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Méthodes et techniques d'étude par Métallographie [+ traduction des mots clés]

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Metallography [= métallographie]
 
Metallography is the science of revealing and evaluating the internal structures of materials. It is one of the most important methods of materials research today, indispensable to the scientist as well as to the engineer.
 



More recently, advanced materials [= nouveaux matériaux] such as high tech metal alloys, ceramics, composites and polymers have appeared which made metallography become an increasingly vital part of modern industry.
 
Through a process of cutting [= découpage], mounting [= enrobage], grinding and polishing [= polissage], a smooth surface [= surface polie] is obtained to reach the materials true structure. In order to achieve high preparation quality and reproducible results; a combination of Right equipment, Correct preparation method and Right consumables, is required.
 

CUTTING [= découpage]
 
The first step in preparing a specimen for metallographic or micro structural analysis is to locate the area of interest. Sectioning or cutting is the most common technique for obtaining this area of interest. Proper sectioning guarantees minimal micro structural damage. Excessive subsurface damage and damage to secondary phases [= secondes phases] (e.g. graphite flakes, nodules or grain pull-out) should be avoided.
 



Sectioning can be categorized into two areas: Abrasive Cutting [= decoupage par abrasion] and Precision Wafer Cutting [= decoupage de précision]. Abrasive cutting is generally used for metal specimens and is accomplished with silicon carbide [= carbures de silicium] or alumina abrasives [= grenailles d’alumine] in resin or resin rubber bonds. Proper blade selection is required to minimize burning and heat generation during cutting which degrades both the specimen surface as well as the blade cutting efficiency. Precision wafer cutting is accomplished with thin diamond blades [= particules diamantées]. Wafer cutting is especially useful for cutting ceramics and minerals as well as some metallic materials.
 
ABRASIVE CUTTING [= Découpage abrasif]
 
Sample preparation starts with cutting and good cutting means a good start. Abrasive cutting is primarily used for ductile materials. Examples include metals, plastics, polymer matrix composites, metal matrix composites, etc.. Proper selection of abrasive blades requires an understanding of the relationship between the abrasive particle, abrasive bonding and the specimen properties.
 
Selecting the right cut-off wheel ensures freedom from burn and distortion and is the best way to save time and consumables. Correct cutting produce specimens which are in perfect condition for the next preparation steps. The most commonly used abrasives for the cutting of different materials are SiC and Al2 O3.
 
Silicon carbide [= Carbures de silicium] is suitable for non-ferrous metals [= métaux non ferreux] whereas Aluminium oxide is preferred for ferrous metals [= fer et alliages ferreux]. Hard wheels [= disques abrasives durs]  are used for cutting soft materials while soft wheels are recommended for cutting harder materials [= materiaux plus durs].
  
Lubrication [= Lubrification] during abrasive cutting is required to minimize damage and to remove the cutting debris [= copaux]. It should have a relatively high flash point because of the sparks produced during abrasive sectioning.
 
MOUNTING [= enrobage]


Following cutting or sectioning the specimen is typically mounted. Mounting provides the following advantages:
 
•  Convenient means to hold the specimen
•  Provides a standard means to mount multiple specimens
•  Protect edges [= bords, pour les échantillons de forme complexe]
•  Provide proper specimen orientation
•  Provides a means to label and store the specimens
 
Hot Mounting [= enrobage à chaud]
 
Hot mounting is quick and easy to produce, requiring several minutes at the appropriate mounting temperature. Most of the time required in compression hot mounting occurs in the heating and cooling stages. Hot mounting resins [= resines d’enrobage à chaud] include:
 
•  Phenolic Resins
•  Acrylic Resins
•  Epoxy Resins (glass filled)
•  Diallyl Phthalate Resins
•  Conductive Resins
 
The most important properties of a hot mounting resin are; Hardness [= dureté], Shrinkage [= rétrécissement ] and Viscosity [= viscosité].
 



The Hardness of the compound should match the hardness of the specimen in order to avoid uneven abrasion during grinding. If the shrinkage during curing is large, a gap between the specimen and the mount will occur and edge will not be adequately protected. Viscosity is important to reach to all areas.
 
Cold Mounting [= enrobage à froid]
 
Cold mounting is preferred for samples which are sensitive to damage from heat and pressure [= chaleur et pression](like coatings [revêtements], polymers, etc.) Cold mounting resins [= resines d’enrobage à froid] are cross-linked polymers [= polymers partiellement réticulés] that are easy to use and require mixing of powder and liquid. The mix is then poured into a mould [= moule] and allowed to set.
 
