strathSEEA

Crystal Lattice Expansion

 

One method by which a gauge of the amount of diffused carbon within the sample can be calculated is to measure the amount by which the crystal lattice has expanded. The greater the interstitial supersaturation of the carbon, the more the lattice will expand. The hardness and roughness of the material can also increase with an increased amount carbon. This can be verified by comparisons with the RST and micro-hardness results.

 

During the investigations, XRD analysis was performed on both the polished and unpolished sides of both the treated and untreated samples. This allowed for conclusions to be drawn regarding;

 

• Any changes in the microstructure due to the sample preparation (polishing process)

• The expansion of the lattice due to carburisation

• Comparisons of the effectiveness between the K22 and K33 carburisation treatments

 

The XRD lattice expansion analysis can be further divided into four investigations:

 

• Untreated Polished Vs. Untreated Unpolished Expansion

• Treated Polished Vs. Treated Unpolished Expansion

• Treated Polished Vs. Untreated Polished Expansion

• Treated Unpolished Vs. Untreated Unpolished Expansion

 

Clicking on the red link will take you to the relevant analysis, or a summary can be found by pressing the adjacent summary button.

 

XRD Output Spectrum

 

The lattice expansion can be calculated by noting the theta angle on the x-axis of the three main peaks marked by the red lines in Figure 11 below.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Note that the values at the peaks are the intensity in terms of counts. 

 

The 3 main observed peaks are representative of the austenite phase present, with the first, second and third peaks representing the {[111]}, [{200]} and [{220]} lattice planes respectively.

By taking the change in the positions of the peaks, the magnitude of the lattice expansion can be calculated, where a larger shift equals a larger expansion. An example of the relative peaks shifts post and prior to carburisation on both the polished and unpolished sides of a sample is shown in Figure 12.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

XRD Phase Identification

 

 

A secondary use of the XRD analysis is the ability to identify different phases and/or compounds within the microstructure. Although it was known that the primary phase was made up of austenite due to thehaving an FCC structure, the introduction of carbon to the lattice could also cause the formation of carbides. These can be identified by matching the positions of newly formed peaks on the x-axis to that of previously known carbides from the data base. An example of an additional compound, represented by a newly formed peak, post carburisation is highlighted by the red circle below in Figure 25. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Systermatic Machine Error

 

It was noted that in the majority of the samples, a peak of relatively small intensity was noted at a position of approximately 1° - 2° higher than the 1st austenite peak ({111} plane). This is highlighted by the blue circle in Figure 25. To confirm the presence of this peak, an XRD analysis was carried out on a previously analysed sample; Stainless Steel 904L. It was known that this additional peak should not occur in the XRD patternevaluation. However the results again revealed the presence of the shoulder on the first austenite peak. This lead to the conclusion that there was a systematic error within the machine. As there was no time available to recalibrate the machine and re-run all of the experiments, the results were still used, but the peak was to be considered an anomaly and excluded from the evaluation.

 

Full analysis of the phase constituents proocess can be found by pressing the button below: