متالورژي سمنان


عوامل مؤثر در حلاليت

Factors Affecting Solubility  عوامل مؤثر در حلاليت 

There are four major factors that determine whether a solid solution will indeed arise between two metals. The foremost is the atomic size factor. If a foreign atom is substituted into a lattice, there will naturally be a distortion of the lattice. This will cause the formation of a new phase. Well, more closely in size this atom is to the lattice atoms, the easier it will be to assimilate. Research has shown that the atomic sizes of the two components have to be within 15% to get complete solubility. However, size compatibility does not entail solubility. There are other forces at work here. The elements have to have similar electrochemical factors. The more electropositive one element and the more electronegative the other, the more there will be a tendency to form an intermetallic compound instead of a solid solution. The third factor of relevance is the relative valency of the two elements. The higher the valency of one component, the more likely will it tend to dissolve in the other component. And finally, the two potential solid solution members need to have the same crystal structure for complete solubility.

 

شرايطي كه دو عنصر در حالت جامد در تمام نسبتها در يكديگر محلولند عبارتند از :

1) شرط اوليه براي حلاليت كامل كريستال به صورت جانشيني ، داشتن شبكه كريستالي يكسان است.

2) اختلاف واحد شبكه يا شعاع اتمي يا يونـــــــي انها در حلاليت به صورت جانشيني بايد تا حد امكان كوچك بـــاشد.(حداكثر اختلاف15درصد)

 3) داشتن ظرفيت يكسان :

4 ) داشتن الكترو نگاتيويته يكسان نيز حلاليت اتمها را در هم مناسبتر مي كــــند.

بهترين مثال در مورد حلاليت كامل دو فلز در حالت جامد و  از نظر صنعتي ، حلاليت مس و نيكل و از نظر فلزات قيمتي طلا و نقره است ، of measuring solubility is to look at dHm, the enthalpy of mixing. The more positive this quantity is, the less ideal the solution. The diagram below shows a progression from ideal to less ideal solutions and what the corresponding diagrams look like as the dHm increases.

After that refreshing meal of metallurgical wisdom, we are ready to look at the most prominent type of phase diagram.

Eutectic Reactions   واكنش يا تحول يوتكتيك

 

در جريان سرد شدن مذاب ، اگردر درجه حرارت مشخصي يك فاز مذاب به دو فاز جامد تبديل شود و يا در جريان گرم كردن دو فاز جامد به يك فاز مذاب تبديل گردند تحول يوتكتيك صورت كرفته است .

Above is the typical eutectic diagram. At first, it may look formidable, but fear not, for we shall tame the roynish scoundrel. As you can see, we have all the regions common to a complete solubility system. Now, it is common to call the terminal solution on the left side the alpha and the one on the right the beta. And so it shall be. So we have the alpha, the beta, and the liquid phase. Thus the term three-phase equilibria. We also have the alpha + beta, alpha + L, and beta + L. The liquidus line in a binary eutectic is the curve Ta-E-Tb. The solubility curves ac and bd delimit the solid phases (a.k.a. the solvus curves). The solidus curve is the line Ta-a-E-b-Tb. Note that the solubility curves slope inward, in accordance with the fact that solubility increases with temperature.

Now lets consider what happens when a solution of certain composition cools. The simplest case is that of a compound of eutectic composition, denoted by line 1 on the diagram. The eutectic reaction occurs at Te, the eutectic temperature, and is denoted by:

L -> alpha + beta


where L is the liquid phase. As the compound cool, it will reach Te. Here, the eutectic reaction will proceed until all of the liquid has been consumed. At this point, the alpha and beta will be in equilibrium with each other. Now the resulting solid will cool, where the relative compositions of the phases will be controlled by the solubility curves ac and bd. Remember the lever rule and tie lines and all that? It is explained elsewhere in the analytic section, Interpretation of phase diagrams by that hardworking Chris Lattin, our Analytic Group leader.

Now, a eutectic mixture will have a characteristic microstructure. These structures are called lamellar, globular, rod and acicular, depending on the appearance of the two phases (see main analytical page). However, this classification is somewhat misleading, since it has been shown that the structure of the eutectic composition is dependent upon the conditions of solidification as well as the effect of impurities in the liquid. The solidification will initiate with the nucleation of one of the phases. This will cause the other phase to be rejected at the interphase of the first phase with the liquid. At some further point, the other phase will begin to nucleate. In the case of a lamellar eutectic, the second phase nucleates on the first phase, thus creating the lamellar growth which give it its name.

