許多學者利用實驗方式探討形狀記憶合金擬彈性效應，藉由循環加載不同熱處理或不同鎳含量之形狀記憶合金，但對於加載次數增加與相變應力的影響結論不盡相同，對於造成相異結果的機制有許多推想，然而實驗中對微結構的變化觀察並不容易，因此本研究藉由分子動力學模擬來探討此現象，藉由循環加載鎳鈦記憶合金，觀察結構缺陷與相變應力的關係，解釋擬彈性行為。
分子模擬的形狀記憶合金模型，採用鎳原子與鈦原子各為50%的體心立方單位晶格，晶格常數為3.01Å，沿著晶格方向[1-10]、[110]、[001]分別疊加20、20、30次，組成原子總數為48000顆的模型。為了達到鎳含量不同的組成比例，隨機將50%模型中的鈦原子以鎳原子取代，使鎳含量分別為50.5%、51%和51.5%，首先探討不同鎳含量對於相變溫度的影響。勢能函數採用Finnis-Sinclair勢能，施加三方向的週期性邊界，模擬無限大的鎳鈦合金塊材，先在高溫500K進行平衡，設定時間步階為1fs，以NPT系綜平衡20ps，且不拘束其盒子大小及角度，使模擬中週期性盒子可以自由調整長度及角度使平均壓力為零，接著進行升降溫模擬，將高溫平衡後模型從500K降溫至10K，再從10K升溫至500K，以10K為一個區間，在每個溫度區間皆為NPT系綜，且不拘束盒子大小及角度平衡20ps，升降溫速率為0.5K/ps，紀錄模擬取最後2ps的各項數據取其平均值，為該溫度下的平衡值，觀察結構變化並定出相變溫度。在循環加載模擬中，在溫度300K下，固定週期性盒子的角度維持90度進行NPT系綜平衡20ps，達到平衡，再對平衡後的原子模型逐步施加單軸壓縮應力負載，採用NPT系綜平衡，固定週期性盒子之角度及非壓縮方向之壓力為零，每次加載的模擬時間為20ps，取最後的原子平衡的5ps的應變值與應力值平均，記錄其應力應變關係，在以相同方式逐步釋放應力觀察模型的形狀。本研究欲探討循環壓縮加載對於擬彈性效應的影響，因此將釋放負載後的模型再重複以上的加、卸載模擬步驟，即可得到循環壓縮模擬的結果。為了觀察塑性變形產生時原子內部位置的變化情況與形狀回復的關係，因此使用滑移向量法進行原子觀察，當滑移向量長度達0.8Å以上即為發生差排，並結合W參數分析法進行相變的觀察，比較不同比例及型態之缺陷模型的相變與差排行為。
由升降溫模擬結果得知，鎳鈦合金在升降溫過程中原子體積變化符合熱脹冷縮的現象，由W參數法進一步觀察原子變化得知，鎳鈦合金晶相從高溫時的沃斯田體相轉變為麻田散體相，即發生相變行為。也觀察到隨著鎳含量的組成比例上升，相變溫度有下降的趨勢，在50%鎳含量時沃斯田體相變與麻田散體相變的溫度區間不同，產生相變溫度的遲滯現象，而此現象到51%鎳含量時兩相變溫度區間就會相同。從循環加載的模擬結果得知，若加載過程中無產生差排，則會使原子能量與相變應力下降的現象；若加載過程中生成差排，會使相變時原子較難穿越差排層，導致相變應力提升。在不同缺陷模型的分析中，觀察到模型中的缺陷會產生應力集中的現象，導致相變提前發生，相變應力會隨著缺陷比例的上升而下降的趨勢，也發現隨機點缺陷對於相變應力的影響，會比中心球型缺陷還要劇烈的現象。

