Effect of Aging on Yield Stress and Corrosion Resistance of Die Cast Magnesium Alloy

This page summarizes the research paper "Effect of Aging on Yield Stress and Corrosion Resistance of Die Cast Magnesium Alloy". This study investigates how aging at 160°C affects the corrosion behavior of die cast magnesium alloy AZ91D, linking these changes to alterations in the material's microstructure and composition.

1. Overview:

  • Title: Effect of Aging on Yield Stress and Corrosion Resistance of Die Cast Magnesium Alloy
  • Authors: Guangling Song, Amanda L. Bowles, David H. StJohn
  • Publication Year: 2004
  • Publishing Journal/Academic Society: Materials Science and Engineering A
  • Keywords: Magnesium, Corrosion, Microstructure, Heat treatment
Fig. 1. Average weight loss rate of die cast AZ91D aged at 160 ◦C after immersion in 5 wt.% NaCl solution for 7 days.
Fig. 1. Average weight loss rate of die cast AZ91D aged at 160 ◦C after immersion in 5 wt.% NaCl solution for 7 days.

2. Research Background:

  • Social/Academic Context of the Research Topic:
    • The automotive industry is increasingly utilizing magnesium alloys due to their high strength-to-weight ratio, addressing energy and environmental concerns.
    • Magnesium alloys are considered promising alternatives to aluminum and steel in automotive components.
    • These components are often exposed to moderate temperatures (60–200°C) in service.
    • Exposure to such temperatures leads to the precipitation of the β phase (Mg17Al12) primarily at grain boundaries, altering mechanical properties.
    • The effect of microstructure on magnesium alloy corrosion resistance is a well-documented area of research.
    • The β phase in AZ91 is generally recognized as a corrosion barrier, enhancing corrosion resistance in solution-heat-treated and aged alloys.
  • Limitations of Existing Research:
    • Limited research exists on the effect of aging on the corrosion resistance of die cast AZ91.
    • Existing studies, like Suman's, have shown limited effects of short-term aging at moderate temperatures, but lacked detailed explanation and correlation with microstructure changes.
    • There is a lack of published data on the long-term effects of moderate temperature aging on the corrosion resistance of die cast AZ91, relevant to real-world service conditions.
  • Necessity of the Research:
    • Understanding the long-term impact of moderate temperature aging on the corrosion resistance of die cast AZ91 is crucial for practical applications.
    • Investigating the mechanisms behind corrosion performance changes during aging is essential for a better understanding of the relationship between microstructure and corrosion resistance in die cast AZ alloys.

3. Research Purpose and Research Questions:

  • Research Purpose:
    • To investigate the corrosion behavior of die cast magnesium alloy AZ91D when aged at 160°C.
    • To understand how changes in microstructure and local composition during aging affect the corrosion resistance of AZ91D.
  • Key Research Questions:
    • How does aging at 160°C influence the corrosion rate of die cast AZ91D over extended periods?
    • What is the relationship between the evolving microstructure (specifically, the precipitation of the β phase and changes in the α matrix) during aging and the observed corrosion resistance?
  • Research Hypotheses:
    • The corrosion rate of die cast AZ91D will change with aging time due to alterations in its microstructure and local composition.
    • Precipitation of the β phase during initial aging stages will act as a corrosion barrier, potentially decreasing the corrosion rate.
    • Changes in the α matrix composition, particularly the depletion of aluminum, during prolonged aging may influence the corrosion rate.

