问题 问答题

形变后的材料经恒温退火,再结晶结束后继续保温以使晶粒长大。当退火30分钟时测得材料的晶粒直径为23μm,其对应的屈服强度为112MPa;退火60分钟时测得屈服强度为103MPa;求退火90分钟时材料的屈服强度。(设完成再结晶所需要的时间以及再结晶结束时的晶粒尺寸可忽略不计)

答案

参考答案:据晶粒直径与退火时间关系:
d2=kt
得 k=17.6
所以退火时间为60分钟时d=32.5μm
退火时间为90分钟时d=39.8μm
根据Hall-Petch公式:σ=σ0+kd-1/2
得 k=272,σ0=55.3
所以退火90分钟时
σ=98.4MPa

阅读理解

阅读理解。

     Young people and old people do not always agree with each other. They sometimes have different

ideas about living, working and playing. But in one program in New York State, adults and teenagers live

together in a friendly way.

     Each summer 200 teenagers and 50 adults are invited to join the program. They live together for

eight weeks as members of a special work group. Everyone works for seven hours a day. They do so

not only to keep busy, but also to find meaning and happiness in work.

     Some teenagers work in the woods or on the farm near the village. Some of them learn to make things

like tables, chairs and something like that and to build houses. It is the adults who teach them to do those.

     There are a few free hours each day and weekends are free, too. During the free hours some of the

teenagers learn to take photos or paint pictures, others sit around and talk, sing or dance. Each teenager

has his own way to spend his free time.

     When people live together,  rules are necessary. In this program the teenagers and the adults make the

rules together.If someone breaks a rule, the problem goes before the whole group. They talk about it and

ask,"What should we do about it?"

     One of the teenagers talks about his experience, "You stop thinking only about yourself, You learn to

think about the group. "

1. In the program in New York State, young and old people _                .

A. teach one another new ways of building houses

B. are friendly to one another

C. do not work together

D. spend a long time together, working as farmers

2. All the members spend some time together every day mainly to _               .

A. learn new skills of farming

B. lead a busy life

C. get used to the life on the farms

D. find value and pleasure in work

3. The last paragraph shows that the teenager thinks his experience in the program is _________.

A. tiring          

B.unpleasant

C. helpful                    

D. boring

4. The best title for the passage is _________.

A. The Rules of Living Together

B. Life in New York State

C. Teenagers in the Special Group

D. Free Hours in the Special Work Group

单项选择题