Mechanical properties, Casting techniques and Characterization of Microstructures of Mg Alloys
Magnesium alloys have been in use for a period of time comparable to that of aluminium alloys; magnesium was isolated prior to aluminium and magnesium alloys being used significantly during the Second World War. However, magnesium consumption is a small fraction of that of aluminium. There has been rapid growth in the application of magnesium alloys in recent years, with growth in pressure die cast applications exceeding 18 percent per year over the last decade. Magnesium is particularly appealing in applications where weight is critical, as it has the lowest density of any structural metal, 1.74g/cm3, compared to 2.70g/cm3 for aluminium and 7.87g/cm3 for iron (pure metals at 258C).
In recent years, the automotive industry has provided a significant impetus for growth in magnesium consumption. Magnesium and other light weight materials can help to offset the tendency for vehicle weight increases caused by the addition of new features. Overall, reducing vehicle weight benefits both cost and performance by improving fuel economy, performance (handling, acceleration, and braking), and durability.
Magnesium, like other common metals, can be wrought and cast. However, the majority is consumed in pressure die castings, with wrought applications accounting for less than 5% of total consumption. This is due, in part, to magnesium's excellent casting properties, which enable the production of complex components withthinwax.
Magnesium alloys are widely recognised as the lightest structural alloys. They are made of magnesium, the lightest structural metal, which is combined with other metal elements to improve physical properties. Manganese, aluminum, zinc, silicon, copper, zirconium, and rare-earth metals are among these elements.
Why Magnesium?
It is a lightweight structural metal (1.8 g/cm3)
Improved dimensional stability
High damping capacity and specific strength
In wet environments, it is extremely corrosive.
Casting with thinner walls than aluminum is possible (1-1.5mm versus 2-2.5mm).
Because of these properties the material lends itself to a range of automotive, aerospace, industrial, electronic, biomedical, and commercial applications.
Types and Designation:
Magnesium alloys are divided into two types:
cast alloys
wrought alloys.
Cast alloys are created by pouring molten liquid metal into a mold, where it solidifies into the desired shape. Common magnesium cast alloys contain varying amounts – but no more than 10% – of aluminum, manganese, and zinc as principal alloying elements. Other alloying elements, such as zirconium and rare-earth metals, have recently been used, primarily to improve creep resistance. Furthermore, heat treatments improve the mechanical properties of cast alloys.
Wrought alloys, on the other hand, are alloys that have been mechanically worked to achieve the desired shape, such as forging, extrusion, and rolling operations. The main alloying elements are also aluminum, manganese, and zinc. Magnesium wrought alloys are classified as heat treatable or non-heat treatable.
To better understand the compositions of the alloys, designation systems displaying the alloying elements and their relative information have been developed. The ASTM Standard Alloy Designation System is one of the most widely used designation systems. It is composed of four parts, as illustrated in the following example.
Magnesium Alloy: AZ91E-T6
The first part (AZ) denotes the two primary alloying elements (aluminum, zinc)
The second part (91) specifies the percentage amount of the primary alloying elements (9 percent and 1 percent , respectively)
The third part (E) distinguishes alloys with the same amounts of the main alloying elements (fifth standardized alloy with the above percentages)
The fourth part (T6) specifies the alloy's condition (temper)
QE22 - Mg alloy contains 2% by wt SIlver and 2% by wt rare earth elements
AC63 - Mg alloy contains 6% by wt Aluminum and 3% by wt Copper
As a result, magnesium alloys are named and grouped in the ASTM designation system based on their main alloying elements. The primary alloying elements and their relative designations are shown in Table 1.
