Understanding the High Creep Resistance of MRI 230D Magnesium Alloy through Nanoindentation and Atom Probe Tomography

Dorothea Matschkal-Amberger,Steffen Lamm,Patrick Tuengerthal,Mathias Göken,Peter Felfer,Heinz Werner Höppel

doi:10.3390/met11111727

Abstract

Due to their low density, magnesium alloys are very appealing for light-weight constructions. However, the use of the most common magnesium alloy, AZ91 (Mg 9 wt.% Al, 1 wt.% Zn), is limited to temperatures below 150 °C due to creep failure. Several alloys with an improved creep resistance have been developed in the past, for example the alloy MRI 230D or Ca-alloyed AZ91 variants. However, there is an ongoing discussion in the literature regarding the mechanisms of the improved creep resistance. One factor claimed to be responsible for the improved creep resistance is the intermetallic phases which form during casting. Another possible explanation is an increased creep resistance due to the formation of precipitates. To gain more insight into the improved creep resistance of MRI 230D, nanoindentation measurements have been performed on the different phases of as-cast, creep-deformed and heat-treated samples of MRI 230D and Ca-alloyed AZ91 variants. These nanoindentation measurements clearly show that the intermetallic phase (IP) of the alloy MRI 230D does not lose strength during creep deformation in contrast to the Ca-alloyed AZ91 variants. High-temperature nanoindentation measurements performed at 200 °C clearly show that the intermetallic phases of the MRI 230D alloy maintain their strength. This is in clear contrast to the Ca-alloyed AZ91 variants, where the IP is significantly softer at 200 °C than at room temperature. Atom probe measurements have been used to gain insight into the differences in terms of chemical composition between the IPs of MRI 230D and the Ca-alloyed AZ91 variants in order to understand the dissimilar behaviour in terms of strength loss with increasing temperature.

Highlights

Since vehicle mass is a key contributor to fuel efficiency, the use of lightweight materials can have a significant impact on their CO2 emissions
The creep strength of the alloys significantly increases with an increase in Ca content, which is in accordance with literature
Nanoindentation measurements revealed that the intermetallic phase (IP) of the alloy MRI 230D is significantly harder at 200 ◦ C than the IP of Ca-alloyed AZ91 variants, while the differences in the hardness between the alpha grains of the respective alloys even at high temperatures are low

Summary

Introduction

Since vehicle mass is a key contributor to fuel efficiency, the use of lightweight materials can have a significant impact on their CO2 emissions. The increased use of magnesium alloys is very appealing, as magnesium is the least dense metal regularly used in structural applications. Mg–Al system and their application is limited to temperatures below approximately 150 ◦ C as a result of the low creep strength of these alloys [1,2,3]. E.g., for automotive powertrain applications, alloys with an increased creep strength for use up to 200 ◦ C under long-term loading conditions are needed. Several successful attempts have been made to increase the creep strength of Mg–Al alloys by adding rare earth elements or alkaline earth elements, see for example [4,5,6,7,8]. There is a lack of understanding and an ongoing debate in the literature on the mechanisms responsible for the increased creep strength

Results

Discussion

Conclusion