the introduction of martensitic shape memory ( sm ) nickel - titanium ( niti ) alloy in the manufacturing of rotary endodontic instruments has led to relative increase in their flexibility , cutting efficiency and cyclic fatigue resistance .
these alloys were popularly termed as m - wire , cm - wire ( controlled memory ) and thermal memory wire by different manufacturers .
the unique nature of the martensitic sm alloy is the presence of stable martensite phase at body temperature . as the austenite is relatively more rigid ,
they remain in the deformed state even after the withdrawal of stress due to the absence of conventional stress - induced martensitic transformation as observed in superelastic ( se ) niti alloys .
although , deformed martensitic niti instruments recover their shape when heated during sterilization , few instruments undergo permanent plastic deformation making them destined for single use .
also , separation of rotary instruments during instrumentation without warning is a major hindrance for the success of root canal treatment .
therefore , a supplementary treatment that increases the martensite content of sm alloy would be ideal to overcome the drawbacks and improve the clinical performance of these instruments . cryogenic treatment ( ct )
is a supplementary procedure of exposing metals to very low temperatures , which affects the entire cross - section , unlike purely surface treatment . based on the soaking temperature ,
the process may be deep ct ( dct ) or shallow ct ( sct ) .
dct of se niti endodontic instruments has shown improved cutting efficiency and microhardness . the two main mechanisms responsible for the improvement in steel tool properties following ct are a complete transformation of retained austenite to the martensite phase and precipitation of micro - carbide particles . despite extensive reports on the enhancement of mechanical properties of steel tools following ct ,
their impact on the microstructure of sm niti alloys is not studied so far in the literature .
therefore , the aim of this preliminary study was to investigate the role of dry ct conditions on the microstructure of martensitic sm niti alloy .
experiments were conducted on ni-50.8 wt% ti sm alloy with austenite finish temperature ( af ) of 46c in the form of sheet ( 2.2 mm thickness ) and rod ( 9 mm diameter ) of same ingot which complies with the specifications of american society for testing materials ( astm ) f2063 - 05 .
five cylindrical specimens ( 10 mm height ; 9.0 mm diameter ) and five sheet specimens ( 15 mm  15 mm square ) were sectioned from the as - received niti sheet and rod with the help of wire electrical discharge machine ( aq300 l ; sodick , shaumburg , il ) .
the protocol for dry ct and the experimental setup has been followed earlier in the previous study .
the specimens were randomly divided into four experimental groups based on the soaking temperature and time with a control group as follows : dct 24 group : 185c ; 24 h , dct 6 group : 185c ; 6 h , sct 24 group : 80c , 24 h , sct 6 group : 80c , 6 h and control ( ctrl ) group : no treatment .
the constant cooling rate and warming rate used during dry ct was 1c / min ( 3 h ) and 0.6c / min ( 6 h ) respectively .
the cross - sections of five niti cylindrical specimens ( one specimens from each group ) were polished with a standard sequence of abrasives ( carbimet paper discs , buehler : 240 and 600 grit ) and alumina paste ( alpha micropolish , buehler : 6 m , 1 m and 0.5 m particle sizes ) .
the polished surfaces were etched using an etching solution of composition hf + hno3 + h2o with a volume ratio of 1:4:5 by swabbing technique .
the microstructures of the specimens were examined with an optical microscope ( leica dm4000 m led ; leica microsystems cms gmbh , germany ) and an scanning electron microscope ( sem ) ( cx-200 ; coxem ltd . , daejeon , south korea ) .
forty - five lines per mm were used for the calculation in both vertical and horizontal direction .
the elemental composition of different zones namely , the matrix , grain boundaries , martensitic variants and precipitates was determined for the sm alloy using scanning electron microscope ( cx-200 ; coxem ltd . ,
daejeon , south korea ) coupled to energy dispersive x - ray spectrometer ( sem - eds ) .
the quantitative analysis was performed in nonstandard analysis mode employing zaf correction methods using the software .
x - ray diffraction ( xrd ) study was performed on polished sheet specimen from each group at room temperature using x - ray diffractometer ( miniflex ii - c , rigaku , tokyo , japan ) .
