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附錄一
澆鑄鈦和金的顯微結(jié)構(gòu)和機(jī)械性能
摘要：通過(guò)感應(yīng)熔化的方法而獲得的Ti21523合金，研究熱處理和冷卻凝固率對(duì)其顯微結(jié)構(gòu)和機(jī)械性能的影響和作用。結(jié)果表明：通過(guò)增加冷卻凝固率，可以使Ti21523合金的顯微結(jié)構(gòu)從單一化特征及大尺寸的粒狀結(jié)構(gòu)變成了具有優(yōu)良性能的小尺寸粒狀結(jié)構(gòu)。通過(guò)采用不同的方法和對(duì)不同時(shí)期的合金進(jìn)行處理，合金相位逐漸在粒狀晶體的內(nèi)部和粒狀晶體的邊界上沉淀。由于沉淀物晶相的改變，合金承受拉力的性能和伸長(zhǎng)率同時(shí)被改良。在σb=1. 406Gpa、δ=4. 5%時(shí)，將會(huì)獲得一種具有良好性能的合金,在臨界區(qū)域里使用這種合金會(huì)讓我們收到滿意的效果。
關(guān)鍵字 澆鑄Ti21523合金；冷卻凝固率；機(jī)械性能
1 介紹
鈦合金以其優(yōu)良的機(jī)械性能，在飛機(jī)、航空航天和其它領(lǐng)域中，受到了人們的關(guān)注和認(rèn)可，尤其是在較高特殊作用力的環(huán)境之下。在降低航天器的質(zhì)量并改進(jìn)的它的運(yùn)輸適宜性上，該合金受到了關(guān)注。為了滿足以上兩種情況，一種被叫做貝它鈦的重要鈦合金逐漸得到發(fā)展和優(yōu)化。由于其具有高抗力、彈性系數(shù)和伸長(zhǎng)率等良好的綜合性能，合金 Ti215V23Cr23Sn23Al(Ti21523) 已經(jīng)變成了潛在的選擇材料被用于在那些貝它類型合金之中。從以上的論述中我們可以知道，Ti21523合金在室溫有較好的可使用性，同時(shí)也適用于寒冷的工作環(huán)境之下。不幸地是，由于合金的高處理成本以及諸如低可塑性和高剛度等缺點(diǎn)，使其在制造復(fù)雜的聯(lián)合體和薄壁件時(shí)存在許多問(wèn)題，成為影響其在航空航天業(yè)中廣泛應(yīng)用的關(guān)鍵所在。為了降低其合成成本并達(dá)到其易于重新塑造的彈性，精密鑄造技術(shù)被引入到了這個(gè)領(lǐng)域中。但是由于鑄造出來(lái)的合金其貝它晶粒較大且機(jī)械性能很低，故此Ti21523合金的使用受到了極大的限制。由于熱處理對(duì)Ti21523合金的力有影響，因此Ti21523合金還是可以改善其伸長(zhǎng)率并提高它的機(jī)械性能的。關(guān)于熱處理對(duì)Ti21523合金的影響的研究首先在美國(guó)和前蘇聯(lián)開(kāi)展。他們指出：在熱處理之后，在阿爾法晶相的內(nèi)部和邊界上均出現(xiàn)了矩陣式的沉淀物，阿爾法相的出現(xiàn)與分布戲劇性地改善了合金的機(jī)械性能。這篇文章的目地就是要找出在不同的冷卻凝固率和熱處理?xiàng)l件下鈦合金的機(jī)械性能和微觀結(jié)構(gòu)的變化，以找到一個(gè)科學(xué)合理的方法來(lái)測(cè)量和進(jìn)一步提高合金的機(jī)械性能。
2 實(shí)驗(yàn)
實(shí)驗(yàn)的原料來(lái)自海棉狀的鈦,礬和鋁的合金，高純凈的鋁塊，鉻粉和錫塊。 然后他們?cè)谝黄鹪诟袘?yīng)爐里被融化，依照合金名義上的組成成份，其組成成份有15% V、3% Al、3% Cr、3% Sn，其余的全部是Ti。裝料的總重量是18千克。我們?cè)O(shè)置旋轉(zhuǎn)式噴灌器工作轉(zhuǎn)速為200轉(zhuǎn)/分，分布的溫度大約是1750℃。為了研究合金不同的冷卻凝固率對(duì)于其機(jī)械性能和微觀結(jié)構(gòu)的影響，熔化的合金離心后被澆注到一個(gè)長(zhǎng)235mm，寬100mm，厚度分別為50mm、25mm、10mm的一系列金屬模具內(nèi)。用于分析合金的微觀結(jié)構(gòu)和機(jī)械性能的樣品就來(lái)自于其中。熱處理的試樣在800℃下被加熱20分鐘，然后水冷。其他用空氣冷卻的試樣也是如此。合金的顯微結(jié)構(gòu)被放在高倍顯微鏡下和TEM機(jī)上進(jìn)行研究。拉伸試驗(yàn)后的物理斷面也被放在SEM機(jī)下進(jìn)行研究。它的機(jī)械特性是在Instron 1186電子拉伸機(jī)上進(jìn)行測(cè)試的。
3 結(jié)果及討論
3.1冷卻凝固率對(duì)合金微觀結(jié)構(gòu)的影響