Mg Alloys for Automotive Applications:
AZ91D Alloy (Most WIdely used alloy)
Properties:
Excellent room temperature strength
excellent die casting
excellent corrosion resistance
AM60, AM50, AM20 Alloys (higher temperature alloys)
Properties:
high ductility
good impact strength
good die castability
good corrosion resistance
ZE41, AC63 Alloys (Heavy duty & high temperature alloys)
Properties:
Sand casting
high strength at room temperature
improved castability
Mg Alloys Casting Processes:
High Pressure Die Casting:
High-Pressure Die Casting (HPDC) is a highly efficient manufacturing method for creating a wide range of product forms. The process forces molten metal into a closed steel die cavity at high speed and pressure. Both the stationary and moving halves of the die are mounted to the platens of the die casting machine. The injection end of the die casting machine uses hydraulics and pressurised gas to move a piston forward, injecting molten metal into the closed steel die.
The die casting machine also has a clamping end that absorbs injection pressure and holds the die shut while the part solidifies using hydraulics and mechanical toggles. In seconds, the process can transform molten metal into a solid near-net-shape part. After solidification, the die is opened and the metal is released. The mold cavity is resealed after removal for the next cycle.
In a fraction of a second, molten metal is injected into the mold cavity (usually under 100 ms). Once the die cavity has been filled, extremely high pressure (often over 1000 bar) is applied to a molten metal true injection plunger. This is known as the intensification phase. This pressure compresses any trapped gases in the metal (due to the extremely fast and turbulent cavity filling) and feeds more metal into the mold to partially compensate for the metal's shrinkage while solidifying.
Everything from engine blocks to aerospace components and heavy-duty fasteners is made using high-pressure die casting. While there are several approaches, most high-pressure die casting processes include the four steps listed below:
Preparing Mold:
Mold preparation is the first step in high-pressure die casting. The manufacturing company applies lubricant to the interior walls of the mold during this initial step. This is significant because the lubricant regulates the temperature of the mold while also forming a film between the molten metal and the mold, allowing for easier casting removal.
Injection
The manufacturing company injects molten metal into the die mold after it has been prepared. During this step, the mold must be completely closed and sealed. It will not be able to "accept" the highly pressurised molten metal otherwise. Depending on the application, molten metal can be injected into the mold at pressures ranging from 1,500 to 25,000 pounds per square inch (PSI). This pressure is maintained by the mold until the molten metal has cooled and solidified.
Cavity ejection
The manufacturing company then removes the newly formed cavity from the mold. The mold itself usually has ejector pins that, when engaged, allow the cavity to be released. Of course, for it to eject, the cavity must be solid. The manufacturing company must wait for the raw metal to cool before ejecting the cavity from the mold if it is still liquid.
Shakeout
Shakeout is the fourth and final step in high-pressure die casting. The manufacturing company separates any scrap metal from the newly created cavity during this step. Excess scrap metal is not uncommon in high-pressure die casting. To put it another way, not all of the molten metal is used to make the casting. Some is still trapped inside the mold. As a result, before the mold can be reused, the scrap metal must be removed.
In addition to traditional high-pressure die casting methods, there have been a number of process improvements in recent years. As an example,
vacuum die casting.
squeeze casting
semi solid casting
Aluminum, zinc, and magnesium are the three primary metal groups used in this die casting process. CWM uses both cold chamber and hot chamber high-pressure die casting processes to cast all three metals.
High pressure die-casting: Hot and cold chamber systems
Two different systems can be used to inject molten metal into the mold:
hot chamber system
cold chambers system
Let's learn more about these two procedures.
Hot chamber system:
Metals such as zinc, magnesium, and lead are used in the hot chamber system. A hot chamber machine's injection system is immersed in molten metal from the blast furnace. When the piston pumps, metal is forced through the nozzle and into the die.
Cold Chamber system:
Metals that melt at high temperatures, such as aluminum, copper, and magnesium, are used in the cold chamber system.
Magnesium parts can be manufactured using either system, though small parts are typically produced in hot chamber systems and large parts in cold chamber systems due to the limited dimensions of hot chamber machines.
When compared to the hot chamber system, this system uses higher pressure. A hot chamber machine has a higher production rate than a cold chamber machine because the pouring process takes less time.
