The effects of alloying additions and heat treatment on creep properties and microstructure of high-pressure die-cast magnesium-rare-earth alloys

Poor elevated temperature creep properties have limited the use of traditional Mg alloys, such as Mg-Al alloys, in the automotive industry primarily to ambient temperature applications in automobiles. Grain boundary reinforcement was commonly used to improve elevated creep properties of Mg alloys by preventing grain boundary sliding. However, the main creep mechanism is still under debate. More recently, precipitation hardening, solid solution strengthening and/or the diffusion rate of solute in the matrix have been proposed as the key factors that influence creep. High-pressure die-cast (HPDC) Mg-rare earth (Mg-RE) alloys are an ideal choice for elevated temperature automotive parts, such as powertrains, due to their excellent elevated temperature creep resistance and high production rate. In this investigation the differences in elevated temperature creep properties of three high-pressure die-cast Mg-rare earth (Mg-RE) alloy series, Mg-La-Nd, Mg-La-Y and Mg-La-Gd have been determined. A consistent concentration of La (0.45at.%) was used to maintain similar grain boundary strengthening for all the alloys and also to assist with castability. The ternary RE concentration was varied and the RE elements were chosen to have differing solubility in Mg. This was done to investigate the influence of solid solution strengthening and precipitation hardening on creep. The creep stress exponent in combination with electron microscopy was used to determine that the main creep mechanism was diffusion-controlled dislocation climb. Dislocations were shown to be decorated by precipitates in all three alloy series. It was shown that the Mg-La-Nd alloy series had significantly worse creep resistance at 177°C and 90 MPa when compared with the Mg-La-Y or Mg-La-Gd alloys series. For alloys with approximately 0.2 at.% ternary RE or greater, the Mg-La-Y and Mg-La-Gd alloys had minimum creep rates of approximately 5 x 10^-10 s^-1 in comparison with Mg-La-Nd alloys which had 1 x 10^-8 s^-1. Alloys with high concentrations of ternary RE such as Mg-0.45La-1.18Y (at.%) and Mg-0.45La-0.87Gd (at.%) reached 0.1% creep strain following 600 h of creep testing at 90 MPa and 177°C. The relatively high Nd-concentrated alloy Mg-0.45La-0.63Nd had significantly worse creep properties reaching 1 % creep strain in less than 350 h. The morphology of the microstructure of three HPDC Mg-La-RE alloy series with varying ternary alloying concentrations was compared. It was found that all alloys had relatively similar morphologies with respect to average grain size, volume fraction of eutectic present at grain boundaries and the intermetallic phase present in the eutectic. The age hardening response of the alloys revealed that the Mg-La-Nd alloys reached peak-aged conditions sooner than Mg-La-Y or Mg-La-Gd but also overaged more rapidly. This was used to indicate that Mg-La-Nd alloys had the poorest thermal stability/fastest diffusion rate of solute out of the three alloys series investigated. The precipitates formed dynamically during creep testing and were finer in Mg-La-Y and Mg-La-Gd alloys than in Mg-La-Nd alloys. It was shown that it is possible to solution treat for a relatively short duration, or solution treat and then age HPDC Mg-La-RE alloys without causing any surface blistering to the casting. However, creep properties as well as yield strength were negatively affected unless relatively high concentrations of solute in solid solution or a relatively high number density of precipitates were present. This was the result of a reduction in grain boundary reinforcement that was caused by the intermetallic becoming less continuous and also by the removal of the supersaturated region of solute in the matrix near the grain boundaries. It was concluded that improvements to elevated temperature creep resistance of HPDC Mg-La-RE alloys could be achieved by building on a base Mg-RE alloy (Mg-La) that had sufficiently good castability and grain boundary reinforcement with low diffusive/thermally stable soluble ternary RE additions in solid solution

The effects of alloying additions and heat treatment on creep properties and microstructure of high-pressure die-cast magnesium-rare-earth alloys