合金在冷卻凝固后的微觀結(jié)構(gòu)如圖1所示.帶有少許氣體和熱力孔的等軸β晶粒在合金晶粒的內(nèi)部和邊界上均有分布。帶有黑色的第二幅圖被認(rèn)為是一個(gè)不平衡的冷卻凝固結(jié)構(gòu)。隨著冷卻凝固率的不斷增加，晶粒的尺寸變得越來(lái)越小。越靠近模型的內(nèi)表面晶粒的尺寸越小，越小的鑄造尺寸結(jié)果也是如此。這是因?yàn)樵谀Ｐ偷膬?nèi)表面以及較小的鑄造尺寸時(shí)，激冷作用對(duì)合金的晶粒尺寸并沒(méi)有多少不同之處
3.2冷卻凝固率對(duì)合金機(jī)械性能的影響
下表1列出了不同的冷卻凝固率對(duì)合金延展性的影響。隨著冷卻凝固率的增加，合金承受的拉力也隨之增加，與此同時(shí)，合金的延伸率也逐漸升高。延展率的增加主要?dú)w因于較小的晶粒尺寸。比較較薄的部分而言，中等厚度及較厚區(qū)域在延展性方面并沒(méi)有太大的不同之處。
3.3熱處理后的合金微觀結(jié)構(gòu)
在正常情況下，通過(guò)空冷和小冷的合金單β是可以區(qū)分開(kāi)的。經(jīng)過(guò)不同時(shí)期和不同方法的處理之后，針狀的α相出現(xiàn)在了晶粒的內(nèi)部以及邊界上，良好的拉伸和延伸率的結(jié)合是可以通過(guò)恰當(dāng)?shù)責(zé)崽幚韥?lái)達(dá)到的。圖2的（a）和（b）中展示了TEM假想的在450℃到650℃時(shí)加熱8小時(shí)后的合金顯微結(jié)構(gòu)。隨著溫度的逐漸升高，針狀的α相變得粗糙。α相與其基體之間存在著互不相干的相互關(guān)系。圖2（c）中顯示出α相析出于晶粒的邊界上，α相與晶粒邊界所成的角度估計(jì)30°。α相之所以很容易在晶粒的邊界上析出的原因可以歸結(jié)為較低的成核能量以及不完全處理后在晶粒的邊界上所產(chǎn)生的元素相互排斥作用。在晶粒的邊界上大量的析出α相將導(dǎo)致合金的脆性。圖3（a）和（b）中展示出了TEM機(jī)假想的在450℃下加熱6小時(shí)和24小時(shí)時(shí)的合金相圖。隨著加熱時(shí)間的增加，α相變得越來(lái)越粗糙。同時(shí)，α相也會(huì)部分的增加。圖3（）中展示出了TEM機(jī)假想的雙期處理后的合金的對(duì)比。雙期處理后的α相變得更加粗糙，在α相的析出物上有很長(zhǎng)一段的距離。
3.4熱處理后合金的機(jī)械性能
圖4（a）中表示了在不同的加熱溫度下加熱8小時(shí)后合金機(jī)械性能的變化。隨著加熱溫度的逐漸增加，伸長(zhǎng)率和屈服強(qiáng)度下降而伸展率增加。導(dǎo)致合金機(jī)械性能變化的主要原因是晶粒的大小，數(shù)量以及基體上的α相。隨著加熱溫度的不斷增加，α相變得越來(lái)越粗糙，從而導(dǎo)致了該相越來(lái)越容易析出并附著在原有的晶粒上。在進(jìn)行機(jī)械性能測(cè)試的過(guò)程中，α相最終導(dǎo)致了合金的低強(qiáng)度和高的延展性。當(dāng)加熱溫度為450℃時(shí)，等于1.406GPa,當(dāng)加熱溫度為650℃時(shí)，等于905MPa。對(duì)比于強(qiáng)度而言，合金延展率的變化有不同的傾向，延展率從4.5%（450℃）變?yōu)?4.4%（650℃）。
圖4（b）中展示出了在450℃下加熱不同時(shí)間后合金機(jī)械性能的變化。隨著加熱時(shí)間的不斷增加，合金的伸長(zhǎng)率和屈服強(qiáng)度稍有增加而延伸率卻下降了。隨著加熱時(shí)間的繼續(xù)增加，析出物的距離變得特別小，從而很難再析出，以致于導(dǎo)致了在測(cè)試的過(guò)程中，金屬的強(qiáng)度升高而延伸率下降。
圖5顯示出了在450℃和650℃下加熱8小時(shí)后的合金破碎形態(tài)。結(jié)果顯示出破碎是在內(nèi)部出現(xiàn)的表現(xiàn)出漣漪的特性。雖然破碎是內(nèi)部的微粒，但是相對(duì)較小的微粒尺寸也許是高延展性的最好解釋，合金二期加熱后，強(qiáng)度增加而延展性下降，下降到了386兆帕。延展率提高到3.5個(gè)百分點(diǎn)，二期處理之后，析出物之間較長(zhǎng)的距離是導(dǎo)致合金低強(qiáng)度和高延展性的主要原因。
3.5熱處理后冷卻凝固率對(duì)合金機(jī)械性能的影響
圖6顯示出了熱處理之后合金中等厚度和較厚部分的機(jī)械性能，試樣在800攝氏度下完全處理20分鐘之后在不同的溫度下加熱8小時(shí)。隨著加熱溫度的逐漸增加，中等厚度和較厚區(qū)域合金的拉伸力下降而延展率升高，總的來(lái)說(shuō)，合金中部的伸長(zhǎng)率和延展率都比較厚部分的高，但是當(dāng)溫度厚度部分高于510攝氏度時(shí)加熱8小時(shí)后，合金中等厚度部分的伸長(zhǎng)率和延展率卻都比較厚部分的低。
