Phương pháp điều trị hiện tượng như những người được nêu trong Module, năng suất và dòng chảy nhựa (Module 20) là rất hữu ích cho việc dự đoán kỹ thuật, nhưng họ chỉ cung cấp cái nhìn sâu sắc hạn chế các cơ chế phân tử cơ bản năng suất. Sự hiểu biết phân tử là một cấp cao hơn của cái nhìn sâu sắc, và cũng hướng dẫn chế biến điều chỉnh có thể tối ưu hóa các vật liệu. | The Dislocation Basis of Yield and Creep David Roylance Department of Materials Science and Engineering Massachusetts Institute of Technology Cambridge MA 02139 March 22 2001 Introduction Phenomenological treatments such those outlined in the Module on Yield and Plastic Flow Module 20 are very useful for engineering predictions but they provide only limited insight to the molecular mechanisms underlying yield. Molecular understanding is a higher level of insight and also guides processing adjustments that can optimize the material. As discussed in the Module on Atomistics of Elasticity Module 2 the high level of order present in crystalline materials lead to good atomistic models for the stiffness. Early workers naturally sought an atomistic treatment of the yield process as well. This turned out to be a much more subtle problem than might have been anticipated and required hypothesizing a type of crystalline defect the dislocation to explain the experimentally observed results. Dislocation theory permits a valuable intuitive understanding of yielding in crystalline materials and explains how yielding can be controlled by alloying and heat treatment. It is one of the principal triumphs of the last century of materials science. Theoretical yield strength In yield atoms slide tangentially from one equilibrium position to another. The forces required to bring this about are given by the bond energy function which is the anharmonic curve resulting from the balance of attractive and repulsive atomic forces described in Module 2. The force needed to displace the atom from equilibrium is the derivative of the energy function being zero at the equilibrium position see Fig. 1 . As a simplifying assumption let us approximate the force function with a harmonic expression and write . T xA T Tmax sin 2- - a J where a is the interatomic spacing. The stress reaches a maximum a quarter of the distance between the two positions dropping to zero at the metastable position midway .