In addition, the cold chamber system employs two injection systems in the process:
horizontal injection
vertical injections
The molten metal is poured into an opening leading to a channel feeding the cold chamber, either manually or automatically, in this system. A hydraulically activated piston moves down this steel channel, blocking light and forcing metal into the mold at high speed and pressure. Following flow solidification, the piston is retracted, the mold is opened, the flow is expelled, and the system is ready to be activated again.
Advantages of HPDC:
High-Pressure Die Casting is capable of producing large, light-alloy parts in large quantities at a high rate and efficiency.
The HPDC process also produces parts with excellent surface finish, uniformity, and mechanical properties.
Another advantage of High-Pressure Die Casting is that, due to its excellent dimensional accuracy and smooth surfaces, it generally does not require much machining.
The fusions have a good surface finish, which is a basic requirement for plating, and the reduced thickness of the plate walls is possible because the overall weight of the fusion is reduced.
The molds are long-lasting, lowering unit costs, and more complex parts can be produced, reducing the number of components requested each time.
When compared to other manufacturing processes, high-pressure die casting is an efficient and cost-effective method that:
Produce strong, lightweight components that require less machining than fabricated components.
Complex and intricate shapes can be created in a single piece, eliminating the need for assembly or welding.
Provide a greater variety of shapes than other metal manufacturing techniques
Close tolerances are attained.
Sand Casting:
Sand casting is a manufacturing process that involves pouring liquid metal into a sand mold with a hollow cavity of the desired shape and then allowing it to solidify. Casting is a manufacturing process that involves pouring a liquid material into a mold that contains a hollow cavity of the desired shape and then allowing it to solidify. Metal, concrete, epoxy, plaster, and clay are examples of casting materials. The subject of this article will be sand casting.
The sand casting process is well known for its versatility. Sand castings can produce castings in a wide range of sizes and weights with extremely complex geometries and metals. The sand casting process is distinguished by the use of sand as the molding media.
The significant cost savings gained by using sand instead of other materials to make molds is a significant advantage. Mold production costs constitute a significant portion of the costs associated with alternative casting procedures. The molds used in the technique, however, are disposable and non-reusable due to the nature of sand.
It is impossible to keep the sand mold intact when removing a casting. Sand casting, on the other hand, is ideal for metals with high melting points, such as titanium, steel, and nickel. It is the only casting process that can work with these materials. As a result, the technology is the preferred method for producing low-cost small-series parts in the aerospace and automotive industries.
Components of a Sand Casting Mold
The sand casting mold is typically made up of four components, which are as follows:
Base Sand:
The base sand is the sand used to make the mold in its purest form. To keep it together, a binding agent is required. The core is made of base sand as well. The following are the most common types of base sand:
Silica sand
Olivine sand
Chromite sand
Zircon sand
Chamotte sand
Binders or Binding Agents:
The glue that holds the sand particles together is the binding agents. The most common types of binders are as follows:
Clay and water
Oil
Resin
Sodium silicate
Improvement Through Additives:
Additives are used to improve the surface finish of the mold, as well as its strength, refractoriness, and cushioning.
Parting Compounds:
This can be a fine powder or liquid used to help remove the pattern from the mold.
How Sand Casting is done?
Several steps are taken when sand casting, including:
Place the Mold Pattern in the Sand
The first step in sand casting is to place the mold pattern in sand. The mold has a direct impact on the size and shape of the casting. As a result, manufacturers must develop new molds to produce metal products and components in specific sizes and shapes.
Setting Up the Gating System
Sand casting, like most other casting processes, makes use of a gating system. It is used to funnel the molten mold into the mold cavity and consists of a pouring cup and tunnels or "gates" to the mold. Manufacturing companies will install a gating system like this after placing the mold pattern in sand.
