ME 215 ENGINEERING MATERIALS ICHP.8HIGH TEMPERATUREPROPERTIES OF MATERIALSINTRODUCTIONWhile technolgy is a!ancing "any enginee#ing a$$lications #e%&i#e "ate#ials 'o#(ing at high te"$e#ates)*igh te"$e#ate "eans ho' close is the 'o#(ing te"$ to "elting te"$ o+ the "ate#ial itsel+),o# e-a"$le. co&l a "ate#ial 'ith 5//C "elting $oint 0e &se in an a$$lication 'he#e 'o#(ing te"$ is 1//C +o# a #elati!ely long $e#io o+ se#!ice2The e!elo$"ent o+ "ate#ials to #esists high te"$e#ates the#e+o#e has 0een an i"$o#tant technological a!ence o+ #ecent yea#s)De"ans +o# alloys to #esist high te"$e#ates co"e +#o" "any ne' a$$lications. incl&ing 3s$ace !ehicles.3#oc(ets .3gas tines.3i"$#o!e"ents in con!entional $o'e# $lants +o# inc#ease the#"oyna"ic e++eciency.3che"ical $#ocesses.etc)It can $#o0a0ly 0e sai that high te"$e#ate a$$lication #e$#esents one o+ the &lti"ates in enginee#ing esing) 4The co##osion an c#ee$ #esistance li"its o+ the a!alia0le "ate#ials #est#icts the conte"$late high $#esse #eactions an high te"$e#ates conitions in che"ical $#ocessing)4In ai# c#a+t an s$ace !ehicles.high te"$e#ates a#ise +#o" the heat gene#ate in the engines an +#o" the ae#oyna"ic heating o+ ai#+#a"es at s&$e#sonic s$ees)The $oint o+ choosing co##ect "ate#ial +o# high te"$ a$$lication is the#e+o#e !e#y i"$o#tant)5eca&se the#e is the g#eatest !a#iety o+ $#actical consie#ations 'hich ete#"ine the choice o+ "ate#ial +o# high te"$e#ate a$$lications )These incl&e63 En!i#on"ent3 St#ess syste" an #ange3 7e#"issi0le c#ee$ e+o#"ation3 7#o&ction #e%&i#e"ent3 Weight an si8e #est#ictions3 Cost an a!aila0ility3 Se#!ice te"$e#ate #ange1. Environment9 it co!e# a 'ie #ange +#o" an ine#t gas to co"$letely hostile en!i#on"ental conitions) In the latte# case this 0eco"es a $#o"inent +acto# in the choice o+ "ate#ials)2. Stress system and range9this can !a#y +#o" hea!y loas in 0&l(y st#&ctes to 8e#o a$$lie loas in co!e#s an heat shiels),atig&e.&ne# +l&ct&ating st#esses o# the the#"al st#esses is al'ays o+ $#i"e conce#n)3. Permissible creep deformation9 this again is co"$letely e$enent &$on the +&ntional #e%&i#e"ents)4. Production requirements9 it sho&l 0e note that so"e o+ the high te"$e#ate "ate#ials a#e not a"ena0le to #eg&la# "ethos o+ $#o&ction to $#o&ce the esi#e sha$es)5. Weigt and si!e restriction9 these a#e o+ i"$o#tance in ai#c#a+t an.es$ecially. s$ace li(e a$$lications)". #ost and availability9 the#e a#e "any e-cellent high te"$e#ate "ate#ials 0&t thei# cost li"its thei# &se in "ost con!entional a$$lications$.Service temperature range9 $#o0a0ly the "ost i"$o#tant +acto# in eciing the essantial $#o$e#ties): STATIC CREE7 7RO7ERTIES4C#ee$ is a $#og#essi!e e+o#"ation o+ a "eta#ial &ne# st#ess) 4It is a ti"e e$enant $e#"anent e+o#"ation4In c#ee$. at constant st#ess. st#ain contin&es to inc#ease 0y ti"eTi"eSt#essSt#ainThe#e a#e th#ee "ain (ins o+ c#ee$64Loga#ith"ic c#ee$4Di++&sion c#ee$ 4*igh;te"$e#ate c#ee$The loga#ith"ic c#ee$ occs at lo' te"$e#ates 'he#e no #eco!e#y o+ "echanical $#o$e#ties is $ossi0le)It is gi!en the na"e elongation@ a#e ta(en at s&++iciently +#e%&ent inte#!als to e+ine the st#ain 3 ti"e c!e till #&$te)#reep time curves ave four stages64 .nitial instantaneous stage in 'hich total e+o#"ation "ay 0e +&lly elastic.o# $a#tly elastic an $a#tly $lastic94 / transient stage 'he#e the #ateo+ c#ee$ e+o#"ation ec#eases'ith ti"e as a #es&lt o+ st#ain ha#ening >$#i"a#y c#ee$ @4 / stage of constant minimum creep rate 'he#e the e++ect o+ st#ain;ha#ening is 0alance 0y an annealing in+l&ence >secona#y c#ee$ @4 / final stage 'he#e the c#ee$ #ate accele#ates) The accele#ation can ha!e "any ca&ses)It is atti#i0&te to the $#og#essi!e inc#ease o+ the t#&e st#ess acting on the c#oss;sectional a#ea o+ the s$eci"en.an to "etallgical changes an the +o#"ation o+ inte#nal ca!ities >te#tia#y c#ee$@)Sa"$le e+o#"ation at a constant st#ess >@ !s) ti"e7#i"a#y C#ee$6 slo$e >c#ee$ #ate@ ec#eases 'ith ti"e)Secona#y C#ee$6 steay;statei)e). constant slo$e)Te#tia#y C#ee$6 slo$e >c#ee$ #ate@ inc#eases 'ith ti"e. i)e) accele#ation o+ #ate),/tC#ee$ &s&ally occs at ele!ate te"$e#ate. T H /)A T"An e++ect o+ te"$e#ate ince#ase on the c!e is seen in +ige +o# constant st#ess conitionAa$te +#o" ,igs) K)2E. Callister 7e.elastic$#i"a#ysecona#yte#tia#y Creep tests can be done under different stresses too.When tension-creep tests are conducted at various stress values (and a constant temperature), a family of creep curves is obtained as shown in figure5oth te"$e#ate L a$$lie st#ess a!e#sely a++ect the c#ee$ st#ains) Us&ally &ne# the sa"e te"$e#ate i++e#ent st#ess le!els a#e a$$lie L the c#ee$ st#ain !s ti"e c!es a#e ete#"ine.Creep StrainT1 or 1TimeT2 or 2T3 or 3T4 or 4T1