這個(gè)不期望的后果可以由晶粒從邊界向晶粒內(nèi)部逐漸混合，從而導(dǎo)致了內(nèi)部應(yīng)力起作用而獲得解釋
σ s= σ i+ kL d- 1/2 (1)
其中σ i是斷層混亂運(yùn)動(dòng)中磨擦力的反作用力。
kL是一個(gè)常量，d是晶粒直徑。
晶粒內(nèi)部應(yīng)力可以由Ashby的Orowan公式來(lái)描述。
τ=Gb/2 π( D- l) ln l/ r0 (2)
其中G是剪切模量，b是伯格斯向量，r0=4b,D是析出物之間的距離，L是析出物的厚度。當(dāng)考慮到拉伸屈服作用力的時(shí)候，我們便可以推斷出多晶材料時(shí)：
=1/2 σ
晶粒邊界應(yīng)力和內(nèi)部應(yīng)力的混合作用關(guān)系式可以表示為：
σ s= σ i+ kL b- 1/2+ Gb/ π( D- l) ln l/ r0(3)
增加晶粒的尺寸將會(huì)導(dǎo)致屈服強(qiáng)度的下降，但同樣可以導(dǎo)致在晶粒內(nèi)部的析出物的密度變大，從而使析出物之間的距離減小，比較較小尺寸的晶粒而言，較大尺寸的晶粒在晶粒內(nèi)部的析出物在對(duì)伸長(zhǎng)率的影響與作用上占有優(yōu)勢(shì)。當(dāng)在510攝氏度下加熱8小時(shí)后，大尺寸晶粒的伸長(zhǎng)率在晶體內(nèi)部的析出物中要遠(yuǎn)遠(yuǎn)超過(guò)小尺寸的晶粒。
4 結(jié)論
（1）凝固后的合金的微觀結(jié)構(gòu)是由各方等大的β晶粒和在晶體邊界和內(nèi)部的一些氣泡和熱力孔所組成。隨著冷卻凝固的增加，合金的晶粒尺寸變小，伸長(zhǎng)率和屈服強(qiáng)度增加。同時(shí)，合金的延展率升高。
（2）隨著加熱溫度的升高和加熱時(shí)間的增加，針狀的相變得粗糙。同時(shí)，相的碎片數(shù)量也隨之增加，二期處理后相變得更加粗糙。
（3）隨著加熱溫度的升高，伸長(zhǎng)率和屈服強(qiáng)度下降而延展率升高，隨著加熱時(shí)間的升高，伸長(zhǎng)率和屈服強(qiáng)度稍有升高而延展率下降而伸長(zhǎng)率下降。
（4）總體來(lái)說(shuō)，中等厚度部分的合金的伸長(zhǎng)率和延展率均比較厚部分的高，各項(xiàng)性能的最好結(jié)合是在等于1.406GPa,σ為4.5%時(shí)獲得的。它可以滿足臨界領(lǐng)域這種合金的使用要求。
附錄2
Microstructure and mechanical properties of high strength as cast Ti21523 alloy
Abstract：The effects of heat treatment and solidification cooling rate on the microstructure and mechanical properties of as cast Ti21523 alloy prepared by induction skull melting method were investigated. Results show that the microstructure of as2cast Ti21523 alloy changes from the features of simplified and larger size of beta grains to finer grain size with increasing solidification cooling rate. After solution treatment and different ageing treatment, alpha phase precipitates in grains interior as well as in grain boundaries. Due to the modification of the precipitate phase, the tensile strength and elongation of the alloy are improved simultaneously. A good combination of the values of of and 4. 5 % of was obtained , which will be satisfied the use of this kind of alloy in critical areas.
Key words：cast Ti21523 alloy; solidification cooling rate; mechanical properties
1 INTRODUCTION
