Another real, live materials scientist/engineer on this thread, cool.
DBTT, ductile-brittle transition temperature...this is bringing back memories from my undergrad classes.
For others on this thread, the sudden-onset ductile-brittle transition is an interesting property of some metals/alloys. Essentially, the material undergoes a sudden transition from ductile to brittle at a certain temperature. This change from ductile to brittle also means there is a sudden, large drop in the toughness of the material.
It was largely an unknown phenomenon up until about the 1940s. It was actually a major problem with the so-called "Liberty ships" that sailed in the northern Atlantic Ocean during WWII. The metal alloys behaved perfectly at ambient temperatures when tested during construction, but would undergo a sudden, substantial transition from ductile to brittle at a certain temperature, the DBTT. The problem in those days was that the DBTT was higher than the temperatures of north-Atlantic waters, so these ships suddenly became very brittle when they sailed out into cold water, leading to catastrophic failures. This is one of the issues that lead to much of the study of metallurgy (and later materials science) as we know it today.
Nowadays, we know and understand a lot more about the behavior of materials, so to a competent engineer, things like service temperature are well accounted for.
Folks knew that embrittlement was an issue back in the 1800's but it was only in the 20th century that people started figuring out what was causing it and how to address it.
Titanium alloys are also susceptible to this.