particles have greater centroclinal velocity than that of primary Si particles due to the lower density of the Mg2Si particles. The primary Si particles are pushed by blocky primary Mg2Si particles. The two kinds of particles move toward the inner wall of the tube together during the solidification. A model of particles movement has been established according to the experimental results.

Influences of Morphology and Size of Mg2Si on Microstructural Evolution of Mg–6.2Si Alloys during Partial Remelting

After isothermally treated at 660°C for 20 min, both dendritic Mg2Si in unmodified alloys and polyhedral ones in KBF4-modified alloys transformed to nearly globular morphology, while the degree of spheroidization in KBF4-modified alloy was lower than that in unmodified alloy. This result might be attributed to both restriction effect of boron and stability of octahedral primary Mg2Si crystals. After isothermal treatment at 660°C for 20 min, however, the initial grain sizes of dendritic Mg2Si crystals in unmodified alloy or polyhedral ones in modified alloy obtained by different cooling rates (copper mold with the holes of φ20 and φ6 mm) only caused a little effect on the partial remelting microstructure. The fine primary Mg2Si particles obtained from φ6 mm hole, however, were more readily to evolve to globular crystals, which might be attributed to the more active Ostwald ripening and/or coalescence.

Effect of solidification cooling rate on the morphology and number per unit volume of primary Mg2Si particles in a hypereutectic Al-Mg-Si alloy

Growth morphology and number $$\bar N_v $$ per unit volume have been determined vs withdrawal velocity V over the range 0.1 to 4 mm/s for primary Mg2Si in a Bridgman-solidified hypereutectic Al-Mg-Si alloy. Primary Mg2Si shows a transition from irregular or regular polyhedral to dendritic with increasing V, and $$\bar N_v $$ increases with solidification cooling rate $$\dot T$$ according to the relationship $$\bar N_v /\dot T = 270 \pm 110mm^3 (K/s)^{ - 1} $$ . These results are compared with corresponding ones for Al-Mg-Si alloy wedge castings and for primary silicon in hypereutectic Al-Si alloys.

High-resolution electron microscope observation of interface microstructure of a cast Al-Mg-Si-Bi-Pb(6262)/Al2O3p composite.

High-resolution electron microscopy was employed to characterize the interface structure of a cast Al-Mg-Si-Bi-Pb aluminium(6262)-based composite reinforced by alpha alumina particles with a trace of beta alumina in order to investigate the behaviour of alloying elements in cast composites. Except for a few primary Mg2Si particles, few reaction products were detected at the interface of Al/alpha-Al2O3 due to the unfavourable reaction kinetics during the squeeze-casting process. The Mg2Si particle has an orientation relationship with alpha-Al2O3 of [011]Mg2Si//[1210]alpha-Al2O3 (111)Mg2Si//(0006)alpha-Al2O3. A significant amount of MgAl2O4 was found on the surface of the beta-Al2O3 particles, which is in contrast to the small degree of reaction found on alpha-Al2O3 particles. MgAl2O4 and beta-Al2O3 particles have the following orientation relationship: [011]MgAl2O4//[1210]beta-Al2O3 (111) MgAl2O4//(0006)beta-Al2O3. The similar crystal structure of beta-Al2O3 to MgAl2O4 favours MgAl2O4 nucleation and growth on the surface of beta-Al2O3. Interfacial energy minimization dominates the atomic structure of the interface with the result that close packed planes and directions in the Al2O3 reinforcement and reaction products are parallel to the interfaces. Bi and Pb were found in the form of metallic nanometre particles between Al2O3 particles, or between the MgAl2O4 and Al2O3 particles, or in the open channels of beta-Al2O3 filled by the Al matrix.

J. Japan Soc. Powder and Powder Metallurgy: I. Minato et al (Diamond Electric co, Osaka, Japan) Vol 39, No 12, 1992, 1076–1079. (In Japanese)

Mg2Si/Mg–Zn–Si composite was in situ synthesized and modified by the addition of Ba and Sb; the microstructure and tensile properties of the resulting composites were investigated. Results show that the co-modification with Ba and Sb effectively refined the morphology of Mg2Si as follows: The primary Mg2Si particles were modified to fine polygonal particles <15 μm; when the Ba content was 1.0 wt.%, the average size of primary Mg2Si particles decreased with increasing Sb mass fraction. The ultimate tensile strength and elongation values of the composites increased with increasing Sb content. This was attributed to the formation of fine Ba2Sb particles that acted as the heterogeneous nucleation sites for the primary Mg2Si particles, resulting in a refined distribution of these precipitates. The dispersion of the refined Mg2Si particles strengthened the composite and improved the ductility.