 

The diagram has been repeated above for convenience. This is a friendly page, afterall. Now we come to the case of hypo-eutectic alloys. These are ones whose Xb component is less than the eutectic composition. There are three different cases worth noting, denoted by 2a, 2b, and 2c. Lets look at 2a. This is the same case as for a two-phase system. The solid starts to nucleate when the cooling crosses the liquidus line and when it crosses the solidus line, there is no more liquid. So we get the regular grain structure of a metal. The 2b case is more interesting. It begins in the same way, freezing as an alpha solid solution. When it crosses the ac solvus line, however, the beta phase starts to deposit from the alpha phase. The beta phase can precipitate at the alpha grain boundaries, at certain crystallographic planes of the alpha phase, or around inclusions and dislocations. the final case is illustrated by 2c. Here, upon crossing the liquidus line, the alpha phase begins to deposit. When the composition reaches the eutectic temperature, the remaining liquid undergoes the eutectic reaction. The alpha phase is then present as primary (that fraction present before the eutectic reaction) and as one of the phases in the eutectic mixture.

Hyper-eutectic alloys are ones where the Xb composition is greater than the eutectic composition. They behave in a similar way to hypo-eutectics, the only change being that now the dominant phase is the beta phase. And thats all I have to say about that.

 

Actually, no its not. One more point will be made. Above is a representation of the evolution of the limiting form of a eutectic diagram. The diagram on the right is an impossibility, since it implies complete immiscibility. it also implies a melting range of Ta to Te for the alpha phase. Thus such a diagram cannot be. The complete immiscibility of two metals does not exist.

 

Peritectic Reactions   واكنش(تحول) پريتكتيك 

 

هر گاه در جريان سرد شدن آلياژي ، يك فاز جامد به همراه يك فاز مايع ، تبديل به يك فاز جامد متفاوت با فازجامد قبلي گردد ويا برعكس در جريان گرم كردن يك فاز جامد تبديل به يك فاز مايع و يك فاز جامداز نوع ديگر گردد تحول پريتكتيك صورت گرفته است.

 

Above is the picture of a typical phase diagram incorporating a peritectic reaction. We basically have all the same regions as in a eutectic. The only difference is that there is no eutectic reaction. instead we have a peritectic reaction:

L + alpha -> beta

 

دردياگرام آهن و سمانتيت تحول پري تكتيك در دماي 1493 درجه سانتيگراد صورت ميگيرد كه دراثر آن ،( در جريان سرد شدن ) يك مذاب به همراه يك جامد  ( آهن دلتا ) ، تبديل به اوستنيت كه خود جامدي است متفاوت با آهن دلتا تبديل ميگردد .


Instead of a eutectic temperature, we have the peritectic temperature, located at Tp in the diagram. P is called the peritectic point. Here we have three types of alloy, just like in the eutectic system. These are alloys of the peritectic composition, Xb = P, alloys with Xb<P, and as the clever reader might have guessed, alloys with Xb>P. The terms hypo- and hyper- can be used, but usually these are reserved for the eutectic case.

Let us examine the elusive conundrum of the peritectic reaction. Going down line 1 on the diagram, we cross the liquidus curve Ta-b-Tb. The alpha phase starts to separate and increases in fraction according to line Ta-a. When Tp is reached, the peritectic reaction occurs. When the reaction is completed, all the liquid has been consumed and all the alpha in the solution has been converted to the beta phase. So now we have 100% beta solution. As the solution cools further, the alpha phase begins to precipitate back from the beta phase. Unearthly, would you not say? had the curve Pd been sloping to the left, the solution would remain 100% beta.

Alloys with Xb<P are only interesting for the case a<Xb<P. These alloy undergo the peritectic reaction. As the liquid cools, it forms the alpha phase until it hits Tp. Then all the liquid reacts with some of the alpha solid to form the beta phase. Thus, after the peritectic reaction is complete, consists of the leftover primary alpha phase together with the newly formed beta.

For alloys with Xb>P, the same thing applies. For P<Xb<b, the peritectic reaction occurs. In these cases, there is an excess of the liquid phase over that required to form the necessary amount of beta. That is the only difference.

A common type of peritectic diagram is one with an intermediate phase. This intermediate phase has a composition between those of the terminal solutions. It usually as a different crystal structure which differs from the components. Also, there is usually a eutectic reaction between this phase and the beta phase. These phases can be classified into three broad categories:

  1. normal valency compounds: these are phases that obey the valency rules and have much in common with ionic compounds
  2. size factor compounds some phases have structures which are determined by the relative sizes of the constituent atoms
  3. electron compounds: with similar electrochemical properties and size factors, the composition occurs at one of three specific ratios. These are 3:2, , 7:4
Limited Solubility in the Liquid Phase  حلاليت محدود در فاز مايع 

موقعي آلياژ خوب توليد مي شود كه دو فلز در حالت مذاب بخوبي در يكديگر حل شده باشند كه يك مايع كاملا يكنواخت بدست بياييد . البته استثناء وجود دارد مثلا وجود ذرات سرب در آلياژهاي زود تراش لازم است تا ماشينكاري را آسانتر كند ، مذاب فلزاتي مثل مس و سرب هيچگونه انحلالي در حالت مذاب ندارند ضمن اينكه بايد دانست وقتي مذاب فلزاتي كه در حالت مذاب در همديگر حل ميشوند دليلي ندارد كه در حالت جامد نيز كاملا در همديگر محلول باشند بنابر اين چند حالت ممكن است پيش آيد.