Removing the Mold Pattern
After installing the gating system, manufacturing companies can remove the mold pattern from the sand. At this point, the mold pattern is no longer relevant. When a mold pattern is placed inside sand, the sand takes on the shape of the mold. As a result, the mold pattern is no longer necessary.
Filling the mold cavity with molten metal.
The molten metal is now ready to be poured into the mold cavity. Sand casting is compatible with a wide range of metals and alloys, including iron, steel, aluminium, bronze, magnesium, zinc, and tin. Manufacturing companies may need to heat up to 3,000 degrees Fahrenheit depending on the metal or alloy used. When the metal or alloy has melted and become liquid, it is poured into the mold cavity.
Allowing the metal to cool.
Manufacturing companies must wait for the molten metal to cool after it has been poured into the mold cavity. Again, different metals take different amounts of time to cool. As the molten metal cools, it will transition from a liquid to a solid state.
Breaking the mold open to get the metal casting out.
Breaking open the mold to remove the newly created metal casting is the sixth and final step in sand casting. Mold patterns are usually reusable, but the actual molds are not. As a result, every time a new metal product or component is created using sand casting, manufacturers must create a new mold.
Applications of Sand Casting:
The applications of sand casting include:
Pump bodies
Bearings
Bushings
Air compressor pistons
Impellers
Electronic equipment
Engine crankcases
Fittings
Engine oil pans
Gears
Flywheel castings
Gas and oil tanks
Machine parts
Applications of Sand Casting wrt Mg Alloys:
Aerospace applications make use of magnesium alloy sand castings. Significant research and development on these alloys has resulted in some spectacular improvements in general properties. Although magnesium-aluminum and magnesium-aluminum-zinc alloys are generally easy to cast, they have some limitations.
The two alloys ZE41A and EZ33A are finding the most use in relatively moderate temperature applications (up to 160°C).
An alloy known as QE22A was created as a result of further research aimed at improving both room-temperature and elevated-temperature mechanical properties.
The most recent alloy to emerge from this research was WE54A, which contained about 5.0 percent Y in combination with other rare earth metals, replacing both thorium and silver.
Advantages of Sand Casting:
As much as sand casting has drawbacks such as:
Low material strength - Because of the excessive porosity, the material strength is low when compared to a machined item.
Dimensional precision is particularly poor when shrinking and finishing the surface.
Surface roughness on the internal sand mold wall results in poor surface finishes.
Defects or differences in quality, such as shrinkage, porosity, pouring metal defects, and surface defects, are unavoidable in any metallurgical process. Sand casts have a high level of porosity when compared to other casting methods such as die casting and investment casting.
Post-processing — If a tighter tolerance is required to interface with other mating parts, additional machining is frequently required. Processing costs are significant when compared to tooling and material costs.
The benefits outweigh the drawbacks of sand casting. Sand casting has several advantages, including:
It is used to create complicated shapes.
It is capable of producing very large parts.
Tooling is very inexpensive.
Reuse and recycle scrap
It is adaptable to all metals, including those with high melting points.
Simple to scale
Geometries with complex geometry and thin wall sections
Low manufacturing costs
Geometries with complex geometry and thin wall sections
Twin Roll Casting:
Mg-based alloy strips are traditionally produced by hot-rolling slabs prepared by direct chill (DC) casting. Because the slabs are coarse-grained, a sophisticated rolling process must be carefully carried out in order to fabricate much thinner strips. These repeated steps result in inefficient production and high production costs. To overcome the difficulties associated with a solid state deformation process, a twin roll casting (TRC) process for producing Mg alloy strips directly from liquid Mg alloys has been developed. In recent years, a TRC technique has been extensively developed, and much research has been conducted to improve our understanding of the TRC process's solidification mechanism.
The traditional TRC process combines casting and rolling into a single process, emphasising large deformation in the hot-rolling process. The major drawbacks of the conventional TRC process are coarse columnar grains of approximately 1mm in size, severe chemical segregation at the strip's centre, large amounts of defects such as bleeding, and only casting dilute alloys with a narrow freezing range. The majority of such issues arise during the deformation process.
