外文翻譯---材料的選擇

1、<p><b>　　1.材料的物理性能</b></p><p>　　在選擇工業(yè)用材料時，許多工程師通常都只考慮其平均的宏觀性能，因?yàn)檫@些特性是由工程實(shí)驗(yàn)確定的，而很少考慮其微觀特性。但有些工程師由于其專業(yè)范圍或所從事工作的需要，也考慮其微觀性能。</p><p>　　材料的平均性能包括制造過程的波動引起的缺陷、組織的變化以及密度變化。微觀性能則與原子、分子以

2、及它們的相互作用有關(guān)。為了使材料可直接用于工業(yè)，也為了在開發(fā)新材料時對可有的性能進(jìn)行評估，因而需對材料的這些問題進(jìn)行研究。</p><p>　　當(dāng)研究物質(zhì)的微觀性能與材料的平均性能之間的關(guān)系時，為了不至于將明顯矛盾的概念混淆，就應(yīng)了解造成材料具有各種性能的原因。這些性能有一般的，微觀的、宏觀的。一般性能則是以上提到的平均性能，這些性能屬經(jīng)典物理學(xué)的范疇，微觀特性主要是由量子力學(xué)理論來解釋，而宏觀特性則是要由相對論

3、來說明。</p><p>　　相對論涉及的是類似行星那樣大的物體或接近于光速的物體，同時也適合于較小的物質(zhì)，如高速運(yùn)行的亞原子粒子。對于核工程師、研究粒子加速器的電氣工程師，相對論也有一定的地位。而對于生產(chǎn)工程師，相對論僅是學(xué)術(shù)上的興趣，或是只因?qū)W科上的完整性而被提及。</p><p><b>　　2.材料的機(jī)械性能</b></p><p>　

4、　在確定了材料的重要物理性能之后，就必須考慮其機(jī)械性能（如屈服強(qiáng)度和硬度）。機(jī)械性能具有結(jié)構(gòu)敏感性，因?yàn)樗鼈內(nèi)Q于晶體結(jié)構(gòu)的類型及結(jié)合力，特別是取決于晶體內(nèi)容或晶粒邊界上缺陷的特性。</p><p>　　區(qū)分金屬與其他材料的一個重要特性是其塑性及在強(qiáng)度沒有減少的條件下塑性變形的能力。設(shè)計時，延伸率為5—15%則可使其承受突然的動態(tài)過載。為了適應(yīng)這類載荷而不破壞，材料應(yīng)具有動態(tài)韌性、較大的彈性模量以及經(jīng)過斷裂之前產(chǎn)

5、生足夠的塑性變形以消耗能量的能力。</p><p>　　為了預(yù)測材料受載后的特性，工程師需要材料機(jī)械性能的可靠數(shù)據(jù)。手冊中的數(shù)據(jù)是普通合金在68°F時的平均特性。設(shè)計時最常用的數(shù)據(jù)是拉伸屈服強(qiáng)度、硬度、彈性模量，在其他溫度時（除68°F之外）的屈服強(qiáng)度。設(shè)計者很少用到抗蠕變特性、缺口敏感性、沖擊強(qiáng)度以及疲勞強(qiáng)度。材料廠商的樣本中通常也提供最新的或完整的數(shù)據(jù)。</p><p&

6、gt;　　在手冊中很難找到生產(chǎn)工程中所需的象強(qiáng)重比、單位體積的成本以及對特定工作環(huán)境的承受能力等數(shù)據(jù)。</p><p>　　材料的主要機(jī)械性能及其對設(shè)計的重要影響的簡單回顧可使讀者熟悉每一種試驗(yàn)的重要性。</p><p><b>　　3.材料的選擇</b></p><p>　　各類實(shí)用的材料在不斷增長，而每種材料都具有各自的特點(diǎn)、應(yīng)用范圍及優(yōu)缺

