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Effect of Mg-Controlled Solidification Pathways on Microstructure and Properties of As-Cast Al-1Y-xMg Alloys

This study demonstrates that increasing magnesium content in as-cast Al-1Y alloys systematically alters solidification pathways to form a continuous τ + β eutectic network, which significantly enhances tensile strength and enables tunable thermal and electrical properties, thereby establishing a quantitative framework for optimizing these alloys for combined structural and thermal management applications.

Original authors: Yulong Ren, Xuexuan Yang, Peng Tang

Published 2026-07-23

📖 6 min read🧠 Deep dive

CC BY 4.0

Original authors: Yulong Ren, Xuexuan Yang, Peng Tang

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you are trying to build the ultimate sandwich. You want the bread to be strong enough to hold a heavy load without crumbling, but also soft enough to chew without breaking your teeth. Now, imagine that the "bread" is a block of aluminum, a metal used everywhere from soda cans to airplane wings. Scientists have long known that adding a pinch of magnesium (Mg) to aluminum makes it stronger, kind of like adding extra cheese to a sandwich. But there's a catch: if you add too much magnesium, the sandwich gets brittle and snaps, or worse, it starts to rust away quickly.

For years, researchers have been trying to find the perfect "recipe" to get that sweet spot of strength and flexibility. They've tried sprinkling in expensive rare metals like scandium, but that's like trying to make a gourmet sandwich with gold leaf—it costs too much and is hard to control. This paper dives into a different, more affordable ingredient: Yttrium (Y). Think of Yttrium as a special seasoning that helps organize the magnesium. The big question the scientists wanted to answer is: "If we keep the Yttrium steady and just change how much Magnesium we add, what happens to the invisible structure inside the metal, and how does that change how the metal behaves?" They aren't just guessing; they are looking at the microscopic "architecture" of the metal to see how the ingredients arrange themselves as the liquid metal cools down and turns solid.

The Recipe for a Super-Metal Sandwich

In this study, the researchers at Guangxi University cooked up a series of aluminum alloys. They started with a base recipe containing 1% Yttrium and then added varying amounts of Magnesium, ranging from 0% all the way up to 8%. They poured this molten mixture into molds to let it cool and solidify, creating what they call "as-cast" alloys. Think of this as letting a cake batter cool in the pan; the way it sets depends entirely on the ingredients and the cooling process.

The Magic of the "Solidification Pathway"
The most exciting discovery is how the magnesium controls the "solidification pathway." Imagine the metal cooling down as a construction crew building a city.

Low Magnesium (0-2%): The city is mostly empty space (the aluminum) with a few isolated, bright white buildings (Yttrium-rich phases) scattered around. It's a bit weak but very stretchy.
Medium Magnesium (4-6%): As they add more magnesium, a new type of building starts to appear. First, a specific structure called τ-phase (tau-phase) forms. Then, as the magnesium level hits the sweet spot, a second structure called β-phase (beta-phase) joins in. These two start to link up, forming a continuous, interconnected network along the edges of the metal grains. It's like the construction crew building a strong, continuous highway system that connects all the neighborhoods.
High Magnesium (8%): The highway system gets too crowded. The β-phase becomes so dominant that it forms a thick, brittle wall around the neighborhoods, blocking movement and making the whole city prone to cracking.

The Sweet Spot: 6% Magnesium
The researchers found that the 6% Magnesium alloy was the champion.

Strength: It could withstand a pulling force of 158.0 MPa (MegaPascals). That's about 95% stronger than the alloy with no magnesium at all.
Flexibility: Even though it was super strong, it could still stretch by 11.0% before breaking. This is a rare "strength-ductility balance." Usually, when something gets stronger, it gets brittle (like glass), but this alloy stayed tough.
The "Serrated" Flow: When they pulled on the metal, it didn't stretch smoothly. Instead, it moved in tiny, jerky jumps, like a car with a sticky accelerator. This is called the Portevin-Le Chatelier (PLC) effect. The magnesium atoms were acting like little magnets, temporarily sticking to the moving defects (dislocations) in the metal, then letting go. This "stick-and-slip" behavior actually helped the metal get stronger as it was being pulled. However, at 8% magnesium, this helpful "stickiness" disappeared because the magnesium was all tied up in the brittle walls, leading to a drop in strength and a loss of stretchiness.