How is TRC done?
Molten magnesium metal is heated to liquid temperature and flows down the cooling slope in the TRC process. As the melt reaches the launder's tip, it rapidly cools (nozzle). As an adiabatic boundary condition in the tundish, the slope is kept constant at a constant temperature. The melt is dragged onto the lower roll's surface by the tundish. Solidification begins immediately after the melt leaves the tundish; thus, in the TRC process, the tip keeps the melt constant, improving the integrity of the solidified strip, minimising turbulence, and distributing the melt to the required strip width.
The melt solidifies into a continuous casting strip when it comes into contact with water-cooled rolls. A semi-solid region is included in the liquid-to-solid transformation. When the solidifying metal reaches the required strength, as determined by its 'rigidity point,' it undergoes some hot working before leaving the roll bite. As a result, the TRC process integrates solidification and hot rolling deformation into a single operation. Figure 1 depicts a schematic diagram of the TRC process.
Mg Alloys Processed by TRC:
The vast majority of research, pilot scale TRC tests, in- cluding commercial-scale manufacturing of Mg strip, involves the AZ31 alloy. Research with other commercial and experi- mental alloys is rather conducted at the laboratory scale. The strip made from high strength Mg alloys is rarely reported.
AZ31 Alloy:
The research is mainly focussed on structure of the strip and its inhomogeneity, surface segregation or post-TRC processing.
There are reported attempts of chemistry modification of the AZ31 alloy through minor additions of other elements. Additions of 0.08 wt.% Ca into AZ31 melts, significantly reduced the average grain size within the TRC strip from 100 μm to 30 μm through formation of Al 2 Ca.
Alloy with high Al contents:
Although 3 wt.% is the common Al level present in the AZ31 alloy, there are reported trials with TRC of Mg alloys with higher Al content reaching 9 wt.%, typical for cast alloys.
The Mg–4.5Al–1.0 Zn (AZ41M) alloy sheets produced by twin roll casting, sequential warm rolling and annealing at 350 °C showed higher strength and lower elongation after sequential warm rolling
TRC experiments with Mg–6Zn–1Mn (ZM61) alloys having varying Al contents showed microstructure refinement with increasing Al and changes in a type of precipitates: from MgZn 2 in ZM61 alloy to Al 8 Mn 5 in the ZM61–1%Al (ZMA611) and ZM61–3% Al (ZMA613) alloys,
The ZMA611 alloy showed a better combination of tensile properties than the ZM61 alloy. However, the ZMA613 alloy developed poor ductility due to formation of coarse Mg 12 (Al, Zn) 17 phase.
Alloys with rare earths:
TRC trials of Mg alloys with rare earths cover ZE10, ME21 and WE43 grades, which are known as high strength alloys with good corrosion behavior and creep properties.
The strength of WE43 (4.04 wt.% Y, 2.59 wt.% Nd and 0.52 wt.% Zr) 315 mm wide strip exceeded values reported for AZ31 with larger scatter of results, explained through higher microstructural inhomogeneity.
Experimental Alloys:
To improve performance during TRC and then strip prop- erties the commercial grades are modified with small addi- tions of alloying elements or novel chemistries are developed. In addition to rare earths a particular interest is created by calcium. The clustering tendency of Ca promotes GP zone formation and evenly-distributed fine precipitates, including grain boundary regions.
AZ61, AZ31 with Ca, Sn
To address oxidation and ignition resistance the AZ31 and AZ61 alloys were modified with additions of calcium
Mg-3Al-1Zn-1Mn-0.5Ca
The newly developed Mg-3Al-1Zn-1Mn-0.5Ca (wt.%) al- loy marked as AZMX3110 shows a much improved com- bination of formability with the Index Erichsen (IE) value of 8 mm and yield strength of 219 MPa
Mg-6.1Zn-0.4Ag-0.2Ca-0.6Zr
The Mg-6.1Zn-0.4Ag-0.2Ca-0.6Zr (wt.%) alloy with a des- tination for TRC was developed through additions of 0.4 wt.% Ag and 0.2 wt.% Ca to the Mg-6.1Zn-0.6Zr grade [89]. The purpose of Ag and Ca micro-additions was to enhance the age hardening response after T6 heat treatment.
