Modeling the Dislocation-Precipitate Interaction in High Temperature Materials

There is an increasing demand to improve the efficiency of thermal power plants due to the growing energy demand and inevitable requirement of reducing the CO₂ level. One of the most effective way to improve the efficiency is by increasing the steam temperature and pressure. In India, the maximum steam temperature at which power plants are operated is in the range of 575-600 ⁰C. Thus pushing the steam temperature to beyond 700 ⁰C is targeted to create advanced ultrasupercritical (AUSC) power plants. However at such high temperatures, the material properties become a constraint as it is well known that the property of a material deteriorates as we advance towards higher temperature. Hence there is an increasing need to develop heat resistant materials which can operate at temperatures in the excess of 700 ⁰C. Some of the components used in power plant applications is shown in Fig. 1.

Fig. 1 : Power plant components requiring heat resistant materials

The material properties which are needed for high temperature applications are good creep and fatigue resistance, high fracture toughness and good oxidation resistance. Presently the 9-12 % Cr steel is used in most of the power plant components as these have excellent creep resistance up to a temperature of 600 ⁰C. However beyond these temperatures, the properties fall off and hence austenitic stainless steel and Ni based superalloys are being proposed for use at higher temperatures. The operating temperature of different classes of materials for power plant applications is shown in Fig. 2.

Work done so far and work plan

For high temperature applications, austenitic stainless steel and Ni based superalloys are being considered to provide good high temperature strength and stability. 304HCu austenitic stainless steel is one of the candidate materials for superheater and reheater tubes in AUSC power plants. Excellent high temperature strength in 304HCu is achieved from the precipitation of nanoscale Cu rich phase and Nb(C,N) phase in the austenite matrix during service at high temperature. Cu rich precipitates were regarded to provide the most effective strengthening effect in this steel. While a lot of experimental and simulation studies have been devoted to understand the precipitation and dislocation-precipitate interaction of Cu precipitates in BCC Fe, the dislocation-precipitate interaction mechanism due to Cu rich phase in austenitic stainless steels such as 304HCu has not been studied in details.

In the present study, molecular dynamics (MD) simulation has been employed to understand the interaction between an edge dislocation and copper precipitate in austenitic stainless steel (18Cr-9Ni). The effect of precipitate size and temperature on the strengthening has been explored in this work. Shearing of the precipitates by the dislocation was observed at all sizes. The maximum stress associated with dislocation overcoming the precipitate increases with increasing precipitate size. There is critical size beyond which increase in strength due to precipitates is significant at both 300K and 973K. The critical size is associated with a transition in the mechanism controlling the strengthening from trailing partial detachment up to critical size to leading partial detachment beyond that.

Following this, an attempt has been made to understand the contribution of different mechanisms (stacking fault strengthening, interfacial strengthening, coherency strengthening and modulus strengthening) to the overall strength. Stacking fault energy (SFE) of the precipitate was varied and it was observed that the peak stress value decreases with increasing value of stacking fault energy of the precipitate beyond the critical size for both 300 K and 973 K. However, the stress value associated with entry of the dislocation into the precipitate increases with increasing value of SFE. Although SFE affects the overall strength of the system, its contribution was found to be low. Contributions were also found to be minimal for the case of interfacial and coherency strengthening. Hence, modulus strengthening was proposed to be the major contributor to precipitation strengthening in the present system since copper is an elastically softer phase as compared to the austenitic matrix. It is further evidenced by the decreasing strength of the alloy with decreasing value of G_matrix/G_precipitate of the system.

The effect of different cutting planes on the precipitation strengthening was studied since the relative position of the precipitate is arbitrary with respect to the slip plane. It was found that the strengthening due to copper precipitate is asymmetric with respect to the plane passing through the centre of the precipitate due to the interaction between the hydrostatic stress field of the dislocation and the precipitate. Further, the spacing between the precipitates was varied (by changing the simulation box dimension) to comprehend its effect on strength and it was found that the strength is inversely proportional to the spacing between the precipitates in the slip plane. An attempt has been made to explain the results obtained in the present simulation studies with the help of Russell Brown model (line tension model taking into account different modulus of the matrix and the precipitate) using some simple assumptions. The model was used to predict the strengthening contribution of copper rich precipitates in 304HCu alloy.