the xrd system was operated under the following conditions : cu - k rotating anode (  = 1.5406  ) , 40 kv accelerating voltage , 40 ma beam current , 10-100 2-theta range at 0.02 steps and 2 s acquisition time per step . a silicon standard was used for calibration of the diffractometer ( 640b silicon powder xrd spacing , standard reference material ; nist , gaithersburg , md , usa ) . the ( hkl ) planes corresponding to the peaks of austenite , martensite and precipitates were identified using the icdd database ( pdf release 2004 ; international centre for diffraction data , newton square , pa , usa ) .
the protocol for dry ct and the experimental setup has been followed earlier in the previous study .
the specimens were randomly divided into four experimental groups based on the soaking temperature and time with a control group as follows : dct 24 group : 185c ; 24 h , dct 6 group : 185c ; 6 h , sct 24 group : 80c , 24 h , sct 6 group : 80c , 6 h and control ( ctrl ) group : no treatment .
the constant cooling rate and warming rate used during dry ct was 1c / min ( 3 h ) and 0.6c / min ( 6 h ) respectively .
the cross - sections of five niti cylindrical specimens ( one specimens from each group ) were polished with a standard sequence of abrasives ( carbimet paper discs , buehler : 240 and 600 grit ) and alumina paste ( alpha micropolish , buehler : 6 m , 1 m and 0.5 m particle sizes ) .
the polished surfaces were etched using an etching solution of composition hf + hno3 + h2o with a volume ratio of 1:4:5 by swabbing technique .
the microstructures of the specimens were examined with an optical microscope ( leica dm4000 m led ; leica microsystems cms gmbh , germany ) and an scanning electron microscope ( sem ) ( cx-200 ; coxem ltd . , daejeon , south korea ) .
forty - five lines per mm were used for the calculation in both vertical and horizontal direction . the elemental composition of different zones namely , the matrix , grain boundaries , martensitic variants and precipitates was determined for the sm alloy using scanning electron microscope ( cx-200 ; coxem ltd . ,
daejeon , south korea ) coupled to energy dispersive x - ray spectrometer ( sem - eds ) .
the quantitative analysis was performed in nonstandard analysis mode employing zaf correction methods using the software .
x - ray diffraction ( xrd ) study was performed on polished sheet specimen from each group at room temperature using x - ray diffractometer ( miniflex ii - c , rigaku , tokyo , japan ) .
the xrd system was operated under the following conditions : cu - k rotating anode (  = 1.5406  ) , 40 kv accelerating voltage , 40 ma beam current , 10-100 2-theta range at 0.02 steps and 2 s acquisition time per step . a silicon standard was used for calibration of the diffractometer ( 640b silicon powder xrd spacing , standard reference material ; nist , gaithersburg , md , usa ) .
the ( hkl ) planes corresponding to the peaks of austenite , martensite and precipitates were identified using the icdd database ( pdf release 2004 ; international centre for diffraction data , newton square , pa , usa ) .
optical microscopic images [ figure 1a ] of all etched specimens showed equiaxed grains with well - defined boundaries .
the average diameter of the grains was approximately 25 m with astm grain size number of 7.7 in all the groups .
table 1 shows the volume of martensite that is observed in the images of different groups . ( a ) optical micrograph of shape memory nickeltitanium ( niti ) alloy ( black circle denotes the martensite variant of the grain ) , ( b ) scanning electron microscope micrograph ( white circle denotes the martensite variant ) ; holes ( black arrow ) and precipitates ( white arrow ) in the niti matrix are present , ( c ) eds spectrum showing the elemental composition comparison of martensite and austenite phase in all the groups the martensite variants were well appreciated under higher magnification with the sem images [ figure 1b ] .
the eds spectrum reveals the elemental composition of the niti specimen , which is nickel - rich [ figure 1c ] .
the average elemental composition of the as - received sm alloy was ni  50.8 wt% and ti  48.4 wt% with trace elements [ table 2 ] .
the regional variations in the composition revealed an increase in ni content , especially in the precipitates and twinned regions .