Titanium alloys have received appreciated attentions in the fields of aircraft, aerospace, and others owing to their excellent mechanical properties, especially the high specific strength. With regards to lower the mass of aircraft and improving their suitability for transportation , an important class named beta titanium alloys are developed to meet the requirement of the above situations[1 ,2 ] . As the result s of good properties combination of high eremitic strength, elastic modulus and elongation, the alloy Ti215V23Cr23Sn23Al (Ti21523) has become a potentially selective material to be used among those beta type alloys [3] .From Ref. [4], it is known that the alloy Ti21523 has good workability at room temperature and suitable for cold working. Unfortunately, the high processing cost and drawbacks of low plasticity and high deformation force of the alloy have made it difficult to produce complex and thin walled components that are being the keynotes for aero applications [5]. In order to reduce the processing cost and reach the flexibility of shaping Ti21523 alloy, the technique of precision casting has been involved in the field. But due to the large beta grain size and lower mechanical properties under casting condition, the usage of the as2cast Ti21523 alloy is limited. Because of the strengthen effects of heat treatment on the beta type titanium alloys, the Ti21523 alloy can somewhat be strengthened to the extent of high level of the mechanical properties. The investigations on the effect s of heat treatment on titanium alloys have been carried out by America and the former Soviet Union [6, 7]. As it is pointed out that after heat treatment, the matrix precipitates alpha phase in grain interior and at grain boundaries as well. The appearance and distribution of alpha phase improve the mechanical properties of the alloy dramatically [8]. The purpose of this article is to investigate the effect of different solidification cooling rates and heat treatment on the microstructure and mechanical properties of the alloy in order to find an efficient measurement to further improve the mechanical properties of the alloy.