4. Research Methodology:

  • Research Design:
    • Experimental study using die cast AZ91D alloy aged at 160°C for varying durations (up to 585 hours).
    • Comparison with solution-heat-treated permanent mould cast binary Mg-Al alloys with different aluminum contents (2.00, 3.89, 5.78, and 8.95 wt.% Al).
  • Data Collection Method:
    • Weight Loss Measurement: Immersion tests in 5 wt.% NaCl solution for 7 days and salt spray tests for 8 days were conducted to determine corrosion rates. Weight loss was measured after removing corrosion products using chromate acid.
    • Electrochemical Testing: Polarization curves and Electrochemical Impedance Spectroscopy (EIS) were performed in Mg(OH)2 saturated 5 wt.% NaCl solution to assess corrosion behavior and electrochemical characteristics.
    • Microscopy: Scanning Electron Microscopy (SEM) was used to observe and analyze the microstructure of aged and corroded specimens, including β phase precipitation and corrosion morphology.
    • Nuclear Magnetic Resonance (NMR): NMR spectroscopy was employed to quantify the fraction of β phase and measure the aluminum content in the α matrix as a function of aging time.
  • Analysis Method:
    • Correlation Analysis: The study aimed to correlate the corrosion rate (from weight loss and electrochemical measurements) with aging time, microstructure evolution (β phase fraction, morphology, and distribution), aluminum content in the α matrix, and yield stress data from previous studies.
    • Electrochemical Data Interpretation: EIS spectra were analyzed to understand impedance characteristics and changes in corrosion resistance. Polarization curves were used to determine pitting potentials (Ept) and assess corrosion susceptibility.
    • Microstructural Analysis: SEM images were used to qualitatively and quantitatively assess the β phase precipitation, distribution, and corrosion morphology. NMR data provided quantitative measurements of phase fractions and compositional changes.
  • Research Subjects and Scope:
    • Materials: Die cast AZ91D magnesium alloy and permanent mould cast binary Mg-Al alloys (2.00, 3.89, 5.78, 8.95 wt.% Al).
    • Aging Conditions: 160°C in air for periods up to 585 hours for die cast AZ91D. Solution heat treatment at 413°C for 24h for Mg-Al alloys.
    • Corrosion Environments: 5 wt.% NaCl solution (immersion and salt spray), Mg(OH)2 saturated 5 wt.% NaCl solution (electrochemical tests).