The "Rust" and "Heat" Connection
The study also looked at how electricity and heat move through these metals.

Conductivity: Pure aluminum is great at conducting electricity and heat. Adding magnesium acts like adding speed bumps to a highway. The electrons (electricity) and phonons (heat) have to bounce around more, slowing them down.
The Trade-off: As magnesium increased, the electrical conductivity dropped from 51.8% IACS (a standard measure of conductivity) down to 28.2% IACS. The thermal conductivity dropped from 218 W·m⁻¹·K⁻¹ to 148 W·m⁻¹·K⁻¹.
The Mechanism: At low magnesium levels, the slowdown was mostly because of individual magnesium atoms floating in the metal. But at high levels (6-8%), the slowdown was caused by the massive "highway networks" (the eutectic network) of the τ and β phases. These networks created so many boundaries that electrons and heat had a hard time getting through.
Corrosion: The study also found that as the magnesium network became continuous (at 6-8%), the metal became more prone to "pitting" (tiny holes caused by rust) in salty water. The continuous network acted like a fast lane for corrosion to spread, shifting the metal's electrical potential to a more negative value (from -0.72 V to -0.85 V).

What They Ruled Out
The researchers made it clear that simply adding more magnesium isn't always better. They showed that once you pass the 6% mark, the benefits stop and the metal actually starts to get weaker and less stretchy. They also ruled out the idea that the metal's strength comes just from the magnesium atoms floating around; the arrangement of the phases (the network) is the real hero.

The Bottom Line
This paper doesn't just say "add magnesium." It provides a precise map. By controlling the magnesium content, engineers can now "dial in" the exact properties they need. If they need a part that is super strong and can handle some heat, they aim for that 6% magnesium sweet spot. If they need something that conducts electricity really well, they stick to lower magnesium levels. It's like having a volume knob for the metal's properties, turning the strength up or the conductivity up depending on what the job requires. The study proves that by understanding how the metal "freezes" and forms its internal network, we can design better, cheaper, and more versatile aluminum alloys for everything from cars to heat sinks.

Technical Summary: Effect of Mg-Controlled Solidification Pathways on Microstructure and Properties of As-Cast Al-1Y-xMg Alloys

Problem Statement
Al-Mg alloys are critical for lightweight structural applications due to their corrosion resistance and formability, yet they face a fundamental trade-off between strength and ductility. High magnesium content, while beneficial for solid-solution strengthening, often leads to the inhomogeneous precipitation of brittle $\beta$ -Al $_3$ Mg $_2$ phases along grain boundaries during conventional casting. This continuous network promotes stress concentration, microvoid nucleation, and crack propagation, drastically reducing ductility and increasing susceptibility to stress corrosion cracking. Furthermore, existing microalloying strategies using expensive elements like Scandium (Sc) or limited-solubility elements like Erbium (Er) and Cerium (Ce) have constraints regarding cost, thermal stability, or strengthening potential. While Yttrium (Y) offers potential for multiphase strengthening, the specific mechanisms by which varying Mg content controls solidification pathways, phase selection sequences, and the resulting structure-property relationships in Al-Mg-Y systems remain poorly understood, particularly under non-equilibrium solidification conditions.

Methodology
To address these gaps, the authors designed and fabricated a series of as-cast Al-1Y-xMg alloys with Mg content ranging from 0 to 8 wt.% (nominal composition). The alloys were prepared via conventional gravity casting using a crucible resistance furnace, followed by air cooling. The study employed a comprehensive multiscale characterization approach:

Microstructural Analysis: Optical microscopy (OM), Scanning Electron Microscopy (SEM) with Energy Dispersive Spectroscopy (EDS), and X-ray Diffraction (XRD) were used to identify phase evolution, grain size, and precipitate morphology.
Mechanical Testing: Tensile tests were conducted to determine Ultimate Tensile Strength (UTS), elongation, and the presence of Portevin-Le Chatelier (PLC) serrations. Vickers microhardness was also measured.
Physical Property Evaluation: Electrical conductivity was measured and converted to thermal conductivity using the Wiedemann-Franz Law. Differential Scanning Calorimetry (DSC) analyzed solidification temperatures and enthalpy changes.
Electrochemical Testing: Open-circuit potential (OCP) measurements in 3.5 wt.% NaCl solution evaluated surface electrochemical stability.
Theoretical Modeling: The study utilized the Hall-Petch equation for grain boundary strengthening, the Orowan mechanism for precipitation strengthening, and the Matthiessen-Flemming rule to analyze electron scattering mechanisms.

Key Results

Solidification Pathway Evolution: Increasing Mg content fundamentally alters the solidification sequence. At low Mg levels (0–2 wt.%), the microstructure consists of isolated Y-rich phases. As Mg increases to 4 wt.%, blocky $\tau$ -Al $_x$ MgY phases appear. At 6–8 wt.% Mg, the pathway evolves to a sequential precipitation of $\tau$ -Al $_x$ MgY followed by $\beta$ -Al $_3$ Mg $_2$ , culminating in a continuous $\tau$ + $\beta$ eutectic network along grain boundaries.
Mechanical Properties: The alloy with 6 wt.% Mg achieved a peak UTS of 158.0 MPa, approximately 95% higher than the Mg-free alloy (81.2 MPa), while retaining 11.0% uniform elongation. However, at 8 wt.% Mg, strength slightly declined to 151.5 MPa, and ductility dropped to 8.0%. Fracture analysis revealed a transition from ductile dimples (low Mg) to quasi-cleavage dominated by brittle fracture (high Mg).
Dynamic Strain Aging (DSA): Alloys with Mg $\ge$ 2 wt.% exhibited PLC serrations. The amplitude and frequency of serrations increased with Mg content up to 6 wt.%, indicating enhanced solute-dislocation interactions. At 8 wt.% Mg, serration activity decreased as the continuous brittle $\beta$ -network consumed solute Mg, shifting the deformation mechanism toward grain-boundary cracking.
Physical Properties: Electrical and thermal conductivities decreased monotonically with increasing Mg, ranging from 51.8% IACS (218 W·m $^{-1}$ ·K $^{-1}$ ) for 0Mg to 28.2% IACS (148 W·m $^{-1}$ ·K $^{-1}$ ) for 8Mg.
Electrochemical Behavior: Increasing Mg content caused a negative shift in Open-Circuit Potential (OCP), from -0.72 V to -0.85 V, attributed to the formation of anodic, continuous $\beta$ -Al $_3$ Mg $_2$ networks that compromise the passive film integrity.

Key Contributions

Quantitative Processing-Microstructure-Property Relationships: The study establishes a direct link between Mg content, the evolution of the solidification pathway, and the resulting eutectic network topology.
Mechanism Elucidation: It clarifies the competition between solute-controlled electron scattering (dominant at low Mg) and interface-controlled scattering by the $\tau$ / $\beta$ network (dominant at high Mg). Similarly, it details the transition from dislocation-solute interaction (DSA) to grain-boundary cracking as the primary deformation mechanism.
Optimization Strategy: The research identifies 6 wt.% Mg as the optimal composition for balancing strength and ductility in as-cast Al-1Y-Mg alloys, avoiding the excessive brittleness associated with 8 wt.% Mg.

Significance
This work provides a quantitative framework for tailoring cast Al-Mg-Y alloys for applications requiring combined structural and thermal management capabilities. By demonstrating that Mg content acts as a powerful control variable for solidification pathways, the study offers a design guideline for optimizing the trade-off between mechanical performance and functional properties (conductivity, corrosion resistance) without relying on prohibitively expensive microalloying elements. The findings suggest that controlling the formation of the eutectic network is critical for mitigating the inherent strength-ductility trade-off in medium-to-high Mg aluminum alloys.

The Recipe for a Super-Metal Sandwich

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