Grinding & Polishing [= Grenaillage et Polissage]
GRINDING
 

The purpose of the grinding step is to remove damage from cutting, planarize the specimen(s), and to remove material approaching the area of interest.
 
The most common metallographic abrasive used is Silicon Carbide – SiC.  It is an ideal abrasive for grinding because of its hardness and sharp edges. For metallographic preparation, SiC abrasives are used in coated abrasive grinding papers ranging from very coarse [= grossier] 60 grit to very fine [= fin] 1200 grit sizes. Some of the application procedures are given below.
 



•  Soft [= doux, faiblement alliés] non-ferrous metals - Initial grinding is recommended with 320 grit SiC abrasive paper  followed by 400, 600, 800 and 1200 grit SiC paper. Because these materials are relatively soft they do not easily break down the SiC paper. Thus initial grinding with 320 grit is generally sufficient for minimizing initial deformation and yet maintain adequate removal rates. For extremely soft materials such as tin, lead and zinc it is also recommended that the abrasive paper be lightly coated with a paraffin wax. The wax reduces the tendency of the SiC abrasive to embed into the soft specimen.
 
•  Soft [= dur, fortement alliés]ferrous metals - are relatively easy to grind with the depth of deformation being a major consideration. 240 grit SiC abrasives provide a good initial start with subsequent use of 320, 400, 600, 800 and 1200 grit SiC.
 
• Hard ferrous metals - require more aggressive abrasives to achieve adequate material removal. Thus coarse SiC abrasives (120 or 180 grit) are recommended for stock removal requirements. Once planarity and the area of interest are obtained a standard 240, 320, 400 and 600 grit series is recommended.
 
•  Super alloys [= superalliages] - are generally of moderate hardness but have extremely stable elevated temperature characteristics and corrosion resistance. The procedures for preparing super alloys are very similar to that for most non-ferrous metals.
 
• Ceramics - are extremely hard, corrosion resistant and brittle materials. They fracture producing both surface and subsurface damage. Proper grinding minimizes both of these forms of damage. This requires the application of a semi-fixed abrasive which are held rigidly for grinding but can be dislodged under high stress in order to minimize subsurface damage. The abrasive size is also important because very coarse abrasives will remove material quickly but can seriously damage the specimen. For ceramics, consideration of the damage produced at each preparation step is critical to minimizing the overall preparation sequence.
 
•  Composites - are perhaps the most difficult specimens to prepare because of the wide range of properties for the materials used. For example, a metal matrix composite (MMC) such as silicon carbide ceramic particles in an aluminum metal matrix is a difficult specimen to prepare. This composite contains extremely hard/brittle ceramic particles dispersed in a relatively soft/ductile metal matrix. As a rule of thumb, initial grinding should focus on metal planarization and grinding to the area of interest. The secondary grinding steps require focusing on the ceramic particles and typically require the use of diamond abrasives.


Grinding Parameters


The machine parameters which affect the preparation of metallographic specimens are:
 
•  Grinding/polishing pressure,
•  Relative velocity distribution,
•  The direction of grinding/polishing.
 



Grinding Pressure: 
Grinding/polishing pressure is dependent upon the applied force (Newtons) and the area of the specimen and mounting material. Pressure is defined as the Force/Area (N/m2 ). For specimens significantly harder than the mounting compound, pressure is better defined as the force divided by the specimen surface area. Thus, for larger hard specimens higher grinding/polishing pressures increase stock removal rates, however higher pressure also increases the amount of surface and subsurface damage. Higher grinding/polishing pressures can also generate additional frictional heat which may actually be beneficial for the chemical mechanical polishing (CMP) of ceramics, minerals and composites. Likewise for extremely friable specimens such as nodular cast iron, higher pressures and lower relative velocity distributions can aid in retaining inclusions and secondary phases.
 
Rotation Velocity and Direction


The disk speed of the grinder/polisher(Base unit) and the speed of the specimen holder of the Automatic Head(Head Unit) play an important role. This relative rotation allows for a variable velocity distribution depending upon the head speed relative to the base speed.
 
