问题 问答题

何谓成分过冷?用成分过冷理论解释合金的铸造性能(流动性、缩孔的分布特征)与其相图中液/固相线间垂直距离的关系。

答案

参考答案:在合金的凝固过程中,虽然实际温度分布一定,但由于液体中溶质分布发生变化,使液/固界面前沿的液体处于过冷状态,这种由液体成分变化与实际温度分布两个因素共同决定的过冷,称为成分过冷。合金的铸造性能(流动性、缩孔的分布特征)与其相图中液/固相线间垂直距离的大小有关。垂直距离越大,则合金的流动性越差,缩孑L越易呈分散分布。成分过冷的条件为:

,其中,G为液/固界面前沿液体中的实际温度梯度,R为液/固界面推移的速度,D为溶质原子在液相中的扩散系数,m为合金相图中液相线的斜率,C0为合金的成分,k0为合金的平衡分配系数。可以证明,上式中的

就是C0合金在其相图中液/固相线间的垂直距离。显然,液/固相线间的垂直距离越大,合金的成分过冷倾向就越大,液/固界面越倾向于树枝状生长,这阻碍了合金熔液的流动性,也使树枝状晶的枝间处因得不到补缩而形成分散缩孔。

阅读理解

People in the United States honor their parents with two special days: Mother’s Day, on the second Sunday in May, and Father’s Day, on the third Sunday in June.

Mother’s Day was proclaimed宣告 a day for national observance by President Woodrow Wilson in 1915. Ann Jarvis from Grafton, West Virginia, had started the idea to have a day to honor mothers. She was the one who chose the second Sunday in May and also began the custom of wearing a carnation康乃馨.

In 1909, Mrs. Dodd from Spokane, Washington, thought of the idea of a day to honor fathers. She wanted to honor her own father, William Smart. After her mother died, he had the responsibility of raising a family of five sons and a daughter. In 1910,the first Father’s Day was observed in Spokane. Senator Margaret Chase Smith helped to established Father’s Day as a national commemorative纪念的 day in 1972.

These days are set aside to show love and respect for parents. They raise their children and educate them to be responsible citizens. They give love and care.

These two special days are celebrated in many different ways. On Mother’s Day people wear carnations. A red one symbolizes a living mother. A white one shows that the mother is dead.Many people attend religious services to honor parents. It is also a day when people whose parents are dead visit the cemetery. On these days families get together at home, as well as in restaurants. They often have outdoor barbecues for Father’s Day. These are days of fun and good feelings and memories.

小题1:According to the passage, which of the following about the second Sunday in May is not true ?

A.It is a day to show love to mothers

B.It is a day to wear carnations

C.It is a day for people to visit the cemetery

D.It is a day for many people to attend religious services小题2:Choose the right time order of the following events.

a. The first Father’s Day was observed

b. Mother’s Day was proclaimed a day for national observance

c. Father’s Day became a day for national observance.

d. The idea of honoring fathers was bought up

A.a b c d

B.d a b c

C.b a c d

D.d a c b小题3:Who plays the most important role in Father’s Day becoming a national commemorative day?

A.Mrs. Do

B.Margaret Chase Smith

C.Ann Jarvis

D.Woodrow Wilson小题4:From the passage, we know in the U.S ________________.

A.one should wear a red carnation if one’s mother pass away

B.on Mother’s Day, families often go out to have barbecues

C.on Father’s Day, people often stay at home to celebrate the special day.

D.The purpose to have these two special days is to show love to parents小题5:The author’s purpose of writing this passage is to __________

A.call on people to love and respect their parents.

B.introduce Mother’s Day and Father’s Day.

C.tell the difference between Mother’s Day and Father’s Day

D.show how important fathers and mothers are

单项选择题 共用题干题