Influences of Ce addition on the microstructures and mechanical properties of 2519A aluminum alloy
Journal of Alloys and Compounds 491 (2010) 366–371
Contents lists available at ScienceDirect
Journal of Alloys and
Compounds
j o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /j a l l c o
m
In?uences of Ce addition on the microstructures and mechanical properties of 2519A aluminum alloy plate
Wen-tao Wang,Xin-ming Zhang ?,Zhi-guo Gao,Yu-zhen Jia,Ling-ying Ye,Da-wei Zheng,Ling Liu
School of Materials Science and Engineering,Central South University,Changsha 410083,China
a r t i c l e i n f o Article history:
Received 28July 2009
Received in revised form 21October 2009Accepted 22October 2009
Available online 31 October 2009Keywords:Ce addition
2519A aluminum alloy Microstructure
Mechanical properties
a b s t r a c t
The precipitation hardening response,microstructures and mechanical properties of 2519A aluminum alloy plates with additions of 0,0.2and 0.4wt.%Ce were investigated by scanning electron microscopy (SEM),transmission electron microscopy (TEM),differential scanning calorimetry (DSC),X-ray diffrac-tometer (XRD),Vickers hardness test and tensile tests.The results show that 0.2wt.%Ce promotes the precipitation of denser and ?ner ? phase,which improves the tensile strength of the alloy at both room and elevated temperatures.High melting point Al 8Cu 4Ce phase particles are found in alloys with additions of Ce up to 0.4wt.%,which contributes to the mechanical properties at elevated temperature.
? 2009 Elsevier B.V. All rights reserved.
1.Introduction
The high-strength aluminum alloy 2519is used for armor plates applications,such as in American advanced amphibious assault vehicles (AAAV)[1,2].As a new version,2519A alloy was developed with a superior combination of properties including mechanical properties,weldability and stress corrosion cracking resistance [3–5].It is acknowledged that the mechanical properties of the armor plate are main factors in?uencing its ballistic application [6].Accordingly,further improving the mechanical properties,espe-cially which at elevated temperatures,is necessary for expanding the application ?elds of 2519A alloy plate.
It is widely accepted that rare earth elements can enhance the strength of aluminum alloy by inhibiting the recrystallization [7],re?ning grains [8]and in?uencing the precipitation process [9].Li et al.[10]demonstrated that 0.1–0.2%Y improved the tensile proper-ties of 2519alloy at room and elevated temperatures by changing the size and density of ? phase as Y reduced the diffusion of Cu atoms.However,Y has been regarded as an expensive alloying addi-tion,thereby selection of Y would make the relevant products less cost-competitive in industrial application.To develop new alloys at lower costs with superior properties therefore becomes very important.
Ce is considered as an effective micro-alloying element with moderate price,which shows favorable effects on alloys [11–16].?Corresponding author.Tel.:+867318830265.
E-mail address:xmzhang@https://www.wendangku.net/doc/0515374857.html, (X.-m.Zhang).
Zhu et al.[11]investigated the microstructure and microstruc-tural stability of die-cast AE42Mg alloy with Ce-based misch metal addition.According to their work,both A 11RE 3and Al 2RE phases were stable at temperatures up to 200?C,with no decomposition observed after 2weeks.Stanford et al.[12]found that the extrusion texture of M1Mg alloy was weakened by the addition of Ce-rich misch metal.The samples with weakened extrusion textures exhib-ited an increase in the tensile elongation.Wu et al.[13]studied the microstructure evolution of an Al–Mg–Si–Mn–Cu–Ce alloy during homogenization.The enrichment of the Ce was observed in the resulting AlCuSiCe particles after homogenization.Liu et al.[14]obtained the results that adding 0.3wt.%rare-earth elements (con-taining 50wt.%Ce)into Cu–12wt.%Ag alloy formed orthorhombic (Cu,Ag)6(Ce,La)particles which were hardly deformed during cold drawing.Xiao et al.[15]indicated that adding a small amount of the RE elements (containing about 60wt.%Ce)evidently increased the creep-fatigue rupture lifetime of the Sn3.8Ag0.7Cu solder joint,which depended on the re?ning of intermetallic compounds due to the RE elements.Xiao et al.[16]considered the addition of Ce up to 0.45%to an Al–Cu–Mg–Ag alloy induced the precipitation of ?ner and denser and ? phases whilst improved the thermal stability of the phase.This study primarily aims to investigate the in?uence of Ce addition on microstructures and mechani-cal properties of 2519A aluminum alloy plate,for its commercial potential.
