Anodization Replica of Precipitates in Age-Hardening Ti Alloys

.D::mgyan Ding1, Zhaohui Li1

, Yi Yang2, Congqin Ning3, Hegang Liu1, Peng Ge2

, Liang Feng2

I> School of Materials Science and Engineering, State Key Laboratory of Metal Matrix Composites, Shanghai J iao Tong Uni­versity ,Shanghai, 200240 ,China

2> Northwest Institute for Nonferrous Metal Research, Xi' an Shaanxi, 710016 ,China 3 > State Key Laboratory of High Performance Ceramics and Super fine Microstructure ,Shanghai Institute of Ceramics ,Chi­

nese Academy of Sciences, Shanghai, 200050, China

Precipitation-hardening is one of the important strengthening mechanisms which governs the mechanical properties of various kinds of alloys. To

characterize the precipitates in age-hardening Ti alloys, traditional metallography and transmission electron microscopy as well as metallurgical

replica have been employed for either destructive or non-destructive testing. However, these characterization methods usually depend on skilled

sample preparation and analysis, which lacks a good repeatability and sufficient imaging contrast. In the present work.our recent progress is re­ported in developing a novel replica method to characterize the precipitates in age-hardening Ti alloys. Based on a weak anodization and ultrasonic treatment, anodization replicas of various kinds of precipitates and grain boundaries are successfully fabricated in BT22 and TilOCr alloys. The

phase-dependent anodization at different phase areas resulted in the formation of diversified replicas. The in-situ geometric replicas of the precipi­

tates enable us to conduct a multi-scale and multi-dimensional characterization of both the primary and secondary precipitates, which shows great

advantages over traditional characterization methods.

Keywords: Titanium alloy ,age-hardening, precipitation, metallography.replica

1. Introduction

Precipitation-hardening or age-hardening is one of the important strengthening mechanisms which gov­erns the mechanical properties of various kinds of al­loys1·3>. To characterize the precipitates in age-harden­

ing Ti alloys, traditional metallography using grinding/ polishing/ etching, transmission electron microscopy, and metallurgical replica method such as extraction replica have been employed for either destructive or non-destructive testing4

•5>. However, these character­

ization methods usually depend on skilled sample prepara­tion and analysis, which are usually time-consuming and lack a good repeatability and sufficient imaging contrast.

For example, if we want to characterize the ultra­fine secondary precipitates formed after an ageing process, the predominant characterization method at present is either SEM observation of well polished and well etched Ti alloy sample or TEM observation of an ion-thinned alloy sample. One of the main drawbacks of these characterization methods is that the imaging quality may be not good. The SEM observation could be greatly affected by the polishing and etching process especially for hard-to-etch alloys. And the TEM obser­vation will be limited to very small areas of the sample and in many cases we could only obtain poor contrast images of ultrafine precipitates. In addition, rare work has been reported on a metallurgical replica character­ization of ultrafine or even large precipitates in age­hardening Ti alloys, although traditional metallurgical replica based on carbon thin films or polymer films could be employed in non-destructive testing of inac­cessible surface, surface particles and defected areas.

Thus, developing an efficient characterization method rather than employing traditional methods

seems to be highly desirable for rapid evaluation of the precipitates and ageing processes as well as mechanical properties for many age-hardening Ti alloys.

In the present work, our recent progress is repor­ted in developing a novel replica method to characterize various kinds of precipitates in several age-hardening Ti alloys.

2. Experimental

Bulk materials of BT22 ( Ti-5Al-5Mo-5V-1Cr-

1Fe) and TilOCr alloys were solutioned at elevated temperatures (860°C for BT22 and 900°C for TilOCr) and aged at lower temperatures ( 500°C - 600°C for BT22, 400°C - 600°C for TilOCr) to obtain various kinds of age-hardening materials. The age-hardening BT22 and TilOCr alloy samples with a size of 10 X 10 mm were cut from bulk materials. The plate samples were grinded with emery papers and anodized at a OC voltage of 15 V or 20 Vin IM NaH2P04 solution con­

taining 0. 5 wt% HF to obtain anodically oxidized sur­faces. After 1. 5 hours of anodization, the anodized samples were taken out of the electrolyte, dried for di­rect observations or further ultrasonic treatment. The ultrasonic treatment in water lasted for about 10 mi­nutes, which could remove some or all of the anodic ox­ides. Surface morphologies of the as-anodized samples and ultrasonically treated samples were investigated with field-emission scanning electron microscope ( SEM, FEI SIR ION 200).

3. Results and Discussion

BT22 alloy is one of the high-performance Ti al­

loys widely used in industry. And TilOCr alloy is a typ­

ical binary alloy. Both alloys could be strengthened through age-hardening treatment. A solution-treatment

8. Environmental Behavior • 1925 •

and age-hardening at elevated temperatures will lead to

the formation of al~ two-phase microstructures. Anod­ization of pure Ti or Ti alloys to form oxide films at the top surfaces has been widely reported in literatures1

-9>.

These works usually focused on the oxides rather than

the alloy substrate. Our experimental results indicate that, to approach a phase precipitates, a weak anodiza­tion of the age-hardening alloy and lateral ultrasonic treatment could realize in-situ metallography replicas

of the precipitates.