2 EXPERIMENTAL
The experimental raw materials came from spongy titanium, vanadium aluminium master alloy, high purity aluminum block , chrome powder and tin block. Then they were melted in an induction skull melting furnace according to the nominal composition of the alloy which composed of 15 %V, 3 %Al, 3 %Cr, 3 %Sn, and the balance Ti. The total mass of the charge was 18 kg. The pouring parameters were set as the speed of 200 r/ min for rotating table and the pouring temperature of about 1750 ℃. In order to study the effect of different solidification cooling rates on the solidification microstructure and mechanical properties of the alloy , the molten alloy were centrifugally pouring into a step metal mould with the gauge of 235 mm in length , 100 mm in width , and 50 mm , 25 mm , and 10 mm in thickness respectively. The samples for the analyses of microstructure and mechanical properties of the alloy came from the step specimen. The samples for heat treatment was solute treated at 800 ℃for 20 min and then water cooling as well as the treatment of different ageing temperatures and times with air cooling. The microstructure of the alloy was studied with optical microscope and TEM. The morphology of fractures after tensile test was also investigated by SEM. The mechanical properties were tested in model Instron 1186 electric tensile machine.
3 RESULTS AND DISCUSSION
3. 1 Effect of solidification cooling rate on microstructure of alloy The microstructure of the alloy after solidification is shown in Fig. 1. The equiaxed beta grain is found with a few of gas and shrinking holes in grain interior and at grain boundaries as well. The secondary phases with black color were confirmed as a non equilibrium solidification structure. With increasing solidification cooling rate , the grain size becomes smaller. The grain size is small where attached to the inner surface of mould or positions with smaller casting size because of the action of chilling effect of mould inner surface and smaller casting size on the alloy. Compared with the thin section, the middle and thick section have less difference in grain size.
3. 2 Effect of solidification cooling rate on mechanical properties of alloy Table 1 has shown the effect s of different solidification cooling rates on the tensile properties of the alloy. With increasing solidification cooling rate , the tensile strength of the alloy increases. At the same time the elongation of the alloy is improved. The increase of strength and elongation attributes to small grain size [9]. Compared with the thin section, the middle and thick section has less difference in tensile properties.
3. 3 Microstructure of alloy af ter heat treatment
At the normal condition the single beta microstructure for the alloy can be obtained by air cooling and water cooling. After solution treatment and different ageing treatments, acicular alpha phase is observed at grains interior as well as in grain boundaries. A good combination of strength and elongation can be achieved with proper heat treatment .Figs. 2 (a) and (b) show TEM image of the alloy aged at 450 ℃ and 650 ℃ for 8 h. With increasing ageing temperature, the acicular alpha phases become coarse. There exist s incoherent corresponding relationship between the alpha phase and matrix [10]. Fig. 2 (c) shows alpha phases precipitate at grain boundaries. The angles between the grain boundaries and alpha phases were estimated about 30°. The reasons for the easy precipitation of alpha phases at grain boundaries can be interpreted as low nucleation energy and the segregation of alloying element at grain boundaries due to the inadequate solution treatment. The large quantity of precipitates at grain boundaries was responsible for the brittleness of the alloy. Figs. 3 (a) and (b) present TEM image of the alloy aged at 450 ℃for 6 h and 24 h. With increasing ageing time, the alpha phases become coarser. The volume fraction of the alpha phase increases simultaneously. Fig. 3 (c) present the comparison of TEM image of the alloy with duplex ageing treatment. The alpha phases became coarser with a longer distance between alpha precipitates after duplex aged.
3. 4 Mechanical properties of alloy after heat treatment Fig. 4 (a) demonstrates the change of mechanical properties of the alloy aged at different temperatures for 8 h. With increasing ageing temperature , the tensile strength and yield strength decrease and elongation increases. The direct reasons for the change of mechanical properties of the alloy is the size, quantity and distribution of alpha phases in the matrix. With raising ageing temperature, the alpha phases become coarse that leads to the more easiness for dislocations to cross the precipitation during the test process so that the strengthening action of the alpha phase lessens which will cause the lower strength and higher elongation of the alloy. When the ageing temperature is 450 ℃, σb equals to 1. 406 GPa. While the ageing temperature is 650 ℃,σb equals to 905 MPa. Compared with the strength, the elongation of the alloy has different change trends. The elongation is from 4. 5 % (450 ℃) to 14. 4 % (650 ℃) .