Mg–Zn–Zr–Ca-Ag
Twin roll cast and hot rolled Z6 (Mg–6.2 Zn wt.%) alloys containing Zr, Ca, and Ag as microalloying elements showed tensile yield strength exceeding 300 MPa in the T6 (peak- aged) condition with good formability in the T4 condition
Mg-Al Casting Alloy:
The solubility of aluminum in Mg is very high (12.7 wt.%) at 437 °C (eutectic temperature). The α-Mg and γ -Mg17Al12 phases are formed when Al dissolves Mg matrix, leading to solid-solution strengthening. The castability of Mg–Al-based alloys is excellent but has mediocre mechanical properties. It shows good resistance to corrosion which improves with increasing amount of aluminum. Zinc and manganese (Mn) are the common additions to Mg–Al-based extrusion alloys with a concentration of 1 wt.%. The major series of Mg–Al based alloys are AZ and AM among these AZ31, AZ61, AZ81, AZ91 [59] and AM60 are the most investigated as biodegradable materials.In AZ31, alphabets A and Z represent the alloying components aluminum and zinc. The numbers 3 and 1 represent the% composition of each alloying element in the order of their appearance (3% Al and 1% zinc). AM series alloys of AM30, AM40, AM50, AM60 come under the alloys of Mg inclusive of Al, Mn but with higher amount of aluminum (Al). The AM alloys exhibit higher extrudability and mechanical properties than AZ alloys as Zn Containing eutectic ternary phases is absent. In the Mg alloys, zinc is added to strengthen the solute whereas Mn improves anti-corrosion properties by removing traces of Fe from the alloy.
Mg-rare earth alloys:
Cerium (magnesium-Cerium alloy):
REE (rare earth elements) improve magnesium's high thermal strength, corrosion resistance, and creep resistance. Rare earth elements also reduce the freezing range of magnesium alloys. When silicon is added to Mg alloys (magnesium alloys), the anti-corrosion property is reduced. For significant elongation, only 0.2 percent cerium should be added to magnesium. Because of the presence of cerium in magnesium alloys, enhanced plastic deformation capability is observed at recrystallization temperature in extruded rods. The presence of cerium increases the rate of corrosion in Mg-Ce binary alloys. This is due to the Mg12Ce compound, which improves cathodic kinetics. Previous research has found that Ce improves the surface layer stability of Mg hydroxide.
Gadolinium (magnesium- Gadolinium alloy)
At eutectic temperatures, gadolinium has a solubility of 23.49 weight percent. It is highly soluble in Mg (more than 10% by weight). When Gd is added to Mg-Al alloys, the phases Al-Mn-Gd and Al2Gd form, which consume Al and reduce the volume fraction of Mg17Al12. As a result, the reaction rate at the cathode may be reduced. This phenomenon is common in Mg-Al alloys containing rare earth elements. Because heterogeneous mixtures are formed and the content of Al is reduced, the influence of Gd in Mg alloys is complicated. As a result, long-term corrosion tests reveal a higher rate of corrosion in the Mg alloys that comprise Gd. Furthermore, previous research indicates that Gd is usually unfavourable for corrosion
Lanthanum (magnesium- Lanthanum alloy)
Gadolinium has a solubility in Mg of 23.49 wt. percent at eutectic temperature, and when alloyed with Mg, it contributes to solid solution strengthening. Adding La above its limit of solubility in Mg-Al alloys reduces corrosion resistance due to the formation of rough Al-La phase. In comparison to Al and Zn, Gd is an effective solid solution strengthener in Mg due to size misfits and valency effects.