1-    دو فلز در حالت جامد نيز صد در صد بصورت محلول باقي بمانند .

2-    دو فلز حلاليت محدود در همديگر داشته باشند و بقيه بصورت ذرات ريز يا درشت در بين دانه ها رسوب كنند.

3-    دو فلز از حالت انحلال بدر آمده و به دو صورت فلز خالص منجمد شوند.

4-    دو فلز از لحاظ شيمياييبر يكديگر اثر كرده و توليد تركيب فلزي نماييند.

It may indeed happen that in the liquid phase, we have partial immiscibility. If you ever mixed milk and motor oil, you know what I am talking about. Well, the same thing can happen with metals. Above is the diagram of a progression of the appearance of a phase diagram as the dHm, the enthalpy of mixing of the liquids gets more positive.

 

 

Monotectic Reactions  واكنش منو تكتيك 

در تحول منوتكتيك يك مذاب در جريان سرد شدن به يك جامد ويك مذاب از نوع متفاوت با مذاب قبلي خواهد شد ويا برعكس ، يعني هرگاه در جريان گرم شدن يك مذاب به همراه يك جامد در درجه حرارت معيني تبديل به يك مذاب كه از نظر فازي متفاوت با مذاب قبلي باشد گردد تحول منوتكتيك صورت گرفته است ، به مثال زير توجه كنيد .

Above is a diagram of a typical system with a monotectic reaction. The monotectic reaction is:

L1 -> alpha + L2

 

In this reaction, a liquid decomposes into a solid phase and a new liquid phase. Above the monotectic temperature Tm, we have two liquid phases. From the Tm, to Tc, the critical temperature, the two liquids are immiscible and will separate as two layers. Below the monotectic temperature, there is usually a single solid phase in equilibrium with a liquid phase. Examples of this are the Ga-Pb and Ga-Tl systems.

The monotectic alloy goes through point M in the above diagram. At Tm, it undergoes the monotectic reaction. Once again, we see similarities to the eutectic reaction. As the solution passes point M, the alpha phase starts to nucleate. The L2 phase appears as well, dispersed among the alpha matrix. When the solution reaches the Te temperature, the L2 liquid solidifies into the beta phase

 

Syntectic Reactions  تحول يا واكنش سنتيكتيك 

درتحول سنتيكتيك دو فاز مايع در جريان سرد شدن در درجه حرارت معيني تبديل به يك فاز جامد مي گردند ويا برعكس يك فاز جامد در هنگام گرم شدن در درجه حرارت بخصوصي تبديل به دو فاز مايع مي گردد ، در نمودار ذيل دو تحول اتكتيك و يك تحول سنتيكتيك نشان داده شده است .

 

Above, a syntectic reaction lurks within the diagram. This is a rare beast among alloy systems, occurring in systems like K-Zn, Na-Zn, K-Pb, Pb-U and Ca-Cd. The pertinent reaction is:

L1 + L2 -> alpha

 

In a syntectic system, the alpha phase is an intermediate phase formed when the two liquids freeze, for a specified composition, the syntectic composition.

Solid State Reactions  تحولات در حالت جامد 

There are three major solid state reactions that arise:

سه تحول عمده ومهمي كه در حالت جامد صورت ميكيرد عبارتند از :

1-   تحول يوتكتوئيد ، اين تحول هنگامي صورت ميگيرد كه يك فاز جامد در هنگام سرد شدن در درجه حرارت خاصي تبديل به دو فاز جامد از نوع ديگر گردد ويا دو فاز جامد در هنگام گرم شدن در درجه حرارت خاصي تبديل به يك فازديگر شوند بهترين مثال در اين مورد تبديل اوستنيت  به پرليت (سمانتيت و فريت ) در دياگرام آهن و كربن است كه در درجه حرارت 727 درجه سانتيگراد صورت ميگيرد.

2-   تحول منوتكتيك ، در اين تحول يك فاز در جريان سرد شدن در درجه حرارت خاصي به دو فاز جامد كه يكي از آنها با فاز قبلي تفاوت چنداني ندارد و فقط  در نظم و يا بي نظمي متفاوت است تبديل ميشود.