2.Experimental procedures
Investigated 2519A alloys containing nominal amounts of 0,0.2and 0.4wt.%Ce additions,as shown in Table 1,were prepared in an induction furnace by adding pure Al,Cu,Mg and Al–Mn,Al–Zr,Al–Ti–B,Al–Ce master alloys.The alloys were
0925-8388/$–see front matter ? 2009 Elsevier B.V. All rights reserved.doi:10.1016/j.jallcom.2009.10.185
W.-t.Wang et al./Journal of Alloys and Compounds 491 (2010) 366–371
367
melted at 760–780?C for 30min,degassed by C 2Cl 6,then cast into iron moulds to produce billets.The billets were homogenized at 520–530?C for 24h,followed by air cooling,and then hot-rolled from 25to 2.4mm in thickness.Subsequently,the plates were solution-treated at 535?C for 2h,water-quenched,and ?nally aged at 165?C with prior cold-rolled reduction at 15%(T8temper).
The tensile tests were performed on CSS-44100and RWS 50machines for room temperature (20?C)test and elevated temperature (300?C)test,respectively.The specimens were taken in the longitudinal direction from alloy plates with T8temper.The stretching speeds for the tensile tests were 2mm/min.In the test at 300?C,the specimens were held at consistent temperature for 10min to achieve thermal equilibration.The mechanical results reported in this study were the mean values of at least two specimens.The microstructures were analyzed by transmission electron microscopy (TEM,Tecnai G 220)and scanning electron microscopy (SEM,Sirion 200)attached with energy dispersive X-ray spectroscopy (EDS,GENESIS 60S).TEM specimens were cut from the middle segment of the tensile plate samples,polished mechanically to 0.05–0.07mm,then electro-polished in a 70%ethanol and 30%nitric acid solution at ?20?C by a twin-jet equipment operated at 20V.To make a valid comparison of the different Ce containing as-cast microstructures,the specimens for SEM observation were cut from similar regions of the respective castings and etched by Kaller’s reagent.Phase analyses of cast alloys were performed with an X-ray diffractometer (XRD,Rigaku D/max 2500).Differential scanning calorimetry (DSC,NETZSCH DSC200F)was used to examine the role of 0.2wt.%Ce on the precipitation kinetics of strengthening phases.Disc-shaped samples with a thickness about 1mm and diameter 5mm were used.Experiments were performed at a constant heating rate of 10?C/min.The Vickers hardness test was carried out with 3kg loading to investigate the aging response of the alloys.Each hardness value reported in this study was the mean value of at least ?ve measurements.
3.Results
3.1.Microstructures
The as-cast microstructures of the alloys with different Ce con-tents are shown in Fig.1.The segregated phases of Ce-free 2519A alloy (Fig.2(a))are continuous and coarse,which present a retic-ular formation along grain boundaries composed of Al 2Cu and ?(Al)phases.However,when adding 0.2wt.%Ce,these compounds along grain boundaries become discontinuous and thinner whilst some spherical particles less than 8?m can be observed,as arrowed
Table 1
The nominal compositions of the experimental alloys (wt.%).Alloys Cu Mn Mg Ti Zr Ce Al 2519A
5.800.300.200.050.200.00Bal.2519A–0.2Ce 5.800.300.200.050.200.20Bal.2519A–0.4Ce
5.80
0.30
0.20
0.05
0.20
0.40
Bal.
in Fig.1(b).With the addition of Ce increasing,the compounds at grain boundaries become more discontinuous and the quantity of spherical particles increase (Fig.1(c)).Even at 300?C,these spheri-cal particles still exist along grain boundaries (Fig.2).According to the measurement of EDS attached to the SEM,the phase is rich in Al,Cu and Ce with the atomic ratio nearly 8:5:1.To further identify the spherical phase,XRD was used to examine the phase consti-tutions of different as-cast alloys (Fig.3).Compared with 2519A,a new phase of Al 8Cu 4Ce emerges in Ce containing alloys.It can be inferred that the spherical phase is the Al 8Cu 4Ce phase,which is primarily formed in solidi?cation,and dif?cult to be dissolved at high temperature.As a new emerging phase,Al 8Cu 4Ce would bring about an important effect on mechanical properties,espe-cially at high temperature,for its thermal stability.In addition,the phase constitution suggests that Cu and Ce atoms have a chemical af?nity.
Since 2519A alloy plate is mainly served with T87temper,much attention is focused on the role of Ce on microstructure of 2519A–T87plate.The precipitation hardening responses of 2519A and Ce containing alloys are revealed by Vickers hardness curves,which came about after solution heat treatment at 535?C for 2h,cold-rolled and aged at 165?C (Fig.4).It is worth emphasizing that a plateau is observed from 8to 12h before peak-aging for 2519A alloy,but that is not signi?cant in Ce containing curves.The stage of plateau is associated
with the formation of G.P.zone/? phase,thus,addition of Ce up to 0.4wt.%suppresses the precipitation of G.P.zone/? phase of 2519A alloy.Furthermore,the peak-aging time is
Fig.1.SEM images of the as-cast microstructures:(a)2519A,(b)2519A–0.2Ce,and (c)2519A–0.4Ce.
