Duplex Grain Size in Austenitic and Nickel Based Super-alloys

Metals have a crystalline structure. When a metal solidifies from the molten state, a multitude of tiny crystals start to grow. These crystals form the grains in the solid metal.  As a wrought metal is formed, the product goes through repeated cycles of rolling to reduce cross section, and annealing to recrystallize grains.  This not only serves to strengthen the product, but it tends to give the product a very homogeneous, uniform grain.

We pay close attention to grains.  Grains tell us a lot about the product and how it should perform.  Grain size has a measurable effect on most mechanical properties. For example, at room temperature, hardness, yield strength, tensile strength, fatigue strength and impact strength all increase with decreasing grain size.  

Duplex grain structures are sometimes observed, particularly in Ni-base super-alloys.  A duplex grain structure is one in which both relatively coarse and fine grains are present in the same metal matrix, concurrently.  Duplex grain size may occur in some metals and alloys as a result of their thermo-mechanical processing history, often in specimens where recrystalization is incomplete, or at the onset of rapid grain growth.  The duplex nature of the grain size distribution can be highly segregated, in other cases the two distributions are intermixed.  Assigning an average grain size value to a duplex grain size specimen does not adequately characterize the appearance of that specimen, and may even misrepresent its appearance. For example, averaging two distinctly different grain sizes may result in reporting a size that does not actually exist anywhere in the specimen.

Figure A shows a relatively well mixed bimodal distribution of grain sizes.

Figure B, on the other hand, shows an extremely segregated example of a duplex grain size condition. Grain growth has occurred along the surface of this low-carbon steel specimen.

Another segregated form of a duplex condition is the so-called "necklace" type, as shown in Figure C. This is from a highly alloyed stainless steel where fine re-crystallized grains surround the large non-recrystallized grains.

The amounts of the fine and coarse grains can vary considerably. Figure D shows a more equal area percentage of fine and coarse grains in a nickel-base super alloy.

Again, the grain size of such specimens should not be described by a single average value, as it is quite possible that there will not be any actual grains of that size in the specimen.  The best approach is to determine the area percentage of each grain size distribution and the average grain size of each distribution in specimens with a duplex or bimodal grain size distribution.  Test methods for recognizing the presence of duplex grain size are provided in ASTM E118. A common category in the aerospace industry, deals with grades like 718 round bar & forgings.  There, ASTM specs allow for duplex grain size in the metal.  One example might read “grain size of 5 to 6, with occasional grains up to 00”.  This means the majority (typically 80%) of grains are a uniform 5-6, but there may be individual grains, or clumps of grains, with a grain size up to 00, and that is acceptable, provided those are not more than 20% of the total measured grains.  And not all specs are the same; AMS 5662, invokes the 20% rule with respect to the entire product cross section, not an individual grain sample. 

Other terms we may see associated with duplex grain size, as described in ASTM E- 1181, are elaborated upon in the following table from a SNECMA document:

From above, SNECMA defines three classes of grain structure as acceptable, as follows:
class 1: difference in average grain is within 2 grains size numbers = single population structure
class 2: difference within 3 grains size numbers = mixed grain sizes
class 3: difference is 4 or more average grains size numbers = duplex grain structure

Duplex grains are not an immediate reason for rejection of a product, and in many cases, end users and their specifications make allowance for, and openly accept duplex grains.