Fig. 4 (b) demons rates the change of mechanical properties of the alloy aged at 450 ℃ for different times. With increasing ageing time, the tensile strength and yield strength increase a little with the decrease of elongation. With prolonging the ageing time, the distance between the precipitations become nearly that leads to the more difficulty for dislocations to cross the precipitations during the test process, which cause the higher strength and lower elongation.
Fig. 5 shows the fracture morphology of the alloy aged at 450 ℃and 650 ℃for 8 h. It has shown that the fracture is inter granular characterized by dimples. Although the fractures are inter granular , the relatively smaller grain sizes maybe the better explanation of the high elongation. When the alloy is duplex aged, the strength decreases with increase of elongation. σb has been cut down by 386 MPa. The elongation has been improved by 3. 5 %. The longer distance between the precipitations after duplex aging is responsible for the lower strength and higher elongation.
3. 5 Effect of solidification cooling rate on mechanical properties of alloy after heat treatment
Fig. 6 presents s the mechanical properties of the alloy which locate in the middle and thick section after heat treatment. The samples were solute treated at 800 ℃for 20 min and then ageing at different temperatures for 8 h. The tensile strength of the alloy from both the middle and thick section decreases with the increase of elongation when the ageing temperature increases. As a whole, the tensile strength and elongation of the alloy from middle section are higher than that from the thick section. But aged at or above 510 ℃for 8 h , the tensile strength of the alloy from middle section is lower than that of the thick section. This unexpected behavior can be explained by means of model which incorporate the contribution of the grain boundaries to and the grain interior to the tensile strength [8].The contribution of the grain boundaries to σs can be expressed by the well known Hall Petch relationship
σs =σi + kL d - 1/ 2 (1)
Where σi is the friction stress opposing to dislocation motion , kL is a constant , and d is the grain diameter.
The contribution of the grain interior to precipitates (τ) maybe expressed by the Orowan equation modified by Ashby[11] .
τ= Gb/ 2π( D - l) ln l/ r0 (2)
Where G is the shear modulus , b is the burgers vector and r0 = 4 b , D is the distance between precipitates , l is the thickness of the precipitates. When considering the tensile yield strength it is possible to assume that for a polycrystalline material τ= 1/ 2σ.The combined contribution of grain boundaries and the precipitates in the grain interior is
σs =σi + kL b - 1/ 2 + Gb/π( D - l) ln l/ r0 (3)
Increasing the grain size would decrease the yield strength, but would also cause larger precipitate density in the grain interior , which result in a smaller distance between precipitates. Compared with the small grain size , the contribution of precipitates in the grain interior to the tensile strength is the dominating factor for the large grain size. When aged at or above 510 ℃for 8 h , the tensile strength of large grain size has exceeded the small one due to the relatively stronger strengthening action of the precipitates in the grain interior.
4 CONCLUSIONS
1) The microstructure of the alloy after solidification is the equiaxed beta grain with a few of gas and shrinking holes in grain interior and grain boundaries as well. With increasing solidification cooling rate, the size of grain becomes smaller, the tensile strength and yield strength of the alloy increase. At the same time , the elongation of the alloy is improved.
2) With increasing ageing temperature and also prolonging of ageing time, the acicular alpha phase becomes coarse. The volume fraction of the alpha phase increases simultaneously. The alpha phase becomes coarse after duplex aged.
3) With increasing ageing temperature, the tensile strength and yield strength decrease and the elongation increases. With increasing ageing time the tensile strength and yield strength increase a little and the elongation decreases. After the alloy is duplex aged, the strength decreases and the elongation increases.
4) The tensile strength and elongation of the alloy from middle section are higher than that from the thick section in general. A good combination of the values of 1. 406 GPa of σb and 4. 5 % of δ are obtained , which will be satisfied the usage of this kind of alloy in critical areas.
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