5. Main Research Results:

  • Key Research Results:
    • The corrosion rate of die cast AZ91D initially decreases with aging time up to approximately 45 hours at 160°C, and then increases with further aging.
    • A minimum corrosion rate is observed between 15 and 45 hours of aging.
    • The dependence of corrosion rate on aging time is similar under both immersion and salt spray conditions.
    • The change in corrosion resistance with aging time correlates with the change in yield stress observed in previous studies on die cast AZ91D under the same aging conditions. The minimum corrosion rate occurs at approximately the same aging time as the maximum yield stress.
    • Microstructural analysis revealed that β phase (Mg17Al12) precipitates exclusively along grain boundaries during aging.
    • In the initial stages of aging, the volume fraction of β phase increases rapidly, forming a network along grain boundaries.
    • With extended aging, the aluminum content in the α matrix decreases significantly.
  • Statistical/Qualitative Analysis Results:
    • Figure 1: "Average weight loss rate of die cast AZ91D aged at 160°C after immersion in 5 wt.% NaCl solution for 7 days." Shows a decrease in weight loss rate initially, followed by an increase after ~45h aging.
    • Figure 2: "Average weight loss rate of die cast AZ91D aged at 160°C after exposure to salt spraying for 8 days." Similar trend to Figure 1, with initial decrease and subsequent increase in weight loss rate.
    • Figure 3: "Relationship between corrosion rate and yield stress of AZ91D aged at 160°C (the ageing hours are specified in the figure)." Demonstrates a correlation between corrosion rate and yield stress, suggesting both are influenced by microstructure changes during aging.
    • Figure 4: "Influence of the ageing at 160°C on the microstructure of die cast AZ91D. A high magnification image is shown in the bottom right corner. The white arrow indicates a region of discontinuous precipitation and the area marked "A" shows rod shaped precipitates." Illustrates the evolution of microstructure with aging time, showing β phase precipitation at grain boundaries.
    • Figure 5: "Volume fraction of β phase in aged die cast AZ91D and the average aluminium concentration in α matrix." Quantifies the increase in β phase fraction and decrease in α matrix aluminum content with aging time. "The average aluminium content of the α matrix dramatically decreased from about 6.5 to 2 wt.% aluminium in the first 45 h of ageing, and then slowly decreased with extended ageing down to about 1.5 wt.% aluminium after 585h".
    • Figure 6: "Micro-morphologies of die cast AZ91D specimens after immersion in 5 wt.% NaCl solution for 4h. In photo (c), the white tiny dots in the corroded areas are the rod shaped continuous β precipitates." Shows that corrosion mainly occurs in the α matrix, while β precipitates remain relatively unaffected, acting as a barrier.
    • Figure 7: "Average weight loss rates of Mg-Al single phase alloys with various aluminium contents after immersion in 5 wt.% NaCl solution for 3 h." Shows that "the corrosion resistance of α phase decreases as the aluminium level in the α phase decreases".
    • Figure 8: "Electrochemical impedance spectra of Mg-Al single a phase alloys in 5 wt.% NaCl saturated with Mg(OH)2." Demonstrates the dependence of corrosion resistance on aluminum content in α phase alloys.
    • Figure 9: "Electrochemical impedance spectra of die cast AZ91D aged at 160°C for various periods of time in 5 wt.% NaCl saturated with Mg(OH)2." Shows changes in EIS spectra with aging, indicating initial increase and subsequent decrease in impedance (corrosion resistance). "Second, the diameter of the capacitive semicircle in the high frequency region increases first, then decreases with ageing time (the spectrum for 150h slightly deviates from the trend). The maximum appears at an ageing time of 15 h."
    • Figure 10: "Polarisation curves of Mg-Al single α phase alloys in 5 wt.% NaCl saturated with Mg(OH)2." Shows the shift of pitting potential (Ept) with varying aluminum content in α phase alloys.
    • Figure 11: "Polarisation curves of die cast AZ91D aged at 160°C for various periods of time in 5 wt.% NaCl saturated with Mg(OH)2." Illustrates the shift of pitting potential (Ept) for aged AZ91D.
    • Figure 12: "Dependence of "pitting" potential (Ept) on aluminium content in Mg-Al single a phase alloys." Shows that "The α phase, with a higher aluminium content has a more positive Ept".
    • Figure 13: "Dependence of "pitting" potential (Ept) on ageing time for die cast AZ91D aged at 160°C." Shows that "For die cast AZ91D, the shift of Ept to a more negative potential by ageing".
Fig. 2. Average weight loss rate of die cast AZ91D aged at 160 ◦C after exposure to salt spraying for 8 days.
Fig. 2. Average weight loss rate of die cast AZ91D aged at 160 ◦C after exposure to salt spraying for 8 days.
Fig. 3. Relationship between corrosion rate and yield stress of AZ91D aged at 160 ◦C (the ageing hours are specified in the figure).
Fig. 3. Relationship between corrosion rate and yield stress of AZ91D aged at 160 ◦C (the ageing hours are specified in the figure).
Fig. 4. Influence of the ageing at 160 ◦C on the microstructure of die cast AZ91D. A high magnification image is shown in the bottom right corner. The white arrow indicates a region of discontinuous precipitation and the area marked “A” shows rod shaped precipitates.
Fig. 4. Influence of the ageing at 160 ◦C on the microstructure of die cast AZ91D. A high magnification image is shown in the bottom right corner. The white arrow indicates a region of discontinuous precipitation and the area marked “A” shows rod shaped precipitates.
Fig. 5. Volume fraction of  phase in aged die cast AZ91D and the average aluminium concentration in matrix.
Fig. 5. Volume fraction of  phase in aged die cast AZ91D and the average aluminium concentration in matrix.
Fig. 6. Micro-morphologies of die cast AZ91D specimens after immersion in 5 wt
Fig. 6. Micro-morphologies of die cast AZ91D specimens after immersion in 5 wt
Fig. 9. Electrochemical impedance spectra of die cast AZ91D aged at 160 ◦C for various periods of time in 5 wt.% NaCl saturated with Mg(OH)2.
Fig. 9. Electrochemical impedance spectra of die cast AZ91D aged at 160 ◦C for various periods of time in 5 wt.% NaCl saturated with Mg(OH)2.
  • Data Interpretation:
    • The initial decrease in corrosion rate is attributed to the barrier effect of the β phase precipitates formed along grain boundaries, hindering corrosion progression into the α matrix. "The β phase acts as a barrier, resulting in a decreasing corrosion rate in the initial stages of ageing."
    • The subsequent increase in corrosion rate after longer aging times is explained by the decreasing aluminum content in the α matrix, which makes the α matrix more active and less corrosion resistant. "In the later stages, the decreasing aluminium content of α grains makes the α matrix more active, causing an increase in the corrosion rate."
    • Electrochemical testing results (EIS and polarization curves) support the combined effects of β phase precipitation and α matrix compositional changes on the corrosion resistance of aged die cast AZ91D. "Electrochemical testing results also confirm the combined effects of the changes in α and β phases on the corrosion resistance of the aged die cast AZ91D alloy."
    • The correlation between yield stress and corrosion rate suggests that both properties are influenced by the same microstructural changes during aging. "The striking similarity between the curves of yield stress versus ageing time and the corrosion rate versus ageing time strongly suggests that the change in corrosion resistance is closely associated with the change in microstructure that also determines the yield stress."