368W.-t.Wang et al./Journal of Alloys and Compounds
491 (2010) 366–371
Fig.2.SEM image and EDS pattern of 2519A–0.4Ce alloy plate at 300?C.
shorten from 14h for 2519A alloy to 13and 12h for alloys with 0.2wt.%Ce and 0.4wt.%Ce,respectively.The peak-aging hardness of 2519A–0.2Ce reaches 160.3HV,higher than that of 2519A alloy.However,the peak-aging hardness is the lowest with 0.4wt.%Ce addition.The peak-aging time indicates the ? precipitation speed rate whilst the maximum hardness is associated with the volume fraction of ? phase.It can be inferred that up to 0.4wt.%Ce
acceler-
Fig.3.XRD patterns of the as-cast
specimens.Fig.4.Vickers hardness curves of present alloys aged at 165?C.
Table 2
Characteristic points of peak b for experimental alloys by DSC analyses.Alloys
Temperature of peak b (?C)Beginning
Peak Ending 2519A
224.1243.2269.52519A–0.2Ce
212.0
240.4
268.1
ates the ? precipitation process,while only 0.2wt.%Ce promotes the mass ? precipitation.
The conclusion can be con?rmed by DSC measurement.Fig.5shows the DSC curves of 2519A and 2519A–0.2Ce alloys after solution-treatment and being cold-rolled.The DSC curve of 2519A alloy show three major exothermic peaks,indicated as a,b and c.Taking into account the temperature at which the peaks appear and their sequence,it is plausible that the three exothermic peaks relate to the formation of G.P.zone/? phase,? phase and ?phase,respec-tively.In the DSC curve of 2519A–0.2Ce alloy,only two exothermic peaks appear at high temperature (b and c),and peak a is not observed.On the other hand,peak b of the 2519A–0.2Ce curve starts at a lower temperature (
Table 2).These features imply that the addition of 0.2wt.%Ce suppresses the formation of G.P.zone/?
Fig.5.DSC curves of 2519A and 2519A–0.2Ce alloys.
W.-t.Wang et al./Journal of Alloys and Compounds491 (2010) 366–371
369
Fig.6.TEM micrographs of the studied alloy plates peak-aged:(a)2519A,(b)2519A–0.2Ce,and(c)2519A–0.4Ce.
phase,whilst promoting the precipitation of? phase.The forma-tions of G.P.zone and? phase are not explicitly distinguished in the present DSC measurement.
On the basis of the Avrami–Johnson–Mehl equation[17],the activation energies for the formation of? phase in different alloys can be obtained by present DSC curves,which are145.80kJ/mol in 2519A alloy and141.43kJ/mol in2519A–0.2Ce alloy,respectively. Owing to the lower activation energy,it is easier for? to nucleate in the alloy with0.2wt.%Ce addition.These values also reinforce the present interpretation of the in?uence of0.2wt.%Ce addition.
TEM bright?eld images of the studied alloys plates peak-aged at165?C(T87temper)are shown in Fig.6.It reveals that? pre-cipitates,the dominant phase,as platelets on the{001}Al planes are uniformly distributed in the grains of three https://www.wendangku.net/doc/0515374857.html,pared with the Ce-free alloy,the density of? phase is higher in the alloy plate with0.2wt.%Ce addition.However,the density of? phase decreases when Ce content reaches0.4wt.%.
To further understand the difference in the intrinsic feature of? phase at high temperature,the peak-aged alloys were thermal insu-lated at300?C for10min then stretched.Fig.7shows the coarse ? phase within the grains of three alloys.Obviously,the? precip-itates in the2519A–0.2Ce alloy plate are?ner and more densely distributed than that of2519A alloy,whereas the amount of? in the2519A–0.4Ce alloy plate is the least.These direct evidences of TEM images explicitly verify that? phases are prone to precipitate with the addition of0.2wt.%Ce.3.2.Mechanical properties
The tensile properties of the studied alloy plates with T8temper at various test temperatures are listed in Table3.The maximum strength at room temperature is obtained from2519A–0.2Ce alloy plate that is493.0MPa,higher than that of2519A and2519A–0.4Ce alloy plates.At300?C,the tensile strength increases from139.5to 175.0and152.0MPa correspondingly when adding0.2and0.4wt.% Ce,respectively.
Meanwhile the elongations of the studied alloy plates changed appreciably with different Ce contents.At room temperature,the elongations of2519A,2519A–0.2Ce and2519A–0.4Ce are10.8%, 10.7%,12.3%,respectively.At300?C the elongations improve from 22.1%to25.4%and27.0%with an increasing Ce addition.It can
Table3
Mechanical properties of the studied alloy plates at room temperature and300?C.
