什么叫热裂纹？它有什么特征？
参考答案：
习惯上所称呼的热裂纹，一般指焊缝金属，由液态冷却到固相线左右出现的结晶裂纹，其主要的形成原理是随着结晶面的延伸，低熔杂质富集到结晶对称中心位置，造成低熔共晶的富集，冷却收缩而得不到液体金属的补充时，在拉力的作用下，被分隔的自由表面上形成了裂纹，但确切的热裂纹的含意，应包括结晶裂纹，液化裂纹和高温低塑性裂纹，后两种裂纹都发生在热影响区，由于母材受焊接热循环的过热影响，使奥氏体晶粒粗化，并导致出现晶界液膜或晶界弱化，在收缩应力作用下产生了裂纹。
DIY is very poplar in China now. Do you know what it is? Yang Chen works ________ a computer business in Beijing. Though he enjoys his work, it is hard for him ______after work. “ for a long time, I had no choice except going to a bar for a drink and karaoke to relax myself,” he said. One day ________ he saw many toys in a toy shop, he had an idea. “ why _______ some kind of things and relax myself?” Then he began to ________ model planes, model ships and something else. The things he has made by himself don’t look as professionally(专业的) made as those in the shop, ________ he likes doing it. “ Maybe it is not as much fun ______ as things which are bought expensively from the shops,” he said. Now his bookshelves are full of many kinds of things he has made. They are very beautiful and his friends _______ them very much. More and more people are like Yang, making or creating(创造) things of their own. They like to do it __________. Though making things with our own hands may _______ much time, some people enjoy doing it, now known as DIY-do-it-yourself, very much. They can make all kinds of things they like. DIY is becoming more and more popular in China.
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