7、點(diǎn)。當(dāng)今制造業(yè)中常用的材料有：</p><p>　　鐵和鋼（碳鋼、合金鋼、不銹鋼、工具鋼和模具鋼）。</p><p>　　有色金屬及其合金（鋁、鎂、銅、鎳、鈦、超耐熱合金、高熔點(diǎn)金屬、鈹、鋯、低熔點(diǎn)合金以及貴金屬）。</p><p>　　塑料（熱塑塑料、熱固塑料及合成橡膠）。</p><p>　　陶瓷、玻璃陶瓷、玻璃、石墨、金剛石。</

8、p><p>　　復(fù)合材料（增強(qiáng)塑料、金屬基符合材料、陶瓷基復(fù)合材料、蜂窩結(jié)構(gòu)復(fù)合材料）。</p><p><b>　　材料性能</b></p><p>　　當(dāng)選擇產(chǎn)品的材料時，首先要考慮的是材料的機(jī)械性能：強(qiáng)度、韌性、塑性、硬度、彈性、疲勞強(qiáng)度、抗蠕變性。材料的強(qiáng)重比以及剛度與重量比也是重要的特性，特別是用于航空及汽車工業(yè)的材料。鋁、鈦及增強(qiáng)塑料的

9、上述二種比值比鋼和鑄鐵的高。當(dāng)然，產(chǎn)品及其零件材料的機(jī)械性能應(yīng)根據(jù)產(chǎn)品發(fā)揮其預(yù)定功能的工作條件來選定。然后才考慮材料的物理性能如密度、比熱、熱膨脹性、導(dǎo)熱性、熔點(diǎn)、導(dǎo)電特性及導(dǎo)磁特性。</p><p>　　在較差的以及正常的工作環(huán)境下材料的化學(xué)性能也有重要的作用。材料的抗氧化性能、抗腐蝕性、性能的普遍降低、毒性、可燃性也都是要考慮的重要因素。例如，某些商務(wù)航空災(zāi)難性事故中，許多人員的死亡就是由于機(jī)艙內(nèi)非金屬材料燃

10、燒釋放的有毒煙霧所造成的。</p><p>　　材料的制造性能決定了它們是否能用相對簡便的方法鑄造成型、機(jī)械加工、焊接、熱處理等。將材料加工成所需形狀的方法可能對產(chǎn)品的最終性能及使用壽命產(chǎn)生不利的影響。</p><p><b>　　利用率及成本</b></p><p>　　原材料、加工過的材料及制成零件的利用率和成本是制造中的主要問題。材料選擇

11、的經(jīng)濟(jì)性與從技術(shù)上考慮材料性能和特點(diǎn)是同樣重要的。</p><p>　　如果原材料或加工過的材料或制成的零件不適合于所要求的數(shù)量、形狀和尺寸，則應(yīng)選擇其替代品和選擇其他加工方法，這就可能對產(chǎn)品成本起重要的影響。例如，若我們需要一根某一直徑的圓棒，但這種尺寸并不是標(biāo)準(zhǔn)尺寸，那么必須購買較大直徑的棒料，并通過某些加工方法（如切削加工，在模具中拉拔或磨削）加工至所需尺寸。</p><p>　　供

12、需的可靠性也影響其成本，大多數(shù)國家都進(jìn)口許多生產(chǎn)上必不可少的原料，應(yīng)注意，美國也依靠進(jìn)口原料。顯然，這取決于與其他國家的政治關(guān)系。</p><p>　　用不同的方法對材料進(jìn)行加工，其費(fèi)用就不同，有些加工需要貴重機(jī)械，有些需要較大的勞動強(qiáng)度，有些需要具有專門技術(shù)的人員或受過高等教育的人員或經(jīng)過專門訓(xùn)練的人員。</p><p>　　外觀、使用壽命、處理方法</p><p&g