Alloy Testing temperature(?C)Ultimate tensile
strength(MPa)
Elongation
(%)
2519A
25483.210.8
300139.522.1
2519A–0.2Ce
25493.010.7
300175.025.4
2519A–0.4Ce
25447.712.3
300152.027.0
370W.-t.Wang et al./Journal of Alloys and Compounds
491 (2010) 366–371
Fig.7.TEM micrographs of the peak-aging alloy plates stretched at 300?C:(a)2519A,(b)2519A–0.2Ce,and (c)2519A–0.4Ce.
be concluded that the addition of 0.2wt.%Ce increases the ten-sile strength of the alloy at room and elevated temperatures,but slightly affects the elongation.
4.Discussion
The present works shows that the addition of Ce obvi-ously affects the microstructures and mechanical properties of 2519A–T87alloy plate.
2519A–T87alloy is a typical age-hardenable alloy which is strengthened by ? phase.? precipitate is semi-coherent with Al matrix [18],the morphology,size and volume fraction of which control the ?nal performance of 2519A–T87alloy plate [3].In another words,the denser and ?ner the ? precipitates are,the superior the mechanical properties that 2519A–T87alloy has.According to the results of Vickers hardness,DSC curves and TEM images,the density of ? precipitates in 2519A–0.2Ce is the highest and the size is the smallest.It is suggested that 0.2wt.%Ce addition promotes the precipitation of the ? phase.This is the essential rea-son why the tensile strength of 2519A–0.2Ce alloy plate at room temperature and 300?C is optimal.The mechanism of 0.2wt.%Ce which promotes the ? precipitation might be due to the large Ce atoms.Hardy considered that atoms considerable larger than Al could affect nucleation of precipitates [19].The diameter of Ce atom (0.182nm)is larger than that of Al atom (0.143nm).When Ce atoms enter the Al matrix,it inevitably cause the crystal lattices distortion
and raise system energy.In order to keep the system energy low,there might be more oversaturated vacancies aggregating around Ce atoms.Because of the high vacancies binding energy,the Ce atoms would be relatively mobile and able to aggregate suf?ciently to nucleate ? phase.Such movement was described as “amoeba-like”by Embury and Nicholson [20].The nucleus forms where both Cu and Ce-vacancy clusters meet,as Cu and Ce atoms have chemical af?nity.It is possible to predict positions where large Ce atoms with their accompanying vacancies could reduce the mis?t at the edges of ? and hence reduce the interfacial energy.As a result nucle-ation of ? phase is promoted by 0.2wt.%Ce.The present result is in close agreement with what is revealed in Ref.[21].Song,et al considered that Ce improved the nucleation ratio and volume frac-tion of phase in an Al–Cu–Mg–Ag alloy because Ce could act as heterogeneous nucleation centers for phase.
However,with the addition of 0.4wt.%Ce,? precipitates decrease owing that excessive Ce form the Al 8Cu 4Ce phases,which reduces solid solubility of Cu in Al matrix,thereby leading to the reduction of tensile strength of 2519A–0.4Ce alloy plate at room temperature.
It was reported that [22,23]rare earth containing phases played a key role in improving the mechanical properties of alloys.How-ever,referring to Fig.1the spherical Al 8Cu 4Ce particles are at micron level;moreover,they are non-coherent with the Al matrix.Consequently,Ce containing phase makes less contribution than ? to enhance the strength of 2519A alloy at room temperature.Never-theless,Al 8Cu 4Ce phase is of thermal stability,which evidently does
W.-t.Wang et al./Journal of Alloys and Compounds491 (2010) 366–371371
not coarsen evidently at elevated temperature.These high melting point particles strengthen grain boundaries and served as obstacles to slip and creep at high temperature.Due to the Al8Cu4Ce phase the ultimate tensile strength of2519A–0.4Ce at300?C is higher than that of2519A ally although the? precipitates of2519A–0.4Ce alloy is the least at this temperature.
5.Conclusions
(1)The role of0.2wt.%Ce in2519A–T87alloy plate is to suppress
the formation of G.P.zone/? phase,decreases activation energy for the formation of? plates,and promotes the precipitation of denser and?ner? phase.With such a microstructure,the tensile strength at room temperature is improved.
(2)Additions of Ce up to0.4wt.%enhance the thermal stability of
2519A–T87alloy plate at300?C.The improved tensile proper-ties of2519A–0.2Ce at elevated temperature are related to the high density? precipitates and high melting point Al8Cu4Ce phase.The strengthening factor for2519A–0.4Ce at300?C is due to Al8Cu4Ce particles.