Head Speed (rpm)
Base Speed (rpm)
Relative Velocity Distribution
Characteristic
Application
150
300 to 600
High
Aggressive stock removal Differential grinding across the specimen surface
Useful for gross removal on hard specimens
150
150
Minimal
Matching head and base speed in the same direction eliminates relative velocity distributions Uniform stock removal Low stock removal Produces minimal damage
Provides superior flatness over the specimen Useful for retaining inclusions and brittle phases
 
For high stock removal, a slower head speed relative to a higher base speed produces the most aggressive grinding/ polishing operation. The drawback to high velocity distributions is that the abrasive (especially SiC papers) may not breakdown uniformly, this can result in non-uniform removal across the specimen surface. Another disadvantage is that the high velocity distributions can create substantially more specimen damage, especially in brittle phases. In all cases, it is not recommended to have the head rotating contra direction to the base because of the non-uniform removal and abrasive break-down which occurs.


Minimal relative velocity distributions can be obtained by rotating the head specimen disk at the same rpm and same direction as the base platen. This condition is best for retaining inclusions and brittle phases as well as for obtaining a uniform finish across the entire specimen. The disadvantage to low relative velocity distributions is that stock removal rates can be quite low.


In practice, a combination of a high velocity distribution (150 rpm head speed/ 300 - 600 rpm base speed) for the initial planarization or stock removal step, followed by a moderate speed and low velocity distribution (120-150 rpm head speed/ 150 rpm base speed) step is recommended for producing relatively flat specimens. For final polishing under chemical mechanical polishing (CMP) conditions where frictional heat can enhance the chemical process, high speeds and high relative velocity distributions can be useful as long as brittle phases are not present (e.g. monolithic ceramics such as silicon nitride and alumina).


POLISHING [= polissage]


Polishing is the most important step in preparing a specimen for microstructural analysis. It is the step which is required to completely eliminate previous damage.
Ideally the amount of damage produced during cutting and grinding was minimized through proper blade and abrasive grinding so that polishing can be minimized.




To remove deformation from fine grinding and obtain a surface that is highly reflective, the specimens must be polished before they can be examined under the microscope. Polishing is a complex activity in which factors such as quality and suitability for the cloth, abrasive, polishing pressure, polishing speed and duration need to be taken into account. The quality of the surface obtained after the final polishing depends on all these factors and the finish of the surface on completion of each of the previous stages.


Polishing Cloths [= draps de polissage]


There are three types of polishing clothes; Woven, Non-Woven and Flocked.
•  Woven cloths offer ‘hard surface’ polishing properties and guarantee flat pre-polishing,
    without deterioration of the edges.
•  Non-woven cloths, are used on very hard materials for high precision surface finishing such
    as glass, quartz, sapphire and semi-conductors.
•  The Flocked cloths, guarantee a super-polished finish. The polishing duration must be as
    short as possible, to avoid inclusions from being extracted.


Diamond products [produits diamantés]


Diamond, due to its exceptional hardness and cutting capacity, has become the first choice abrasive in metallographic polishing. Diamonds for metallographic grinding and polishing are available in two different crystalline shapes: Polycrystalline (P) and monocrystalline (M). Polycrystalline diamonds provide vast numbers of small cutting edges. In the metallographic preparation process these edges result in high material removal, while producing only a shallow scratch depth.
Monocrystalline diamonds are more block-shaped and provide few cutting edges. These diamonds give high material removal with a more variable scratch pattern. For high requirements, the (P)-type diamonds are chosen. The (M) type diamonds are best suited for all-purpose polishing. Diamond products are usually available in three forms; diamond paste, diamond suspension and diamond spray.
Polycrystalline diamond as compared to monocrystalline diamond provides better surface finishes and higher removal rates for metallographic specimen preparation.



Final Polishing Abrasives [= polissage de finition]


Final polishing abrasives are selected based upon specimen hardness and chemical reactivity. The most common polishing abrasives is alumina. Alumina abrasives are primarily used as mechanical abrasives because of their high hardness and durability. They also exist in either the softer gamma (mohs 8)or harder alpha (mohs 9) phases.



