image: Graph showing how the viscosity of molten Mg-Ca alloy changes with calcium content. The overall rising trend and distinct inflection points (e.g., at 1, 6, 8 wt.% Ca) are directly linked to rearrangements of atomic clusters within the melt, as revealed by molecular dynamics simulations.
Credit: Materials and Solidification, Tsinghua University Press
Magnesium alloys, celebrated as the lightest structural metals, are pivotal for lightweighting in transportation and aerospace. Their widespread adoption hinges on efficient casting processes, which are governed by a crucial yet elusive property: the fluidity of the molten metal. For decades, foundries have known that adding calcium (Ca) improves magnesium's castability and oxidation resistance, but the fundamental reason—how Ca atoms rearrange the melt's inner structure to affect its flow—remained a black box.
A research team from Chongqing University has now illuminated this microscopic world. In a study published in Materials & Solidification, they employed molecular dynamics simulations to map the structural evolution of Mg-Ca alloy melts across a wide range of calcium content (0.1 to 10 wt.%).
The team published their work in Materials and Solidification on January 14, 2026.
The simulation revealed a striking threshold effect. When calcium content is below 4 wt.%, the atomic-scale order within the melt fluctuates wildly with each small addition of Ca. "It's like adding pebbles to a stream; they create local turbulence," explains Ang Zhang, the corresponding author of the study. "The structure is searching for stability, leading to significant oscillations in local bonding patterns." However, once the calcium content surpasses this 4 wt.% threshold, the melt structure settles into a more stable, ordered state.
This structural shift has a direct mechanical consequence: viscosity. The team calculated the melt's resistance to flow (viscosity) and found it generally increases with more calcium, as atomic movement becomes more restricted. Crucially, they identified specific inflection points—notably at 1, 6, and 8 wt.% Ca—where viscosity changes abruptly. "These aren't random noise," notes Zhang. "Each inflection point correlates perfectly with a specific reorganization of the dominant atomic clusters we see in our simulations, such as a sudden change in icosahedral-type bonding pairs."
The power of this research lies in establishing a quantitative link. For the first time, manufacturers can look at a target calcium content and predict not just the melt's approximate viscosity, but also understand the specific atomic arrangements responsible for it. "We've moved from observing a macro effect to understanding its micro-origin," says Zhang. "This allows us to strategically select a Ca content, like near 1 wt.%, if we want to minimize viscosity for thin-wall casting, or a higher content for a more stable, viscous flow."
This work establishes a fundamental theoretical framework for the intelligent design of magnesium alloys, enabling the development of custom-engineered lightweight structural magnesium alloys. The next step is to expand this model to industrially relevant, multi-component alloys containing elements like aluminum and zinc, and to validate the predictions with high-temperature experiments.
About Author
Ang Zhang, Professor, Doctor of Philosophy. He works at Chongqing University and his interests lie in computational materials science.
Journal
Materials and Solidification
Article Title
Correlating melt structure, viscosity, and Ca content in Mg–Ca alloys via molecular dynamics
Article Publication Date
14-Jan-2026