Acknowledgement
This work was supported by the973Program Foundation of China(No.2005CB623700).References
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An army of college students indulge themselves in playing games, enjoying romance with girls/boys or killing time passively in their dorms. When it approaches to graduation, as a result, they find their academic records are less impressive. 4.(an army of, an ocean of; a sea of; a multitude of; a host of; many, if not most)替换many. 注：用many, if not most 一定要小心，many后一定要有词。 Eg. Many individuals, if not most, harbor the idea that….用most, if not all ,替换most. 5. a slice of, quiet a few , several替换some 6. harbor the idea that, take the attitude that,hold the view that, it is widely shared that, it is universally acknowledged that替think(因为是书面语，所以要加that) ,business ,matter 替换thing 8. shared 代common huge fruits 替换get many benefits 10. for my part ,from my own perspective 替换in my opinion 11. Increasing(ly),growing 替换more and more( 注意没有growingly这种形式。所以当修饰名词时用increasing/growing.修饰形容词，副词用increasingly. Eg. sth has gained growing popularity. Sth is increasingly popular with the advancement of sth. if anything, 或little or nothing替换hardly 13..beneficial, rewarding替换helpful, ,client,consumer,purchaser, 替换customer ,extremely, intensely 替换very 16. hardly necessary, hardly inevitable ... 替换unnecessary, avoidable appeals to sb, sth exerts a tremendous fascination on sb 替换sb take interest in / sb. be interested in one's attention替换attract one's attention. ,demension,sphere代aspect indicative of ,be fearful of代indicate, fear rise to, lead to, result in, trigger 替换cause. 22. There are several reasons behind sth 替换..reasons for sth 替换want. attention into 替换pay attention to in mind that 替换remember 26. enjoy, possess 替换have(注意process是过程的意思) 27. interaction替换communication on sth替换be against , disagree with sth name only a few, as an example替换for example, for instance 30. next to / virtually impossible,替换nearly / almost impossible 32. regarding / concerning 替换about. 33. crucial /paramount 替换important. 34. 第一（in the first place/the first and foremost）；第二（there is one more point, I should touch on, that…）；第三（the last but not the least）. 35. assiduous 替换hard-working. 36. arduous 替换difficult. 37. underdeveloped / financially-challenged 替换poor（因为poor通常含有贬义）. 湖南商学院《数据结构与算法》课程设计报告 姓名: 朱晰璋 学号: 160920044 专业: 计算机科学与技术 班级: 计科1602 指导教师: 邹文 职称: 计算机与信息工程学院 20 年月 目录 （结构供参考，请按照自己内容生成，标题宋体小二加粗，居中，段前后距各0.5行）1. 课程设计概（黑体四号，段前后距各0.5行） (1) 1.1问题描述（宋体小四，行距1.5倍） (1) 1.2基本要求 (1) 1.3开发环境 (1) 2. 数据结构及算法分析 (1) 2.1数据结构分析 (1) 2.2算法分析及相关算法介绍 (1) 3. 详细设计与实现 (1) 3.1软件结构 (1) 3.2模块设计 (1) 4. 测试与运行 (1) 5. 总结与收获 (1) 参考文献（要求五篇以上，格式请参考知网上论文） (1) 附录 (1) 课程设计评审表 课程设计题目（宋体小二号，加粗，居中，单倍行距，段前后0.5行） 1.课程设计概述（一级标题段前后距各0.5行） 1.1问题描述（目的，内容，题目等）（黑体小四，二级标题段前后距各0.5行） 1.2基本要求（要完成什么功能，有什么性能，） 1.3开发环境 2.数据结构及算法分析 2.1数据结构分析（采用线性表，树，图等等数据结构的原因及具体结构） 2.2算法分析（比如采用分治法，描述分治法的原理以及应用到此处的理由） 3.详细设计与实现 3.1总体结构（整体包括几个模块，每个模块的功能） 3.2模块设计（具体每个模块的设计实现，可以用流程图或伪码描述等，类似教材的算法描述，包括输入输出，算法功能，以及具体算法描述） 4.测试与运行（测试数据，运行结果，要截图） 5.总结与收获（如果采用多种算法或者数据结构解决同一个问题，请 在总结中用一小节比较各种不同算法空间效率，时间效率，如果没有请省略） 参考文献（要求五篇以上，格式如下，不清楚清参考知网上论文）参考文献通常应为近3年的，至少5篇，其中最好有英文文献。其具体写法为： 1. 专著、论文集、毕业论文、报告等 [序号]作者.书名[文献类型标志].出版地：出版者，出版年.起止页码（任选）.（文 一般过去式 时间状语：yesterday just now (刚刚) the day before three days ag0 a week ago in 1880 last month last year 1. I was in the classroom yesterday. I was not in the classroom yesterday. Were you in the classroom yesterday. 2. They went to see the film the day before. Did they go to see the film the day before. They did go to see the film the day before. 