elemental composition of sm niti alloy at different regions x - ray diffraction pattern [ figure 2 ] of the specimens at room temperature contained the following peaks : ( 110 ) , ( 200 ) and ( 211 ) atomic planes for austenite ; ( 11 ) , ( 002 ) and ( 012 ) atomic planes for martensite ; ( 203 ) , ( 205 ) and ( 220 ) atomic planes for tini3 precipitates ; ( 220 ) and ( 042 ) atomic planes for ti2 ni3 and ti3ni4 precipitates respectively .
average intensities and full width at half - maximum height of the peaks corresponding to austenite ( 211 ) and martensite ( 002 ) were used to describe the qualitative change in crystal size and content of the sm niti alloy [ table 1 ] .
x - ray diffraction patterns for the shape memory nickel - titanium alloy at room temperature . ( a ) all the groups .
optical microscopic images [ figure 1a ] of all etched specimens showed equiaxed grains with well - defined boundaries .
the average diameter of the grains was approximately 25 m with astm grain size number of 7.7 in all the groups .
table 1 shows the volume of martensite that is observed in the images of different groups . ( a ) optical micrograph of shape memory nickeltitanium ( niti ) alloy ( black circle denotes the martensite variant of the grain ) , ( b ) scanning electron microscope micrograph ( white circle denotes the martensite variant ) ;
holes ( black arrow ) and precipitates ( white arrow ) in the niti matrix are present , ( c ) eds spectrum showing the elemental composition comparison of martensite and austenite phase in all the groups
the martensite variants were well appreciated under higher magnification with the sem images [ figure 1b ] .
the eds spectrum reveals the elemental composition of the niti specimen , which is nickel - rich [ figure 1c ] .
the average elemental composition of the as - received sm alloy was ni  50.8 wt% and ti  48.4 wt% with trace elements [ table 2 ] .
the regional variations in the composition revealed an increase in ni content , especially in the precipitates and twinned regions .
x - ray diffraction pattern [ figure 2 ] of the specimens at room temperature contained the following peaks : ( 110 ) , ( 200 ) and ( 211 ) atomic planes for austenite ; ( 11 ) , ( 002 ) and ( 012 ) atomic planes for martensite ; ( 203 ) , ( 205 ) and ( 220 ) atomic planes for tini3 precipitates ; ( 220 ) and ( 042 ) atomic planes for ti2 ni3 and ti3ni4 precipitates respectively .
average intensities and full width at half - maximum height of the peaks corresponding to austenite ( 211 ) and martensite ( 002 ) were used to describe the qualitative change in crystal size and content of the sm niti alloy [ table 1 ] .
x - ray diffraction patterns for the shape memory nickel - titanium alloy at room temperature . ( a ) all the groups .
cryogenic treatment of sm niti alloys for endodontic use appears to be a promising field of research due to the expected transformation of retained austenite to martensite as observed in stainless steel alloys .
the sm alloy regains its shape on heating due to the martensite - austenite transformation , which make the file se before cooling .
deep cryogenic processing uses temperatures around 185c because this is a minimum temperature easily obtainable with liquid nitrogen .
the purposes of selecting raw niti specimens in this study are as follows : 
 the intention was to assess the effect of ct alone.the influence of thermomechanical processing and manufacturing methods ( machining / twisting ) were eliminated.the dimension of the specimens were customized according to the requirement.the complicated geometry of the endodontic files was avoided . 
 
the influence of thermomechanical processing and manufacturing methods ( machining / twisting ) were eliminated .
the complicated geometry of the endodontic files was avoided . the structural analysis of the surface and near - surface ( 50 m )
the grain size of the niti specimens obtained from optical micrographs did not show major variations between the groups .
martensitic variants were observed in specimens of all the groups including the ctrl . this could be attributed to the af temperature of the sm niti alloy which is above the room temperature .
the volume of martensite obtained from micrographs in dct 24 group was relative higher ( 4.1% ) . in our previous study
, specimens treated with dct 24 conditions had showed a significant reduction in vickers hardness and reciprocatory sliding wear resistance of sm niti alloy ( unpublished observations ) .