Erbium (magnesium- Erbium alloy)
Erbium is one of the rare earth elements that dissolves well in magnesium. In the buffer solution of borate, adding 2 to 3 wt. percent Er reduced the rate of corrosion in Mg-2 to 3 Al wt. percent alloys over AM60.
Neodymium (magnesium- Neodymium alloy)
Because of its high solid solubility in Mg and low eutectic temperature of about 552 °C, neodymium has the best response to age hardening when added to Mg.
Both solid solution hardening and precipitation hardening were responsible for the increase in strength with increasing Nd content. Mg-2 Nd alloys in various extruded and heat treated conditions and reported that the Mg-2Nd alloys' high elongation ratios, combined with their low yield strength and low degradation, make them promising for resorbable stent applications and comparable to conventional WE 43 alloys.
Microstructural analysis of EV31A:
EV31A, with neodymium and gadolinium as the main alloying elements, up to 3.1wt percent and 1.7wt percent, respectively. Because of the relevant grain refinement potential of Zr-rich primary particles as a result of its peritectic transformation, zirconium is also present, and its amount is close to the maximum solubility of Zr in molten magnesium (0.6 wt. percent ). Because of the precipitation hardening effect, Zn improves mechanical properties and helps to overcome the corrosive effects of Fe and Ni impurities. Because of the high rare earth content, good mechanical properties can be achieved through precipitation hardening. The alloy investigated in this study is primarily used in the military and aerospace industries.
Figure 1 shows an optical micrograph of the sand cast alloy etched with Nital 2 and SEM magnification, as well as the corresponding EDS analysis. The microstructure is made up of equiaxed grains with an average grain size of 45 µm, which are surrounded by the eutectic compound, as indicated by the arrows. SEM-EDS analyses of the eutectic revealed that it is made up of -Mg and a ternary phase of Mg-Nd-Gd.
In the eutectic, the atomic ratio of Mg to Nd + Gd is between 12 and 13, very close to the stoichiometric ratio of 12. This phase is therefore assumed to be Mg12(Ndx ;Gd1-x), a modification of the metastable phase Mg12Nd in which neodymium is replaced by gadolinium without changing the crystal structure. In fact, the atomic radii of Nd and Gd differ only slightly (rGd=0.1802 nm, rNd= 0.1821 nm). The presence of rod-shaped particles of sub-micrometric and micrometric size inside the grains is also evidenced by the SEM micrograph in Fig.1-b, but they were not clearly visible by SEM on the metallographic samples etched by Nital2.SEM-EDS line profiles of the main elements, across a grain, are reported in Fig. 2, highlighting the presence of Zr clusters inside the grains, not revealed by the OM and SEM images in Fig.1 obtained on samples etched with Nital 2.
Microstructure of the T6 heat-treated alloy:
Figure 5 shows optical micrographs of the T6 heat treated alloy at various magnifications. By comparing representative optical micrographs of the sand-cast (Fig.1) and heat-treated alloys, the microstructural evolution caused by standard T6 heat treatment can be appreciated (Fig.5).
The eutectic compound Mg12(Ndx ;Gd1-x) (highlighted by red circles) did not completely dissolve during the solution treatment, as shown in Fig. 5-a, and the intragranular intermetallic precipitates shown in Fig. 3-a are no longer visible. Furthermore, the presence of particles within the grains (indicated by the red circle) is clearly visible in Fig. 5b; these particles aggregate as clusters and react with Nital2. SEM-EDS analysis of these clusters revealed that they are primarily Zr-Zn based, owing to Zn diffusion during solutioning, as confirmed by the EDS analyses shown in Fig. 6. (spectra 3 to 5).
This Mg-RE alloy's strength is achieved through precipitation hardening, which is induced by the T6 heat treatment, which includes solutioning, quenching, and artificial ageing.
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