3. The man beat his wife yesterday. The man didn’t beat his wife yesterday. 4. I was a high student three years ago. 5. She became a teacher in 2009. 6. They began to study english a week ago 7. My mother brought a book from Canada last year. 8.My parents build a house to me four years ago . 9.He was husband ago. She was a cooker last mouth. My father was in the Xinjiang half a year ago. 10.My grandfather was a famer six years ago. 11.He burned in 1991 The way的用法及其含义（二） 二、the way在句中的语法作用 the way在句中可以作主语、宾语或表语： 1.作主语 The way you are doing it is completely crazy.你这个干法简直发疯。 The way she puts on that accent really irritates me. 她故意操那种口音的样子实在令我恼火。The way she behaved towards him was utterly ruthless. 她对待他真是无情至极。 Words are important, but the way a person stands, folds his or her arms or moves his or her hands can also give us information about his or her feelings. 言语固然重要,但人的站姿,抱臂的方式和手势也回告诉我们他(她)的情感。 2.作宾语 I hate the way she stared at me.我讨厌她盯我看的样子。 We like the way that her hair hangs down.我们喜欢她的头发笔直地垂下来。 You could tell she was foreign by the way she was dressed. 从她的穿著就可以看出她是外国人。 She could not hide her amusement at the way he was dancing. 她见他跳舞的姿势，忍俊不禁。 3.作表语 This is the way the accident happened.这就是事故如何发生的。 Believe it or not, that's the way it is. 信不信由你, 反正事情就是这样。 That's the way I look at it, too. 我也是这么想。 That was the way minority nationalities were treated in old China. 那就是少数民族在旧中 海外留学英语写作测试中的小词大用 朗阁托福考试研究中心 做海外留学英语写作测试教学那么多年，几乎每次课堂中，只要问到学生关于对写作的感受，基本上都会听到差不多的答案，那就是语法和词汇的问题；也就是说大多数学生都认为只要词汇用的高级、有难度，就一定能够得到很高的分数。其实，这只是一个误区，而这个误区在一定程度上也是由于现在国内的教育状况造成的，当学生们已经习惯了这样的考试模式的时候想要进行改变那是比较难的。其实海外留学考试与国内的四六级考试还是有所区别的，在很多高分甚至是满分的作文中，我们都会发现整篇文章都没有一个生词，所以，问题的关键应该是在于如何灵活地运用这些我们非常熟悉的小词，如何让它们在作文中发挥大作用。在下文中，朗阁托福考试研究中心将给大家列举一些在雅思和托福写作中可以用得很精彩的小词以及它们的具体运用。 1.社会生活类 Skip: The poor tend to smoke more and skip breakfast, two factors which have major stress effects on the body. Better of: People of the lower end of the social economic status have stress levels in their bodies that worth 3 times higher than those who are better off. Reputation: Contrary to fears about the violent reputation of some games, there is no firm proof that playing them has a negative impact on children's behavior. Contribution: Video games have a positive contribution for education. Tough: The findings are likely to surprise supporters of tougher regulation of computer games. Midlife review: People need opportunities to make a "midlife review" to adjust to the later stages of employed life, and to plan for the transition to retirement. Update: Others can do voluntary work more effectively if they can update their 1、模板匹配法： 在机器识别事物的过程中，常常需要把不同传感器或同一传感器在不同时间、不同成像条件下对同一景象获取的两幅或多幅图像在空间上对准，或根据已知模式到另一幅图像中寻找相应的模式，这就叫匹配。在遥感图像处理中需要把不同波段传感器对同一景物的多光谱图像按照像点对应套准，然后根据像点的性质进行分类。如果利用在不同时间对同一地面拍摄的两幅照片，经套准后找到其中特征有了变化的像点，就可以用来分析图中那些部分发生了变化；而利用放在一定间距处的两只传感器对同一物体拍摄得到两幅图片，找出对应点后可计算出物体离开摄像机的距离，即深度信息。 一般的图像匹配技术是利用已知的模板利用某种算法对识别图像进行匹配计算获得图像中是否含有该模板的信息和坐标； 2、基本算法： 我们采用以下的算式来衡量模板T（m,n)与所覆盖的子图Sij（i,j)的关系，已知原始图像S(W,H)，如图所示： 利用以下公式衡量它们的相似性： 上述公式中第一项为子图的能量，第三项为模板的能量，都和模板匹配无关。第二项是模板和子图的互为相关，随（i,j）而改变。当模板和子图匹配时，该项由最大值。在将其归一化后，得到模板匹配的相关系数： 当模板和子图完全一样时，相关系数R(i,j) = 1。在被搜索图S中完成全部搜索后，找出R的最大值Rmax(im,jm)，其对应的子图Simjm即位匹配目标。显然，用这种公式做图像匹配计算量大、速度慢。我们可以使用另外一种算法来衡量T和Sij的误差，其公式为： 计算两个图像的向量误差，可以增加计算速度，根据不同的匹配方向选取一个误差阀值E0，当E(i,j)>E0时就停止该点的计算，继续下一点的计算。 最终的实验证明，被搜索的图像越大，匹配的速度越慢；模板越小，匹配的速度越快；阀值的大小对匹配速度影响大； 3、改进的模板匹配算法 将一次的模板匹配过程更改为两次匹配； 第一次匹配为粗略匹配。取模板的隔行隔列数据，即1/4的模板数据，在被搜索土上进行隔行隔列匹配，即在原图的1/4范围内匹配。由于数据量大幅减少，匹配速度显著提高。同时需要设计一个合理的误差阀值E0： E0 = e0 * (m + 1) / 2 * (n + 1) / 2 式中：e0为各点平均的最大误差，一般取40~50即可； m,n为模板的长宽； 第二次匹配是精确匹配。