the conversion of retained austenite into martensite with a substantial increase in the volume of martensite phase could be the reason for such peculiar behavior of the alloy .
although the difference in volume of martensite at a given site among the experimental groups was minimal , they do not actually reflect the exact volume of the entire cross - section of the alloy . in our previous experiments ,
dsc analysis on the as - received sm niti alloy used in this study showed two - stage ( austenite  r - phase  martensite ) phase transformation during cooling ( unpublished observations ) , which is similar to the transformation behavior of sm niti instruments .
ti3ni4 precipitates observed in the micrographs influence the martensitic transformation behavior of the alloy by favoring the formation of r - phase and altering the ni content of the matrix . the ni content in the matrix is reduced due to the presence of ni - rich intermetallic phases , which subsequently results in the increase of af temperature and overall martensite phase transformation temperature range .
the xrd spectra corresponding to 2 range of between 35 and 80 is of special interest as most of the austenite and martensite peaks were located .
the peak intensity of martensite ( 002 ) phase and the additional martensite peaks ( 11 and 012 ) was relatively high in the xrd pattern of dct 24 specimen [ figure 2 and table 1 ] .
this could be attributed to a prominent increase in the martensite volume of the specimen as observed from the micrograph .
crystallites and subsequently the grains in the as - received niti specimens preferentially grow along the martensite ( 002 ) lattice plane following ct . the ( 110 ) peak of the austenite phase was not observed in both control and experimental groups except dct 24 .
this could be due to overlapping of ( 110 ) peak with the prominent ( 11 ) martensite peak .
the presence of martensite phase at room temperature in the niti specimens could be attributed to the composition and heat treatment of the alloy . in dct 24
specimen , the crystallite size and peak intensity of austenite was reduced , whereas they are increased for martensite .
although minor variations in values were observed in other experimental groups , they can not be attributed to the variations in the ct parameters .
it is known that the fatigue resistance and flexibility of the sm rotary instruments is remarkably increased due to the presence of proportionately higher martensite phase .
therefore , the life of the rotary endodontic instrument made out of our new experimental alloy will be extended in a great way .
furthermore , soaking period plays an important role in altering the microstructure of sm niti alloy as far as dct is concerned .
longer soaking time of 24 h provides adequate time for the transformation of retained austenite to the martensite phase .
under the limitations of this study , deep dry ct with 24 h soaking period increases the martensite content of sm niti alloy without altering the grain size . by correlating our previous experiments and the current findings , deep dry ct with 24 h soaking period
could be recommended as a supplementary method in the manufacturing in order to extend the life of sm niti endodontic instruments .
further investigation is required to evaluate the effect of ct on the fatigue properties of the bulk specimens .
 context : recent introduction of shape memory ( sm ) nickel - titanium ( niti ) alloy into endodontics is a major breakthrough . 
 although the flexibility of these instruments was enhanced , fracture of rotary endodontic instruments during instrumentation is an important challenge for the operator . 
 implementation of supplementary manufacturing methods that would improve the fatigue life of the instrument is desirable.aim:the purpose of this study was to investigate the role of dry cryogenic treatment ( ct ) conditions on the microstructure of martensitic sm niti alloy.materials and methods : experiments were conducted on ni-51 wt% ti-49 wt% sm alloy . 
 five cylindrical specimens and five sheet specimens were subjected to different ct conditions : deep ct ( dct ) 24 group : 185c ; 24 h , dct 6 group : 185c ; 6 h , shallow ct ( sct ) 24 group : 80c , 24 h , sct 6 group : 80c , 6 h and control group . 
 microstructure of surface was observed on cylindrical specimens with an optical microscope and scanning electron microscope at different magnifications . 
 subsurface structure was analyzed on sheet specimens using x - ray diffraction ( xrd).results : microstructures of all sm niti specimens had equiaxed grains ( approximately 25 m ) with well - defined boundaries and precipitates . 
 xrd patterns of cryogenically treated specimens revealed accentuation of austenite and martensite peaks . 
 the volume of martensite and its crystallite size was relatively more in dct 24 specimen.conclusions:dct with 24 h soaking period increases the martensite content of the sm niti alloy without altering the grain size . 