關鍵詞：分子動力學、相轉變行為、形狀記憶效應、擬彈性效應、點缺陷及球型缺陷結構、W參數分析法

Aluminum alloys have received broad attention in the automotive community due to its superior lightweight and high-strength properties. With appropriate aging treatments, the formation of secondary phase precipitation impedes dislocation movement and contributes to strength enhancement. The secondary phase precipitation can also induce geometrically necessary dislocation (GND) in the vicinity of precipitation-matrix interface due to deformation compatibility. In this work, we develop a yield stress and work hardening model comprising precipitation, solid solution, and dislocation as physical mechanisms in a crystal plasticity numerical framework. Crystal plasticity (CP) fast Fourier transform model is implemented in an open-source software DAMASK to simulate mechanical response of AA6111 and AA7075 precipitation hardened aluminum. Representative volume elements (RVE) are reconstructed from experimental electron backscatter diffraction (EBSD) data, and the characteristics of precipitation are obtained from transmission electron microscopy (TEM) to capture the precipitation strengthening response. The parameters in materials constitutive models are calibrated automatically via a genetic algorithm to their optimal values. A precipitation kinetics model is incorporated into the CP model to capture the growth of precipitations during deformation. Under various artificial aging conditions for both AA6111 and AA7075 alloys, experimental stress-strain curves are shown to be captured by the CP model with a good agreement. Also, the ability to capture the material response in high temperature deformation is improved with the consideration of precipitation kinetics.

Different from traditional alloys, high-entropy alloys (HEAs) have their unique characteristics by mixing equimolar or near-equimolar multiple principal elements into alloys. Due to these special characteristics, such as high strength, fracture toughness, corrosion resistance, and so on, HEAs have become a hot research issue in recent years. Among many HEAs, CoCrFeMnNi, also known as Cantor alloy, has been studied for many years owing to their excellent mechanical properties. In this study, we implemented a dislocation-based crystal plasticity finite element method (CPFEM) to investigate the strengthening mechanisms of CoCrFeMnNi alloys which mainly result from the precipitates caused by different annealing temperatures. To achieve this goal, we use Dream 3D to construct a polycrystalline representative volume element (RVE) model. Material parameters of CoCrFeMnNi alloys are obtained from experimental data through the curve fitting while the precipitates are analyzed using the SEM-BSE images with the help of the open source Image J. Numerical results demonstrate that the proposed model successfully reflected the influences of precipitates on the work hardening behaviors of the stress-strain curve through the growth of dislocation densities. In addition, reducing the annealing temperature can promote the growth of dislocation density and improving the mechanical properties of the CoCrFeMnNi alloys. Finally, this study provides a method to predict model parameters under different annealing temperatures using regression equations. Based on these predicted model parameters, we can further predict the mechanical behaviors of CoCrFeMnNi alloys at different annealing temperatures.

High entropy alloys (HEAs) have attracted extensive attention due to the complex formation from more than five metal elements and their extraordinary performances, such as high wear resistance, high strength, and high ductility in extreme working temperatures. However, the mechanical properties of HEAs may considerably be changed under different loading conditions and environments. As a result, an accurate simulation method which can be more effective to explore the mechanical behavior of HEAs under different loading conditions and environments is inevitable. In this study, we studied the intergranular creep fracture of FeCoNiCrMn, by exploiting micromechanical modeling. We considered crystal plasticity and the evolution of dislocation density for grain interior and a cohesive zone model for grain boundary sliding and separation. We developed an integrated computational framework to simulate the mechanical properties of FeCoNiCrMn. This framework was constructed by implementing UMAT for grain interiors and UEL for grain boundaries in ABAQUS. We first performed a parametrical study to examine the robustness of the model. The simulation results agree well with the experimental measurements. Furthermore, we discussed the effects of strain rate, grain boundary separation, and grain boundary viscosity. We found that the stress concentration occurs at the grain boundaries with different grain orientation, making the grain boundary prone to separate from alleviating this severe strain inhomogeneous. Finally, the competition mechanisms of creep fracture for FeCoNiCrMn were studied. The simulation results suggested that the creep fracture is dominated by grain boundary separation, consistent with the experimental observation. Thus, our model can be applied to simulate grain interior and grain boundary mechanical behavior at high temperatures for different high entropy alloys with great potential for industrial applications. It can be used to study the design of automobiles, aerospace, and high-performance alloy.