13、t;　　材料被加工成產(chǎn)品之后的外觀對用戶產(chǎn)生較大的影響，其色澤、外觀感覺及表面紋理都是在決定購置產(chǎn)品時要考慮的特性。</p><p>　　使用時間以及與使用相關(guān)的現(xiàn)象（如磨損、疲勞、蠕變及尺寸穩(wěn)定性等）也是重要的因素，這些現(xiàn)象對產(chǎn)品的性能有很大的影響，如不加以控制，就可能使產(chǎn)品完全失效。</p><p>　　同樣，產(chǎn)品中各材料的相容性也是重要的。摩擦、磨損及其它現(xiàn)象都可能縮短產(chǎn)品的使用壽命

14、或?qū)е庐a(chǎn)品破損。例如，由不同材料所制成的配合件間的電蝕作用就會腐蝕零件。</p><p>　　在人們意識到要保持清潔與健康環(huán)境的時代，產(chǎn)品使用壽命期之后材料的回收利用及適當(dāng)?shù)奶幚碜兊萌找嬷匾?。例如，?yīng)注意生物可降解的包裝材料、可重復(fù)使用的玻璃瓶及鋁制飲料罐的回收利用。有毒廢物及材料的恰當(dāng)處理也是一個重要問題。</p><p>　　1 Physical Properties of Mater

15、ials</p><p>　　In the selection of materials for industrial applications, many engineers normally refer to their average macroscopic properties, as determined by engineering tests, and are seldom concerned wi

16、th microscopic considerations. Others, because of their specialty or the nature of their positions, have to deal with microscopic properties.</p><p>　　The average properties of material are those involving m

17、atter in bulk with its flaws, variations in composition, and variations in density that are caused by manufacturing fluctuations. Microscopic properties pertain to atoms, molecules, and their interactions. These aspects

18、of material are studied for their direct applicability to industrial problems and also so that possible properties in the development of the new materials can be estimated.</p><p>　　In order not to become co

19、nfused by apparently contradictory concepts when dealing with the relationship between the microscopic aspects of matter and the average properties of materials, it is wise to consider the principles that account for the

20、 nature of matter at the different levels of our awareness. These levels are the commonplace, the extremely small, and the extremely large. The commonplace lever deals with the average properties already mentioned, and

21、the principles involved are those s</p><p>　　Relativity is concerned with very large masses, such as planets or stars, and large velocities that may approach the velocity of light. It is also applicable to s

22、maller masses, ranging down to subatomic particles, when they move at high velocities. Relativity has a definite place in the tool boxes of nuclear engineers and electrical engineers who deal with particle accelerators.

23、For production engineers, relativity is of only academic interest and is mentioned here for the sake of completeness. </p><p>　　2 Mechanical Properties of Materials</p><p>　　Once the important p

24、hysical properties of a material have been established, mechanical properties such as yield strength and hardness must be considered. Mechanical properties are structure-sensitive in the sense that they depend upon the t

25、ype of crystal structure and its bonding forces, and especially upon the nature and behavior of the imperfections that exits within the crystal itself or at the grain boundaries.</p><p>　　An important charac

26、teristic that distinguishes metals from other material is their ductility and ability to be deformed plastically without loss in strength. In design, 5 to 15 percent elongation provides the capacity to withstand sudden d

27、ynamic overloads. In order to accommodate such loads without failure, materials need dynamic toughness, high moduli of elasticity, and the ability to dissipate energy by substantial plastic deformation prior to fracture.

28、</p><p>　　To predict the behavior of a material under load, engineers require reliable data on the mechanical properties of materials. Handbook data is available for the average properties of common alloys a

29、t 68℉. In design, the most frequently needed data are tensile yield strength, hardness, modulus of elasticity, and yield strengths at temperatures other than 68℉. Designers less frequently use resistance to creep, notch

30、sensitivity, impact strength, and fatigue strength. Suppliers’ catalogs frequently </p><p>　　Production-engineering data that is seldom found in handbooks include strength-to-weight ratios, cost per unit vol