Poor elevated temperature creep properties have limited the use of traditional Mg alloys, such as Mg-Al alloys, in the automotive industry primarily to ambient temperature applications in automobiles. Grain boundary reinforcement was commonly used to improve elevated creep properties of Mg alloys by preventing grain boundary sliding. However, the main creep mechanism is still under debate. More recently, precipitation hardening, solid solution strengthening and/or the diffusion rate of solute in the matrix have been proposed as the key factors that influence creep. High-pressure die-cast (HPDC) Mg-rare earth (Mg-RE) alloys are an ideal choice for elevated temperature automotive parts, such as powertrains, due to their excellent elevated temperature creep resistance and high production rate. In this investigation the differences in elevated temperature creep properties of three high-pressure die-cast Mg-rare earth (Mg-RE) alloy series, Mg-La-Nd, Mg-La-Y and Mg-La-Gd have been determined. A consistent concentration of La (0.45at.%) was used to maintain similar grain boundary strengthening for all the alloys and also to assist with castability. The ternary RE concentration was varied and the RE elements were chosen to have differing solubility in Mg. This was done to investigate the influence of solid solution strengthening and precipitation hardening on creep. The creep stress exponent in combination with electron microscopy was used to determine that the main creep mechanism was diffusion-controlled dislocation climb. Dislocations were shown to be decorated by precipitates in all three alloy series. It was shown that the Mg-La-Nd alloy series had significantly worse creep resistance at 177°C and 90 MPa when compared with the Mg-La-Y or Mg-La-Gd alloys series. For alloys with approximately 0.2 at.% ternary RE or greater, the Mg-La-Y and Mg-La-Gd alloys had minimum creep rates of approximately 5 x 10^-10 s^-1 in comparison with Mg-La-Nd alloys which had 1 x 10^-8 s^-1. Alloys with high concentrations of ternary RE such as Mg-0.45La-1.18Y (at.%) and Mg-0.45La-0.87Gd (at.%) reached 0.1% creep strain following 600 h of creep testing at 90 MPa and 177°C. The relatively high Nd-concentrated alloy Mg-0.45La-0.63Nd had significantly worse creep properties reaching 1 % creep strain in less than 350 h. The morphology of the microstructure of three HPDC Mg-La-RE alloy series with varying ternary alloying concentrations was compared. It was found that all alloys had relatively similar morphologies with respect to average grain size, volume fraction of eutectic present at grain boundaries and the intermetallic phase present in the eutectic. The age hardening response of the alloys revealed that the Mg-La-Nd alloys reached peak-aged conditions sooner than Mg-La-Y or Mg-La-Gd but also overaged more rapidly. This was used to indicate that Mg-La-Nd alloys had the poorest thermal stability/fastest diffusion rate of solute out of the three alloys series investigated. The precipitates formed dynamically during creep testing and were finer in Mg-La-Y and Mg-La-Gd alloys than in Mg-La-Nd alloys. It was shown that it is possible to solution treat for a relatively short duration, or solution treat and then age HPDC Mg-La-RE alloys without causing any surface blistering to the casting. However, creep properties as well as yield strength were negatively affected unless relatively high concentrations of solute in solid solution or a relatively high number density of precipitates were present. This was the result of a reduction in grain boundary reinforcement that was caused by the intermetallic becoming less continuous and also by the removal of the supersaturated region of solute in the matrix near the grain boundaries. It was concluded that improvements to elevated temperature creep resistance of HPDC Mg-La-RE alloys could be achieved by building on a base Mg-RE alloy (Mg-La) that had sufficiently good castability and grain boundary reinforcement with low diffusive/thermally stable soluble ternary RE additions in solid solution

The Effect of Zn Content on the Mechanical Properties of Mg-4Nd-xZn Alloys (x = 0, 3, 5 and 8 wt.%)

The mechanical properties of as-cast Mg-4Nd-xZn (x = 0, 3, 5 or 8 wt.%) alloys were investigated both in situ and ex situ in as-cast and solution-treated conditions. The additions of 3 or 5 wt.% Zn in the base Mg-4Nd alloy did not improve yield strength in comparison to the binary Mg-4Nd alloy. Mechanical properties were shown to improve only with the relatively high concentration of 8 wt.% Zn to Mg-4Nd. The change in intermetallic morphology from a continuous intermetallic to a lamella-like intermetallic was the primary reason for the decreased mechanical properties in Mg-4Nd-3Zn and Mg-4Nd-5Zn compared with Mg-4Nd and Mg-4Nd-8Zn. The dissolution of intermetallic at grain boundaries following heat treatment further indicated the importance of grain boundary reinforcement as shown in both in situ and ex situ compression testing. Azimuthal angle-time plots indicated little grain rotation most noticeably in Mg-4Nd, which also indicated the influence of a strong intermetallic network along the grain boundaries

