It’s important to understand the ductile-to-brittle transition temperatures (DBTT) of metals when considering which materials to use for applications functioning under cryogenic environments. Some more ductile metals at room temperature may become brittle at low temperatures. As a result, they develop fractures, and the structure may fail. For example, the DBTT of the low-carbon steel used in the building of the Titanic may have contributed to the ship’s sinking in an environment that was just -4 °F.
However, is DBTT applicable to all metals, including aluminum 6061? Below, we dive deeper into DBTT and why there actually is no ductile-to-brittle transition temperature of 6061 aluminum.
Understanding the Ductile-to-Brittle Transition Temperature
DBTT refers to the temperature of the material at which it enters into the brittle phase from the ductile phase. At this point, the material can no longer sustain the loading force without developing a fracture.
Pure metals tend to have a definite transition temperature, while in alloys, this transition room temperature is uniquely defined—it may happen over a finite range.
In this regard, the Charpy Impact Test, also known as the Charpy V-notch test, is conducted to determine the toughness of a material. However, surprisingly, pure aluminum does not have a DBTT.
Ductile-to-Brittle Transition Temperature of 6061 Aluminum
Aluminum has a face-centered cubic (FCC) structure. Face-centered cubic materials do not have any ductile-to-brittle transition; they always tend to remain in brittle material condition.
Why? For that, we need to understand the slip system of the material. Metals deform along the most closely packed directions on the most closely packed slip planes, also known as slip systems. The dislocation of a slip happens when a corner atom jumps to the center of the cube.
Let’s take a look at the unit cell of BCC and FCC materials, as shown below.
FCC vs. BCC lattice structure
Both structures have the same number of slip systems (12). However, unlike BCC, FCC has closely packed planes belonging to each slip system.
At high temperatures, both FCC and BCC structures have mobile dislocations, meaning they can go through plastic deformation without undergoing fracture. However, at low temperatures, you need a certain amount of thermal energy to activate dislocations in BCC structures, but dislocations happen in FCC independent of the surrounding temperature. As a result, FCC stays ductile even at low temperatures and displays no DBTT phenomenon.
This is why you can’t find any data regarding the ductile to brittle transition temperature of 6061 aluminum.
Unique Low-temperature Aluminum Applications
Aluminum is one of the more ductile materials while being tough at low temperatures, thanks to the FCC structure of the metal. Such physical properties come as an advantage for offshore installations in the northern hemisphere, which easily reaches -40°F. This makes ductile materials like aluminum quite useful.
5000 and 6000 series aluminum alloys are considered to be the most suitable for cryogenic applications due to their high notch-yield ratio, which is a ratio between notch-tensile strength and tensile-yield strength for a quality control test of material against fracture testing.
However, it’s important to note that not all aluminum alloys behave the same under low temperatures. For instance, the strength of 6061 increases as the temperature is reduced, whereas, for 5456 aluminum, it remains almost constant.
For further clarity, the graph below shows the tensile yield strength at 4K (ksi) force for different aluminum alloys.
Some industries make use of low-temperature fluids to achieve desired working conditions. In this regard, equipment and installations made up of aluminum can offer exceptional strength and high ductility, even at -320.8 °F.
The table below lists different grades of aluminum used over a wide range of cryogenic temperatures.
|Suitable Aluminum Alloys
|Almost all except 7075-T6 and 7178-T6
|2024-T6, 7039-T6, 5456-H343
|2024-T4, 6061-T6, 2219-T87, 5052-H38, 5083-H38
While the absence of a ductile-to-brittle transition temperature of 6061 aluminum is beneficial for industries working with cryogenic applications, its soft nature is a challenge for machine operators. The low melting point of the aluminum alloy can result in gummy build-up around the tool edge.