Magnesium (Mg) is recognized as the lightest structural metal, making it highly desirable in applications where weight reduction is paramount. Material hardness is a measure of a substance’s resistance to permanent deformation, indentation, or scratching. Understanding the hardness of magnesium on various scales provides engineers and manufacturers with the necessary data to predict its performance. This property directly impacts how magnesium parts will withstand wear, abrasion, and localized pressure during their service life.
Quantifying Magnesium’s Hardness
The intrinsic hardness of pure magnesium is relatively low when measured against standard industry scales, reflecting its soft nature as an unalloyed metal. Three scales are commonly used for metals: Mohs, Brinell, and Vickers. The Mohs scale, which measures scratch resistance, places pure magnesium at approximately 2.5. This low value means that magnesium can be scratched easily, even by a copper coin.
For industrial applications, the Brinell and Vickers tests provide more precise measurements of a material’s resistance to indentation under a known load. Pure, cast magnesium typically exhibits a Brinell Hardness (HB) value around 44. This measurement confirms that pure magnesium is quite malleable and easily deformed compared to other common structural metals.
The Vickers Hardness (HV) test often yields values that correspond closely to Brinell measurements for softer materials. The hardness of pure magnesium is generally in the range of 25 to 30 HV. These low numbers are a direct consequence of magnesium’s hexagonal close-packed (HCP) crystal structure, which limits the number of slip systems available for plastic deformation. Consequently, pure magnesium is rarely used in high-stress structural components.
How Alloying Changes Hardness
The magnesium used in most engineering applications is not the pure metal, but a magnesium alloy, which significantly modifies its mechanical properties, including hardness. Alloying introduces other elements to the magnesium matrix to create a stronger, more complex microstructure. Common additions like aluminum, zinc, and rare earth elements are used to increase hardness and mechanical strength.
A widely used alloy, AZ91 (containing aluminum and zinc), demonstrates this substantial increase in hardness. While pure magnesium is around 44 HB, the as-cast AZ91 alloy typically shows a Brinell Hardness ranging from 60 HB to 70 HB. This improvement is often attributed to the formation of the intermetallic compound Mg17Al12 within the microstructure. These hard precipitates impede the movement of dislocations in the crystal lattice, thereby increasing the material’s resistance to indentation.
Another common alloy, ZK60 (using zinc and zirconium), also shows improved properties, often reaching Brinell Hardness values of approximately 64 HB in the as-cast state. Processing techniques such as heat treatment can further modify the hardness of these alloys. For example, a T6 heat treatment on ZK60 can push its Brinell Hardness to around 70 HB by controlling the size and distribution of strengthening precipitates. Cold working is another technique that introduces defects, making the material harder and less prone to deformation.
Magnesium Compared to Common Metals
Placing magnesium’s hardness into context requires comparison with other frequently used structural metals, using a consistent metric like the Brinell Hardness (HB) scale. Even in its alloyed form, magnesium is noticeably softer than many competing engineering materials. A common magnesium alloy like AZ91 (64 HB) is softer than a standard aluminum alloy, such as 6061-T6, which measures around 95 HB.
The difference becomes much more pronounced when comparing magnesium to ferrous and other high-performance metals. Mild carbon steel typically has a Brinell Hardness range starting around 120 HB and can easily exceed 200 HB depending on its composition and heat treatment. Similarly, unalloyed titanium starts at around 70 HB, but common titanium alloys can reach hardness values exceeding 300 HB.
Magnesium’s lower absolute hardness means it is more susceptible to wear and surface damage than steel or harder aluminum alloys. However, its lower density is its defining characteristic, making it approximately one-third lighter than aluminum and one-quarter the weight of steel. This low density, combined with its respectable hardness in alloyed forms, gives magnesium alloys an advantageous strength-to-weight ratio for applications in aerospace and automotive industries.