6. Conclusion and Discussion:

  • Summary of Main Results:
    • Moderate temperature aging (160°C) of die cast AZ91D initially improves corrosion resistance, followed by a decrease with prolonged aging.
    • This behavior is attributed to the interplay between the beneficial barrier effect of β phase precipitation at grain boundaries and the detrimental effect of decreasing aluminum content in the α matrix.
    • Yield strength and corrosion rate of die cast AZ91D exhibit a direct relationship, both being governed by microstructural changes during aging.
  • Academic Significance of the Research:
    • This study provides valuable insights into the long-term corrosion behavior of die cast magnesium alloys under moderate temperature aging conditions.
    • It elucidates the complex role of β phase precipitation and α matrix compositional changes in determining the corrosion resistance of aged AZ91D.
    • The research contributes to a deeper understanding of the microstructure-corrosion property relationship in die cast magnesium alloys.
  • Practical Implications:
    • The findings highlight the importance of considering aging time when evaluating the corrosion performance of die cast AZ91D components operating at moderate temperatures.
    • Optimizing aging treatments for die cast AZ91D should consider the trade-off between the beneficial effects of β phase precipitation and the detrimental effects of α matrix aluminum depletion to achieve optimal corrosion resistance.
  • Limitations of the Research:
    • The study is specific to die cast AZ91D alloy aged at 160°C.
    • Further research is needed to investigate the effects of aging on other magnesium alloys and under different temperature and environmental conditions.
    • The generalizability of the observed yield strength-corrosion resistance relationship to a broader range of casting and aging conditions requires further investigation.

7. Future Follow-up Research:

  • Directions for Follow-up Research:
    • Further investigation is recommended to test a broader range of casting and aging conditions to validate the observed relationship between yield strength and corrosion resistance.
    • Detailed studies are needed to further explore the mechanisms of corrosion in relation to the specific microstructural evolution during aging, including the kinetics of β phase precipitation and aluminum depletion in the α matrix.
  • Areas Requiring Further Exploration:
    • Investigating the influence of other alloying elements and impurities on the aging behavior and corrosion resistance of die cast magnesium alloys.
    • Exploring the long-term corrosion performance under more complex and realistic service environments, including varying temperatures, humidity, and corrosive media.

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9. Copyright:

  • This material is Guangling Song, Amanda L. Bowles, David H. StJohn's paper: Based on Effect of Aging on Yield Stress and Corrosion Resistance of Die Cast Magnesium Alloy.
  • Paper Source: DOI: 10.1016/j.msea.2003.08.060
  • This material was summarized based on the above paper, and unauthorized use for commercial purposes is prohibited.
  • Copyright © 2025 CASTMAN. All rights reserved.
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