3-   تحول پري تكتوئيد ، پري تكتوئيد تحولي است كه در آن دو فاز جامد در جريان سرد شدن تبديل به  يك فاز جامد ديگر شود و يا برعكس ،

  • The eutectoid: alpha -> beta + gamma                       يوتكتوئيد
  • The monotectoid: alpha1 -> beta + alpha2         منوتكتوئيد
  • The peritectoid: alpha + beta -> gamma           پري تكتوئيد        

 

These are exactly as the previously mentioned reactions, except now the liquid phase has been replaced by a solid phase. The only difference is in the kinetics of the reactions, the solid state reactions are slower than their liquid counterparts.

An additional factor must be considered in the free energy calculation. This is the Gstrain, the free energy of straining that occurs as a result of changing crystal structures and lattice parameters. Thus the reaction will occur if -dG.bulk > +dG.interface + +dG.strain.

That completes our tour through the phase equilibria zoo. Yet do not succumb to thanatopsical threnodies, do not sink into lachrymal elegies, for this is not the end. Here's something extra..

 

انواع تركيبهاي آلياژي

1-  تركيب والانسي يا ظرفيتي :

اين نوع تركيبات از قانون شيميايي پيروي شده و يك فلز با خاصيت فلزي زياد با فلز ديگري كه داراي خاصيت فلزي كم و در مرز شبه فلزات قرار دارد تركيب مي شود.مانند:

Mg3Bi-Mg3Sb-Mg2Sn – Mg2Si

 

2      - تركيبهاي بين نشيني (بين شبكه اي ) :

تركيبهاي بين نشيني بين فلزات انتقالي و اتمهاي كوچك مانند هيدروژن ، كربن ، بر ، اكسيژن ، نيتروژن تشكيل ميشود و تفاوت اين تركيبات با محلولهاي بين نشين در اين است كه محلولهاي جامد بين نشين با دامنه بسيار وسيعتري تشكيل مي شوند در حاليكه تركيبهاي بين نشين دامنه بسيار محدودي دارندبطوري كه ميتوان آنها را بوسيله فرمول شيميايي نشان داد.مانند،

W2C   Fe3C Fe2N CrN

اصولا اين تركيبات سختي زياد ، نقطه ذوب بالا و قابليت تورق كمي دارند.

 

 

 

 3- تركيبهاي الكتروني:

  تركيبهاي الكتروني  با نسبت هاي معيني مانبا هم تركيب ميشوند و اختلاف اندازه اتمهاي سازنده اين نوع آلياژها از 15% تجاوز نمي كند.

Cu9Al4 –Cu3Al – Cu5Zn8 – Cu3Sn

( نسبت اين تركيبها از تقسيم تعداد الكترون هاي ظرفيت به تعداد اتم ها به دست مي آيد.)

Cu=1  Zn=2  Al=3  Sn=4 الكترون ظرفيت ،

 به مثال توجه كنيد.

در تركيب الكتروني Cu3Sn   تعداد الكتونهاي ظرفيت  7=(4*1) + (1*3) خواهد بود و تعداد اتمهاي اين تركيب نيز  4=1+3 مي باشد بنابر اين تركيب الكتروني با نسبت 4 به 7 تشكيل شده است.

 

نمودارهاي تعادلـــــي سيستم دو عنـــصري ( دو جـزئي)

با استفاده از نمودارهاي تعادلي مي توان پيش بيني كـــرد كه در يك الياژ يا درصد تركيب (غلظت)

معين در هر درجه حرارت چه فازهايي در حال تعادل است .

و همچنين به كمك ان نمودارها مي توان درصد تركيب شيميايي و مقدار هر فاز را معـــــين كــــرد

اينكه چه فازهايي در يك الياژ و درجه حرارتي بستگي دارد

كاربرد نمودارهاي تعادلي

1) غلظتي را كه در ان مخلوطي از اجزاي سازنده الياژ ، بالاترين و پايين ترين نقطه ذوب را دارد.

2) محدوده حرارتي را كه يك ماده مي تواند حرارت داده شده رابدون اينكه فعل وانفعالات

از نوع كريستال به نوعي ديگر در حالت جــــامد ، تجزيه ويا جدايش در ان انجام گيرد.

3)تعداد و نسبتهاي حجمي فازهايي كه در غلظتهاي معيني شكل ميگيرند.

4) غلظتهايي كه براي الياژهاي ريخته گري مناسب مي باشد.

5) امكان تشكيل فازهاي جــــديد.

6)امكان شكل گيري فاز در درجه حرارت هاي پايين در هنگام ســــرد شدن
نعمت الله طاهری