The Effect of Zn Content on the Mechanical Properties of Mg-4Nd-xZn Alloys (x = 0, 3, 5 and 8 wt.%)

The mechanical properties of as-cast Mg-4Nd-xZn (x = 0, 3, 5 or 8 wt.%) alloys were investigated both in situ and ex situ in as-cast and solution-treated conditions. The additions of 3 or 5 wt.% Zn in the base Mg-4Nd alloy did not improve yield strength in comparison to the binary Mg-4Nd alloy. Mechanical properties were shown to improve only with the relatively high concentration of 8 wt.% Zn to Mg-4Nd. The change in intermetallic morphology from a continuous intermetallic to a lamella-like intermetallic was the primary reason for the decreased mechanical properties in Mg-4Nd-3Zn and Mg-4Nd-5Zn compared with Mg-4Nd and Mg-4Nd-8Zn. The dissolution of intermetallic at grain boundaries following heat treatment further indicated the importance of grain boundary reinforcement as shown in both in situ and ex situ compression testing. Azimuthal angle-time plots indicated little grain rotation most noticeably in Mg-4Nd, which also indicated the influence of a strong intermetallic network along the grain boundaries

Intermetallic Phase Characteristics in the Mg–Nd–Zn System

Neodymium, a Rare Earth with low solid solubility in Mg is an ideal alloying element to improve the yield strength and creep resistance cost effectively. The addition of Zn achieves a further improvement; however, its influence on the intermetallic phases in the Mg–Nd–Zn ternary system is not yet fully understood. A Mg-$_5$Nd alloy modified with 3, 5 and 7 wt% of Zn was investigated with in situ synchrotron radiation diffraction during cooling from the molten state to 200 °C in order to investigate the phase-formation and -transformation characteristics of the alloys. The synchrotron diffraction results have been complemented with TEM investigations on the as-solidified samples. The results suggest that Zn has a strong effect on the microstructure by stabilizing the Mg3Nd phase and accelerating the precipitation formation. The experimental results do not fully comply with the theoretical calculations, indicating the necessity of improving the thermodynamic databank for this alloy system

In Situ Synchrotron Diffraction Analysis of Zn Additions on the Compression Properties of NK30

In situ synchrotron radiation diraction was performed during the compression of as-castMg–3Nd–Zn alloys with dierent amounts (0, 0.5, 1, and 2 wt %) of Zn addition at room temperature.During the tests, the acoustic emission signals of the samples were recorded. The results show thatthe addition of Zn decreased the strength of the alloys but, at the same time, increased their ductility.In the earlier stages of deformation, twin formation and basal slip were the dominant deformationmechanisms. The twins tended to grow during the entire compression stage; however, the formationof new twins dominated only at the beginning of the plastic deformation. In order to accommodatethe strain levels, the alloys containing Zn underwent nonbasal slip in the later stages of deformation.This can be attributed to the presence of precipitates containing Zn in the microstructure, inhibitingtwin growth

Effect of the Zn Content on the Compression Behaviour of Mg5\mathrm{Mg_{5}}Nd(Zn): An In Situ Synchrotron Radiation Diffraction Study

The properties of commercially viable Mg alloys are not sufficient for many of the envisaged applications. The combination of Zn and rare earth metals is one of the most effective ways to enhance the mechanical properties of Mg alloys. In situsynchrotron radiation diffraction is a unique method to investigate the dynamic microstructural processes occurring during deformation. Azimuthal angle–time plots give information on grain structure changes that can be correlated with grain rotation, twinning, recovery and recrystallization. As-cast Mg$_5$Nd, Mg$_5$Nd$_3$Zn, Mg$_5$Nd$_5$Zn and Mg$_5$Nd$_7$Zn alloys were investigated during compression at room temperature, at 200 °C and at 350 °C with a strain rate of 10$^{−3}$ s$^{−1}$ until 10% deformation. The results and post mortem metallography were compared. At high temperatures grain rotation and sub-grain formation are active to obtain the final texture, while at room temperature twinning is the dominant deformation mechanism