Công nghệ cơ khí thường tạo ra các giả lập mô phỏng hoạt động của các đối tượng, như quy trình chế tạo thực tế theo trình tự tối ưu hóa sự thực hiện, hiệu quả kinh tế và chi phí năng lượng trước khi quyết định lựa chọn một thiết kế cụ thể. Các bản vẽ kỹ thuật để chế tạo là sản phẩm cuối cùng của khâu thiết kế. Chúng phải thỏa mãn hai mục đích: bao gồm đầy đủ tất cả các thông tin cần thiết để chế tạo và cũng còn là một tiêu chí kiểm soát. | 120 The Coining of Materials Science Figure . A growth spiral on a silicon carbide crystal originating from the point of emergence of a screw dislocation courtesy Prof. s. Amelinckx . are found for instance. ABCACBCABACABCB for this 15R structure the repeat height must be five times larger than for an ABC sequence. Such polytypes can have 33 or even more single layers before the sequence repeats. Verma was eventually able to show that in all polytypes spiral step height matched the height of the expanded unit cell and later he did the same for other polytypic crystals such as Cdl2 and Pbl2. The details can be found in an early book Verma 1953 and in the aforementioned autobiographical memoir. Like all the innovations outlined here polytypism has been the subject of burgeoning research once growth spirals had been detected one recent study related to polytypic phase transformations dislocation mechanisms have been detected that can transform one polytype into another Pirouz and Yang 1992 . The varying stacking sequences when they are found irregularly rather than reproducibly are called stacking faults these are one of several forms of twodimensional crystal defects and are commonly found in metals such as cobalt where there are two structures cubic and hexagonal close-packed which differ very little in free energy. Such stacking faults are also found as part of the configuration of edge dislocations in such metals single dislocations can split up into partial dislocations Precursors of Materials Science 121 Figure . Projection of silicon carbide on the 0 0 0 1 plane after Verma 1953 . separated by stacking faults and this splitting has substantial effects on mechanical behaviour. William Shockley with his collaborator . Heidenreich was responsible for this discovery in 1948 just after he had helped to create the first transistor. Stacking faults and sometimes proper polytypism are found in many inorganic compounds - to pick out just a few zinc sulphide .