31、ume, and resistance to specific service environments.</p><p>　　A brief review of the major mechanical properties and their significance to design is included to ensure that the reader is familiar with the im

32、portant aspects of each test.</p><p>　　Selecting Materials</p><p>　　An ever-increasing variety of materials is available, each having its own characteristics, applications, advantages, and limit

33、ations. The following are the general types of materials used in manufacturing today:</p><p>　　Irons and steels (carbon, alloy, stainless, and tool and die steels)</p><p>　　Nonferrous metals and

34、 alloys (aluminum, magnesium, copper, nickel, titanium, superalloys, refractory metal, beryllium, zirconium, low-melting alloys, and precious metals)</p><p>　　Plastics (thermoplastics, thermosets, and elasto

35、mers)</p><p>　　Ceramics, glass ceramics, glasses, graphite and diamond.</p><p>　　Composite materials (reinforced plastics, metal-matrix and ceramic-matrix composites, and honeycomb structures).&

36、lt;/p><p>　　Properties of materials</p><p>　　When selecting materials for products, we first consider their mechnical properties: </p><p>　　strength, toughness, ductility, hardness, el

37、asticity, fatigue, and creep. The strength-to-weight and stiffness-to-weight ratios of material are also important, particularly for aerospace and automotive applications. Aluminum, titanium, and reinforced plastics, for

38、 example, have higher ratios than steels and cast irons. The mechanical properties specified for a product and its components should of course be for the conditions under which the product is expected to function. We the

39、n consider the p</p><p>　　Chemical properties also play a significant role in hostile as well as normal environments.</p><p>　　Oxidation corrosion, general degradation of properties, toxicity, a

40、nd flammability of materials are among the important factors to be considered. In some commercial airline disasters, for example, many deaths have been caused by toxic fumes from burning nonmetallic materials in the airc

41、raft cabin.</p><p>　　Manufacturing properties of materials determine whether they can be cast formed, machined, welded, and heat treated with relative ease. The method used to process materials to the desire

42、d shapes can adversely affect the product’s final properties and service life.</p><p>　　Availability and cost</p><p>　　Availability and cost of raw and processed materials and manufactured compo

43、nents are </p><p>　　major concerns in manufacturing. Competitively, the economic aspects of material selection are as important as the technological considerations of properties and characteristics of materi

44、als.</p><p>　　If raw or processed material or manufactured components are not available in the desired quantities, shapes, and dimensions, substitutes and/or additional processing will be required, which can

45、 contribute significantly to product cost. For example, if we need a round bar of a certain diameter and it is not available in standard form, then we have to purchase a larger rod and reduce its diameter by some means,

46、such as machining, drawing through a die, or grinding.</p><p>　　Reliability of supply, as well as demand, affects cost. Most countries import numerous raw materials that are essential for production. Note th

47、e reliance of the United States on imported raw materials. The broad political implication of such reliance on other countries is self-evident.</p><p>　　Different costs are involved in processing materials b

48、y different methods. Some methods require expensive machinery, others require extensive labor, and still others require personnel with special skills or a high level of education or specialized training.</p><p

49、>　　Appearance, service life, and disposal</p><p>　　The appearance of materials after they have been manufactured into products influences .their appeal to the consumer. Color, feel, and surface texture ar

50、e characteristics that we all consider when making a decision about purchasing a product.</p><p>　　Time and service-dependent phenomena such as wear, fatigue, creep, and dimensional stability are important.

51、These phenomena can significantly affect a product’s performance and, if not controlled, can lead to total failure of the product.</p><p>　　Similarly, compatibility of materials used in a product is importan

52、t. Friction and wear, corrosion, and other phenomena can shorten a product’s life or cause it to fail. An example is galvanic action between mating parts made of dissimilar metals, which corrodes the parts.</p>&l

53、t;p>　　Recycling or proper disposal of materials at the end of their useful service lives has become increasingly important in an age conscious of maintaining a clean and healthy environment. None, for example, the use