3. I Replica of Precipitates in Age-hardening BTI2 Alloys Surface morphology of an as-anodized BT22 alloy

is shown in Figure l(a). Large quantity of oxide parti­cles can be seen at the top surface of the as-anodized

sample. There is no trace of precipitates. After an ul­trasonic treatment of the above anodized sample, the

top oxide layer was removed and the replicas of various kinds of precipitates can be found (Figure l(b) ) . It can be inferred from Figure l(b) that the lamellar or plate­

let a phase precipitates, i. e. , ultrafine secondary pre­cipitates, had been in-situ oxidized to form the platelet

replicas. And relatively larger precipitates, i. e. , prima­ry precipitates, had been in-situ oxidized to form parti­

cle replicas consisting of oxide nanotubes. The platelet replicas and the particle replicas offer us a clear image

of the a/~ two-phase structures. With these anodization replicas,it is easy to tell the large primary precipitates from the lamellar secondary precipitates, easy to meas­

ure the precipitate size/ volume fraction and finally cor­relate to mechanical properties such as yield strength.

1''igure I. Anodized BT22 sample and precipitate replicas. ( a) Sur­

face morphology of the as-anodized BT22 sample showing the forma­

tion of oxide particles at top, ( b) Multi-dimensional replicas of the a­

phase precipitations after ultrasonic removal of the particle layer

The anodization replicas also render us great free­dom to investigate the influence of solution-treatment

temperature on the age-hardening process. Figure 2 shows SEM images of the precipitate replicas of the BT22 alloys solutioned at 810°C and aged at either 550°C or 600°C for 5 hours. Quite different precipitate

replicas of the age-hardening alloys could be observed. The alloy aged at 550°C had relatively finer precipitates (whether the primary or the secondary precipitates). Whereas, the alloy aged at 600°C had much larger pre­cipitates. Such a difference directly proves that ageing

at the higher temperature will result in the formation of large precipitates. In addition to the formation of precipitate replica, replica of the grain boundary could be also obtained. As shown in Figure 1 ( b) and Figure 2.

Figure 2. SEM images of the precipitate replicas of the BT22 alloys

solutioned at 810'C and aged at (a) 550'C and (b) 600'C for 5 hours

Figure 3. OM image of the replicas of both the grain bounda­

ries and quite large precipitates for the BT22 alloys solutioned

at 900'C and aged at 550'C for 5 hours

• 1926 • Proceedings of the 12'h World Conference on Titanium

The replicas of the grain boundaries could be directly observed. An optical microscopy of the grain boundary or grain replicas is therefore available. Figure 3 pres­ents OM image of the repl icas of both the grain bound­aries and the precipitates in the BT22 alloys solutioned at 900°C and aged at 550°C for 5 hours. These SEM images and OM images permit us to measure the grain size and thus indirect ly evaluate the precipitation be­havior related to both the aging process and the solu­tion-trea tment process.

3. 2 Replica of Precipitates in Age-hardening TilOCr Alloys Our experimental works further proved that the

above replica method could be used for many other age­hardening Ti alloys like the binary Til OCr alloy. Figure 4 shows SEM images of the precipitate replicas of the TilOCr alloys aged at 500°C and 600°C for 4 hours. Compared with the ultrafine replicas of the precipitates in the alloy aged at 500°C , the replicas of the precipi­tates in the alloy aged at 600°C are much larger, which indicates that ageing temperature has great influence on the precipitation. Figure 5 demonstrates the influ­ence of ageing time on the precipitation of the seconda­ry precipitates and some grain- boundary precipitation. The average size of the secondary precipitates increases with the ageing time and grain- boundary precipitation could be quite different. Similar to the replicas of the BT22 system, the grain boundary replicas could be

Figure 4. SEM images of the precipitate replicas of

the Ti 1 OCr alloys aged at (a) soo·c and ( b ) 600"C

for 4 hours

clearly observed under the optical microscope ( as shown in Figure 6) . T his once again proves that the an­odization replicas here for multi-scale characterization of ul trafine precipitates and grains of age-hardening T i

a lloys. Based on the above microstructura l observations and the great freedom and simplicity of fabricating the anodization replicas, we believe that this anodization replica method could be a useful tool for rapid character­ization of age-hardening Ti alloys in both academic and in­

dustrial labs.

Figure 5. SEM images of the replicas showing the influence of

aging time on the precipitation of the Til OCr alloys aged at

600"C for (a) 30 minutes , (b) 2 hours, (c) 8 hours, (d) 24 hours

and ( e) 48 hours

4. Conclusions

In summary, representative alloys of BT22 (Ti-5 Al-5 Mo-5 V-1 Cr-1 Fe) and Til OCr, which had been so-1 utioned and aged at different stages, were selected as the model systems to demonstrate how anodization rep­

li cas of precipitates could be rea lized. With the anodiza­tion replicas, a multi-scale and multi-dimensional char­

acterization of both the primary and secondary precipi­tates could be realized. uch a novel replica method of­fers great freedom for rapid characterization of precipi­

tates in both industrial and academic labs.

Acknowledgements This work was fina ncia l supported by Shanghai

Pujiang Program ( o. 07 pj14047) , 863 Plan of China (No. 2006AA02Al), National Key Project of China

( o. 2007CB613805) and National Science Foundation of China (No. 51001088) .

8. Environmental Behavior • 1927 •

Figure 6. OM images of the replicas of both the grain

boundaries and the precipitates in the TilOCr alloys aged at 600'C for (a) 30 minutes, ( b) 8 hours, (c) 24

hours and (d) 48 hours

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