Hangbo Alloy Group
Shanghai · Est. 2012
How sigma phase and other intermetallics embrittle nickel alloys — where they form, how to avoid them in welding, and which grades are most at risk.
Few failure mechanisms surprise engineers as much as sigma phase. A component can look perfectly sound, pass pressure test, and then fail by brittle fracture after months at temperature. Sigma is a hard, brittle intermetallic that precipitates silently from the ferrite or from certain austenitic chemistries, and it is worth understanding because it is largely preventable through material selection and weld procedure.
Sigma phase is a hard, brittle intermetallic compound, roughly FeCr in stoichiometry, that forms in the 550 to 900°C range. In a nickel alloy it nucleates preferentially from ferrite or from regions enriched in chromium and molybdenum. A small amount — sometimes a few percent by volume — is enough to turn a ductile, tough component into one that cracks under impact or bending.
The danger is not just the phase itself but what it leaves behind. As sigma grows it consumes chromium and molybdenum from the surrounding matrix, so the adjacent austenite becomes leaner and less corrosion-resistant even before the mechanical embrittlement is obvious. In service this shows up as unexpected brittle fracture or, in aggressive media, as localized attack along previously sound metal.
High chromium and high molybdenum are the ingredients that make sigma likely, which means the most corrosion-resistant superalloys are also the most at risk. Duplex microstructures are especially vulnerable because the ferrite phase transforms rapidly to sigma on prolonged holding. Time at temperature is the lever: a weld that cools through the danger zone quickly may avoid sigma entirely, while a vessel held at 600 to 800°C for thousands of hours will accumulate it.
Weld metal and heat-affected zones are common nucleation sites because compositional segregation concentrates chromium and molybdenum there. Multi-pass welds that repeatedly reheat earlier passes can accumulate more sigma than a single controlled pass, so procedure and interpass temperature matter as much as base chemistry.
Sigma is visible under the microscope as blocky, angular precipitates, and it is confirmed by metallography or X-ray diffraction rather than by hardness alone. Avoidance starts with selection: when long service above roughly 550°C is unavoidable, favor leaner, single-phase austenitic grades and keep molybdenum in check where the application allows.
For welding, control heat input and interpass temperature so the joint spends minimal time in the precipitation window, and avoid over-alloyed fillers that add chromium and molybdenum to an already-rich puddle. Where sigma has formed, a full solution anneal above about 1040°C can redissolve it, but the part must then be cooled fast enough not to re-precipitate.
Sigma is the headline, but it is not alone. Mu phase (a molybdenum-rich intermetallic) embrittles high-molybdenum superalloys in roughly the same temperature band. Laves phase forms in some weld metals and reduces both ductility and corrosion resistance. Continuous grain-boundary carbide networks, while not intermetallics, produce a similar loss of toughness and a path for intergranular attack.
The practical takeaway is consistent across all of them: keep time at 550 to 900°C short, avoid chemistries that over-enrich the matrix, and verify with metallography when the service is critical and the grade is highly alloyed.
The most corrosion-resistant, high-chromium and high-molybdenum grades carry the highest risk — including the C-family Hastelloys and similar super-austenitics. The corrosion case for those grades is laid out in our Hastelloy C22 corrosion resistance article, which is also why weld procedure discipline matters so much with them.
Yes, in most cases. A full solution anneal above roughly 1040°C dissolves the phase back into the matrix, after which rapid cooling prevents re-precipitation. The same furnace discipline used for other high-temperature alloys is described in our furnace fixture alloy selection guide.
Yes. As the phase grows it strips chromium and molybdenum from the surrounding matrix, leaving leaner, more attack-prone metal. This compounds the mechanical embrittlement. The link between alloying and pitting resistance is detailed in our PREN and pitting resistance article.
As a dedicated nickel alloy manufacturer serving chemical processing, power generation, oil & gas sectors, Hangbo Alloy Group supplies the grades discussed here across all standard product forms with full ASTM/ASME documentation and EN 10204 3.1 certification. Our material is specified for applications where corrosion resistance, high-temperature strength, and traceable quality are non-negotiable. See our production & testing facilities for more on how every heat is verified.