Etching [= Attaque chimique]
 
Etchant
Composition
Concentration
Conditions
Comments
Kalling’s No:1
Distilled water CuCl2 
HCl 
Ethanol
33 ml 
1,5 grams 
33 ml 
33 ml
Immersion etching at 20° C
For etching martensitic stainless steels. Martensite will be dark and the ferrite will be coloured
Kalling’s No:2
CuCl2 
HCl 
Ethanol
5 grams 
100 ml 
100 ml
Immersion etching at 20° C
For etching duplex stainless steels and Ni-Cu alloys and superalloys
Kellers etch
Distilled water 
Nitric acid 
HCl 
HF
190 ml 
5 ml 
3 ml 
2 ml
10 –30 seconds immersion. Use only fresh etchant
Excellent for aluminium and alloys immersion for 10 – 20 seconds; titanium alloys immersion for 10-20 seconds
Kroll’s Reagent
Distilled water Nitric acid 
HF
92 ml 
6 ml 
2 ml
15 seconds
Excellent for titanium and alloys. Swab specimen upto 20 seconds.
Nital
Ethanol 
Nitric acid
100 ml 
1-10 ml
Seconds to minutes
Most common etchant for Fe, carbon and alloys steels and cast iron- immerse sample up from seconds to minutes; Mn-Fe, MnNi, Mn-Cu, Mn-Co alloys, immersion upto a few minutes.
Marbel’s Reagent
CuSO4 
HCl 
Water
10 grams 
50 ml 
50 ml
Immerse or swab for 5-60 seconds
For etching Ni, Ni-Cu and Ni-Fe alloys and superalloys. Add a few drops to H2SO4 to increase activity.
Murakami’s
K3Fe(CN)6 
KOH 
Water
10 grams 
10 grams 
100 ml
Pre-mix KOH and water before before adding K3Fe(CN)6
Cr and alloys (use fresh and immerse) iron and steels reveals carbides. Mo and alloys uses fresh and immerse; Ni-Cu alloys for alpha phases use at 75°C; W and alloys use fresh and immerse; WC-Co and complex sintered carbides.
Picral
Ethanol 
Picric Acid
100 ml 
2 – 4 grams
Seconds to minutes Do not let etchant crytallize or gry-expolisive
Recomended for microstructures containing ferrite and carbides.
Vilela’s Reagent
Glycerol 
Nitric acid 
HCl
45 ml 
15 ml 
30 ml
Seconds to minutes
Good for ferrite- carbide structures (tempered martensite) in iron and steel.

 


Hardness Testing [= Essai de Dureté]
Vickers Hardness Test [= Essai de Dureté Vickers]
 
It is the standard method for measuring the hardness of metals, particularly those with extremely hard surfaces: the surface is subjected to a standard pressure for a standard length of time by means of a pyramid-shaped diamond. The diagonal of the resulting indention [= empreinte] is measured under a microscope and the Vickers Hardness value read from a conversion table.
 



Vickers hardness is a measure of the hardness of a material, calculated from the size of an impression produced under load by a pyramid-shaped diamond indenter. 
 

The indenter [= pénétrateur] employed in the Vickers test is a square-based pyramid whose opposite sides meet at the apex at an angle of 136º. The diamond is pressed into the surface of the material at loads ranging up to approximately 120 kilograms-force, and the size of the impression (usually no more than 0.5 mm) is measured with the aid of a calibrated microscope. The Vickers number (HV) is calculated using the following formula: 
 
HV = 1.854 ( F / D² ), 
 
with F being the applied load (measured in kilograms-force) and D2 the area of the indentation (measured in square millimeters). The applied load is usually specified when HV is cited. 
 
The Vickers test is reliable for measuring the hardness of metals, and also used on ceramic materials. The Vickers testing method  is similar to the Brinell test [= Essai de Dureté Brinell]. Rather than using the Brinell's steel ball type indenter [= pénétrateur bille], and have to calculate the hemispherical area of impression, the Vickers machine uses a penetrator that is square in shape, but tipped on one corner so it has the appearance of a playing card "diamond". The Vickers indenter is a 136 degrees square-based diamond cone, the diamond material of the indenter has an advantage over other indenters because it does not deform over time and use. The impression left by the Vickers penetrator is a dark square on a light background. The Vickers impression is more easily "read" for area size than the circular impression of the Brinell method. Like the Brinell test, the Vickers number is determined by dividing the load by the surface area of the indentation (H = P/A).
 



Several loadings give practically identical hardness numbers on uniform material, which is much better than the arbitrary changing of scale with the other hardness machines [= machines de dureté]. A filar microscope is swung over the specimen to measure the square indentation to a tolerance of plus or minus 1/1000 of a millimeter. Measurements taken across the diagonals to determine the area, are averaged. The correct Vickers designation is the number followed "HV" (Hardness Vickers). The advantages of the Vickers hardness test are that extremely accurate readings can be taken, and just one type of indenter is used for all types of metals and surface treatments.



***********************************************************************************

Source : http://www.metkon.com

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