问题 问答题

何为晶粒生长与二次再结晶简述造成二次再结晶的原因和防止二次再结晶的方法。

答案

参考答案:

解析:晶粒生长是无应变的材料在热处理时,平均晶粒尺寸在不改变其分布的情况下,连续增大的过程。在坯体内晶粒尺寸均匀地生长,晶粒生长时气孔都维持在晶界上或晶界交汇处。二次再结晶是少数巨大晶粒在细晶消耗时的一种异常长大过程,是个别晶粒的异常生长。二次再结晶时气孔被包裹到晶粒内部,二次再结晶还与原料粒径有关。 造成二次再结晶的原因:原料粒径不均匀,烧结温度偏高,烧结速率太快。防止二次再结晶的方法:控制烧结温度、烧结时间,控制原料粒径的均匀性,引入烧结添加剂。

阅读理解

It is often said that man has become the enemy of our planet. This is no exaggeration(夸张), for reports show that man’s greed has done much to destroy the earth. Man, in his greedy desire for financial growth, has polluted the air, land and water, and has robbed our valuable natural resources. Man’s industrial plants pour out poisonous waste that pollutes the sea and puts life in the sea under the threat of dying out; at the same time, they release alarming amounts of carbon dioxide, chlorofluorocarbons (CFCs) and other pollutants. Small wonder that the air now is much warmer and more impure than ever before.

Of course, this is not just because of man’s greed—it is man’s pressing need as well. For instance, in developing countries, huge financial foreign debts have forced governments to approve the cutting of forests for agriculture or ranching. As a result, forests are cut down just to meet man’s immediate needs.

Perhaps unknowingly,man has changed the make-up of the earth's atmosphere. Scientific studies have shown that CFCs used in refrigerators and industrial cleaners are fast destroying the ozone layer—a protective layer in the atmosphere that protects us against the harmful effects of the sun’s ultraviolet (紫外线)rays. In fact, documented reports have shown a terrible fact that there are “holes” in the ozone layer over the Antarctic. And they appear to be expanding.

Faced with these environmental problems, world leaders have shown deep concern. The UN Environmental Programme (UNEP) has arranged international agreements like the Vienna convention of 1985 and the Montreal Protocol of 1989 to preserve the ozone layer by controlling the production, use and trade of destructive chemicals.

There may be some doubt as to how effectual these agreements may be, given that some countries still maintain an “I-don’t-care” attitude. Their attitude is probably due to their failure to grasp the importance of the problem. Then, there is no doubt that environmental education on an international scale(规模)is greatly needed, so that all countries, great and small—the “haves” and the “have nots”—may realize their responsibilities for our planet.

In the meantime, the UNEP seems to have taken steps in the right direction, and it is hoped that in the not so distant future, all nations of the world will join hands in saving mother earth.

小题1:Which of the following is a reason for the cutting and burning of forests?

A.To build houses for more population.

B.To have land for agriculture.

C.To export trees to other countries.

D.To build industrial plants.小题2:The holes in the ozone layer are largely due to________.

A.man’s research activities

B.the greenhouse effect

C.the ultraviolent

D.harmful chemicals小题3:Why don’t some countries care about the problem of the environment?

A.They don’t want to put their lives in danger.

B.They just don’t know how to solve the problem.

C.They haven’t realized the seriousness of the problem.

D.They don’t want to break their international commitment.小题4:66. What is the author’s attitude towards the future of the environment?

A.Hopeful.

B.Unconcerned

C.Doubtful

D.Objective

单项选择题