在第一次误差最小点（imin, jmin)的邻域内，即在对角点为（imin -1, jmin -1), (Imin + 1, jmin + 1)的矩形内，进行搜索匹配，得到最后结果。 定冠词the的用法： 定冠词the与指示代词this ,that同源,有“那(这)个”的意思,但较弱,可以和一个名词连用,来表示某个或某些特定的人或东西. (1)特指双方都明白的人或物 Take the medicine.把药吃了. (2)上文提到过的人或事 He bought a house.他买了幢房子. I've been to the house.我去过那幢房子. (3)指世界上独一无二的事物 the sun ,the sky ,the moon, the earth (4)单数名词连用表示一类事物 the dollar 美元 the fox 狐狸 或与形容词或分词连用,表示一类人 the rich 富人 the living 生者 (5)用在序数词和形容词最高级,及形容词等前面 Where do you live?你住在哪? I live on the second floor.我住在二楼. That's the very thing I've been looking for.那正是我要找的东西. (6)与复数名词连用,指整个群体 They are the teachers of this school.(指全体教师) They are teachers of this school.(指部分教师) (7)表示所有,相当于物主代词,用在表示身体部位的名词前 She caught me by the arm.她抓住了我的手臂. (8)用在某些有普通名词构成的国家名称,机关团体,阶级等专有名词前 the People's Republic of China 中华人民共和国 the United States 美国 (9)用在表示乐器的名词前 She plays the piano.她会弹钢琴. (10)用在姓氏的复数名词之前,表示一家人 the Greens 格林一家人(或格林夫妇) (11)用在惯用语中 in the day, in the morning... the day before yesterday, the next morning... in the sky... in the dark... in the end... on the whole, by the way... 英语写作中常用的词语和句型 常用谚语（在议论文中）： 1. As a popular saying goes, “Every coin has two sides”. 2. As a proverb says, “Everything has two sides”. On the one hand, physical exercise is good for your health. On the other hand, if you don’t deal with it properly, it wil l do harm to your health. So we should look at the matter from two sides. 3. As a proverb says, “Where there is a will there is a way.” (有志者事竟成。) 4. As a popular saying goes, “A man is known by the company he keeps.”(观其交友，知其为人。) 5. As is known to all, “No pains, no gains”. (没有苦，就没有甜。) 6. It’s no use crying over the spilt milk. (覆水难收。) 7. More haste, less speed. (欲速则不达。) 8. Look before you leap. (三思而后行。) 9. He who laughs last laughs best. (谁最后笑，谁笑得最好。) ---关于因果关系的句型： 高精度计算模板 注意：减法、除法要用到compare函数 乘法需要加法的部分，加法需要减法部分 #include string ADD_INT(string str1, string str2) { string MINUS_INT(string str1, string str2); int sign = 1; string str; if(str1[0] == '-') { if(str2[0] == '-') { sign = -1; str = ADD_INT(str1.erase(0, 1), str2.erase(0, 1)); }else { str = MINUS_INT(str2, str1.erase(0, 1)); } }else { if(str2[0] == '-') str = MINUS_INT(str1, str2.erase(0, 1)); else { string::size_type l1, l2; int i; l1 = str1.size(); l2 = str2.size(); if(l1 < l2) { for(i = 1; i <= (int)(l2 - l1); i++) str1 = "0" + str1; }else { for(i = 1; i <= (int)(l1 - l2); i++) str2 = "0" + str2; } int int1 = 0, int2 = 0; for(i = str1.size() - 1; i >= 0; i--) { int1 = (int(str1[i]) - 48 + int(str2[i]) - 48 + int2) %10; int2 = (int(str1[i]) - 48 + int(str2[i]) - 48 +int2) / 10; str = char(int1 + 48) + str; } if(int2 != 0) str = char(int2 + 48) + str; } } //运算后处理符号位 if((sign == -1) && (str[0] !='0')) str = "-" + str; return str; } ●I wonder if it’s because I have been at school for so long that I’ve grown so crazy about going home. ●It is because she wasn’t well that she fell far behind her classmates this semester. ●I can well remember that there was a time when I took it for granted that friends should do everything for me. ●In order to make a difference to society, they spent almost all of their spare time in raising money for the charity. ●It’s no pleasure eating at school any longer because the food is not so tasty as that at home. ●He happened to be hit by a new idea when he was walking along the riverbank. ●I wonder if I can cope with stressful situations in life independently. ●It is because I take things for granted that I make so many mistakes. ●The treasure is so rare that a growing number of people are looking for it. ●He picks