Dear Luqing Cui,

Grain boundary hardness measurement is not easy.

First of all, you need to etch to distinguish the grain boundaries, which are more corrosive and are located at a lower level than the intra-granule.

For measurement, I recommend using nanoindenter and AFM.

The nanoindenter will have models that can be measured with indentations of several microns or less.

And AFM is a method of measuring with a smaller indentation.

Search for https://parksystems.com/ and you will find the model you are looking for.

Wish your good luck,

KH Kim

KangHyung Kim Thanks for your valuable advice. I have done some nanoindentation on the AM alloy under different peak stresses (3, 5, 8, 10, 15 and 20 mN) and calculated the circumcircle diameters of residual indentation (1.84, 2.27, 2.91, 3.39, 4.45, 5.23 μm). I have also etched the sample, and it shows that the grain boundaries region is only about 0.5 μm. Therefore, it is difficult to accurately measure the hardness of the grain boundary. However, the grain boundary will definitely affect the measurement of nano-hardness. So I think that the average value of nano-hardness within a certain distance from the grain boundary can be regarded as the hardness of the grain boundary, which can be used to distinguish it from the hardness of the grain. It is also believed that the existence of grain boundaries can affect the shape of the P-h curve (Dislocations or plastic deformation can be transmitted from one side of the grain boundary to the other).

Luqing Cui

If the indenter tip has spherical shape, I suggest to try with the triangular shape tip.

That could be better then spherical tip for narrow boundary, and with smaller load.

regards,

KH Kim

If you look at a grain boundary with a thin foil in the TEM, you will see that it is extremely thin, only a couple of atoms wide. When you etch the specimen to find the grain boundaries, the GB and some surrounding region is attacked and the surface is not flat. Nano-indentation hardness testing is a very difficult method to use and get correct data. It is extremely sensitive to proper specimen preparation and most work simply detects residual deformation. This is an extremely difficult specimen to get valid data.

George Frederic Vander Voort Thanks for your advice. I performed the nano hardness on a well vibration polished samples, not on the etched samples. The relationship between hardness and distance form grain boundaries is shown in Fig. 1. The result is the statistical calculations from 320 nanoindentations, so the results should be reliable. With the increase of distance from GBs, the hardness gradually decreases. Therefore, grain boundaries definitely affect nano-hardness.

For comparison, the average value of nano-hardness within 1 um from the grain boundary can be regarded as the hardness of the grain boundary, which can be used to distinguish it from the hardness of the grain interior (Distance from GBs above1 um), as shown in Fig. 2. Is this available or correct? Looking for your response.

Best Regards,

Luqing Cui

Luqing:

Interesting! I do not understand why you have two plots, one under the other. The x-axis needs to be defined. Exactly where is the grain boundary in the graphs? What was the grade of steel used? Was it heat treated? What was the microstructure and yield strength and tensile strength?

Keep up your research work!

George

George Frederic Vander Voort X-axis is the perpendicular distance from the center of the indent to the grain boundary line, as shown in Fig. 3b. Fig. 3a is the illustration of nanoindentation locations and the black lines in Fig. 3a is grain boundaries. Fig. 1 in my last answer is the plot of distance from grain boundary versus the nanoindentation hardness. In Fig. 2, the higher hardness is the statistical calculations from the nanoindentations within 1 μm from grain boundaries; the lower hardness is the statistical result from the nanoindentations above 1 μm from grain boundaries. Grain boundaries in Fig. 1 and Fig. 2 are located at x=0.

The experimental alloy is as-built SLM 316L stainless steel and was not heat treated.

The microstructures of SLM 316L are shown in Fig. 4, which consists of cell structure with average size of 500 nm. Some textures were formed during the building process with BD//[110], as shown in Fig. 5.

The yield strength and tensile strengths are 583 and 598 MPa at room temperature under strain rate of 0.001.

Looking for your response.

Luqing Cui

Dear Luqing,

Is it the hardness for Y axis?

Or modulus?

I guess the level too high as hardness of 316.

KH Kim

KangHyung Kim I have checked the results of my experiment. Y-axis is hardness. The nanoindentation hardness tests were carried out at room temperature on the UMIS Nanoindentation system with a Berkovich tip. A constant loading rate of 200 μN/s up to a maximum load of 15 mN was used. Hold time at the peak loading is 10s. The result is the statistical calculations from more than 300 nanoindentations.

Best Regard,

Luqing Cui

Thank you, Luqing.

I didn't know the nanoindentation hardness of 316 stainless steel until your comment.

That is reasonable value in nanoindentation by other papers.

Maybe the hardness is almost perfect value without influence of the materials defects such as vacancy, dislocation, impurity etc. in nanoindentation.

Almost defects are move to the nearest grain boundary by energy such as thermal, dynamical driving force.

In metallurgy, the whisker metal fiber has the peculiar strength without the materials defects.

I appreciate your good question.

Best regards,

KH Kim

You should measure the material with a micro-hardness device. Check the trace under the microscope and define depth and symmetry