on the weak mn in order that we may pay attention to him. ●It’s no pleasure being disturbed whena I settle down to my work. ●I can well remember that when I was a child, I always made mistakes on purpose for fun. ●It’s no pleasure accompany her hanging out on the street on such a rainy day. ●I can well remember that there was a time when I threw my whole self into study in order to live up to my parents’ expectation and enter my dream university. ●I can well remember that she stuck with me all the time and helped me regain my confidence during my tough time five years ago. ●It is because he makes it a priority to study that he always gets good grades. ●I wonder if we should abandon this idea because there is no point in doing so. ●I wonder if it was because I ate ice-cream that I had an upset student this morning. ●It is because she refused to die that she became incredibly successful. ●She is so considerate that many of us turn to her for comfort. ●I can well remember that once I underestimated the power of words and hurt my friend. ●He works extremely hard in order to live up to his expectations. ●I happened to see a butterfly settle on the beautiful flower. ●It’s no pleasure making fun of others. ●It was the first time in the new semester that I had burned the midnight oil to study. ●It’s no pleasure taking everything into account when you long to have the relaxing life. ●I wonder if it was because he abandoned himself to despair that he was killed in a car accident when he was driving. ●Jack is always picking on younger children in order to show off his power. ●It is because he always burns the midnight oil that he oversleeps sometimes. ●I happened to find some pictures to do with my grandfather when I was going through the drawer. ●It was because I didn’t dare look at the failure face to face that I failed again. ●I tell my friend that failure is not scary in order that she can rebound from failure. ●I throw my whole self to study in order to pass the final exam. ●It was the first time that I had made a speech in public and enjoyed the thunder of applause. ●Alice happened to be on the street when a UFO landed right in front of her. ●It was the first time that I had kept myself open and talked sincerely with my parents. ●It was a beautiful sunny day. The weather was so comfortable that I settled myself into the “tｈｅwａｙ+从句”结构的意义及用法 首先让我们来看下面这个句子: Ｒead the foｌloｗｉnｇpassagｅａnd talkａbout ｉt wi ｔh your classmaｔes．Try tｏｔeｌl whaｔyｏu thiｎk ｏf Tom aｎd ｏｆｔhe ｗay the chiｌｄrenｔrｅated ｈim. 在这个句子中，ｔhe waｙ是先行词,后面是省略了关系副词that或in whｉch的定语从句。 下面我们将叙述“the ｗaｙ+从句”结构的用法。 1.tｈe wａy之后，引导定语从句的关系词是tｈat而不是hoｗ,因此，<<现代英语惯用法词典＞＞中所给出的下面两个句子是错误的：Ｔhｉs is ｔｈeｗayｈowｉtｈａppened. This is the way how he ａlwａｙs treats me. 2．在正式语体中,thaｔ可被in which所代替;在非正式语体中，ｔhat则往往省略。由此我们得到theｗay后接定语从句时的三种模式：1) tｈe ｗaｙ＋ｔhａt-从句２）the way +ｉn whiｃh－从句3) the ｗay +从句 例如：Ｔｈe way(in whiｃh ，thａt) ｔhｅｓｅｃomｒade ｓloｏｋａｔｐrobｌems is wrong．这些同志看问题的方法 不对。 Ｔｈｅｗａy(that ,ｉn ｗhiｃh)yoｕ’ｒe doinｇit is comple ｔeｌy crａzy.你这么个干法,简直发疯。 Wｅａdmiｒｅd him for thｅｗay ｉｎｗhｉch he fａｃeｓdiｆficultieｓ． Waｌlａｃe ａｎd Ｄarwiｎｇreed ｏn the way ｉｎwhi ｃh ｄｉfｆｅrｅnt forms ｏf life had ｂegｕn.华莱士和达尔文对不同类型的生物是如何起源的持相同的观点。 Thｉs is ｔｈe ｗaｙ（tｈａｔ) hｅdｉd it. I lｉkeｄｔhe waｙ(ｔhat) ｓｈeｏｒganized ｔhe ｍeetｉng． 3.theｗay(tｈat)有时可以与hｏw(作“如何”解）通用。例如： That’s tｈe ｗaｙ(tｈaｔ) shｅspoke. = Tｈat’s how shｅspoke.on the contrary的解析
The way常见用法
英语写作30个最经典的替换词
数据结构与算法课程设计模板2018
英语造句
The way的用法及其含义(二)
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模板匹配算法
(完整版)the的用法
英语写作中常用的词语和句型
高精度模板(算法必备)
学生造句--Unit 1
“the way+从句”结构的意义及用法