Physical Metallurgy of Multi-phase Titanium Alloys

I- Purpose/Objective:

The purpose of this experiment is to examine the impact of heat treatments on the microstructure of two-phase titanium alloys. The specific material to be examined is an alpha+beta Ti-6242 (6%Al, 2%Sn, 4%Zr, 2%Mo, plus Si additions, with all compositions in wt. %). We will relate the microstructure to the structure and properties through x-ray diffraction and hardness measurements, using both Rockwell and Vickers hardness. We will conduct a quantitative image analysis using manual “point-count” and linear intercept methods plus available software packages (e.g., Russ plug-ins for Digital Darkroom Software) and prepare publication-quality gray scale images with programs such as Adobe Photoshop. This experiment involves four major tasks, which have been scheduled in a sequence in order to minimize bottlenecks:

  • Heat treatment of Ti-6242 alloy in Lindberg furnaces
  • Metallographic analysis of heat treated alloys
  • Rockwell and Vickers hardness measurements
  • X-ray diffraction measurements of the two-phase alloys on the Rigaku Miniflex diffraction unit.

II - Experimental Procedure:

Each group will prepare the following specimens of the Ti-6242 alloy, which have been cut from an extruded rod:
  1. As-forged Ti-6242
  2. Transformed beta (Widmanstätten alpha+beta), with alpha needles in a beta matrix by air quenching (With specimens our size, an air quench has a cooling rate about 10°C/s.) from a solutionizing temperature above the beta transus of about 995°C
  3. Alpha+beta sample.

Note that these heat treatments will be conducted in air.  The surface of the alloy will absorb oxygen, an alpha-stabilizer, such that the alloy should develop an alpha case about 80 microns thick. The diffusivity of oxygen in beta-Ti at 1025°C is about 10-8 cm2/sec, so (Dt)1/2 after 2 hours is about 84 microns. Look up data for the diffusivity of oxygen in titanium to verify this calculation.  Note such information in the microstructures you observe. Is there an alpha case on the as-forged sample?  Why or why not? How would you prove this quantitatively?

A.  Heat Treatment and Metallography

1. Heat Treatments:

  • Transformed Beta: Prepare this sample by air quenching from the beta-anneal temperature.  Anneal two samples in a Lindberg furnace at a temperature above 1000°C for at least 20 minutes to solutionize the as-forged samples and bring them into the single-phase beta field. Note thermocouple reading at the specimen position and note the time of the beta anneal. Withdraw one sample for an air-quench to room temperature. [Note: to expedite this part of the experiment, your lab instructor may commence the heat treatment prior to the start of the lab period. If you wish to participate in this part of the experiment, please consult your instructor in advance.]
  • Alpha+Beta: Prepare this sample by air quenching a rod from an alpha+beta temperature. Examine the phase diagram, and choose a temperature for your group’s alpha+beta annealing temperature [approximately 980-850°C].  Consult with the lab instructor for various options.  With the second sample still in the furnace, change the furnace set point to your chosen alpha+beta annealing temperature. After the temperature re-equilibrates, anneal for 2 hours  to produce primary alpha and beta.  Air quench to room temperature to obtain primary alpha and transformed beta. Do not forget to record the temperatures.  Note thermocouple reading at the specimen position and note the time of the alpha+beta anneal.
    SOP: Lindberg Furnace


2. Cut metallographic samples from the as-forged sample and from the center of the two annealed rods.  You will need to save a piece of the sample for x-ray diffraction analysis.
SOP: LECO MSX205M2 Benchtop Sectioning Machine (Cut-off Saw)

3. Mount, polish, and etch with Kroll's reagent. Use the abrasive wheel to section the coupons and mount them in bakelite.  Prepare a good metallographic polish, using the LECO automatic grinding and polishing conditions.  In either case, note your preparation conditions for your report. Etch your samples with Kroll's reagent, which is a general-purpose etch for alpha-beta titanium alloys. Kroll’s reagent is a solution of 100 ml H2O+ 5 ml HNO3+ 3 ml HF.
SOP: ATM Saphir 530 Dual Wheel Specimen Preparation System (Grinder/Polisher)

B.  Optical Microscopy

Obtain images of the following:

1. Images of the appropriate sample:

Transformed Beta:

      1. Use a high enough magnification to view the Widmanstätten microstructure.
      2. Use a lower magnification for observing prior beta grains. Take enough images to provide at least 20-30 prior beta grains.

Alpha+Beta:

      1. Obtain at least two images at magnification appropriate to view the primary alpha and the prior beta grains, possibly 800-1000X.
      2. Also use a high enough magnification to view the Widmanstätten microstructure.

As-forged:

Obtain one or more micrographs illustrating the microstructure in the as-forged alloy prior to our heat treatments.

Alpha case:

Take an image at about 50X or higher to view the alpha case on the surface of the specimen.

C. Hardness
Determine the Vickers hardness of the as-forged, alpha+beta, and transformed beta samples, as well as that of its alpha case.  For comparison, also measure the Rockwell hardness of the as-forged, alpha+beta, and transformed beta sample.  In order to obtain statistically significant data, perform ten of each hardness measurement.  We will share all hardness data electronically. Consult your instructor for details.

SOP: Clark Microharness Tester (Vickers/Knoop) CM-400AT

D.  Microstructural Analysis
Examine the individual specimens for a qualitative analysis, identifying the regions of primary alpha and Widmanstätten, and examining the surface case.  Note the difference in the as-forged sample.  Microscopy can either be done on the optical microscopes or the teaching SEM.   Prepare high quality images at a high enough magnification, possibly about 800X, to analyze the transformed beta in both an alpha + beta and a transformed beta sample.  Also prepare an image at a lower magnification, possibly about 400X, of an alpha+beta sample to analyze the primary alpha.  Store these in your folder for report preparation.

Either during laboratory periods or at other times, students should access their images and conduct a quantitative image analysis using both manual quantitative stereology (point-count) and with the assistance of the Russ plug-ins determine the following:

1. Primary alpha in alpha+beta alloys

  1. The manual point-count method for volume fraction of primary alpha is to be done separately by each student on the same images as follows:
    1. Discriminate primary alpha by outlining a printed image
    2. Conduct a point count with five applications of a 10 x 10 grid
    3. Compare volume fractions determined by your fellow group members.
  2. Automated analysis using the Russ plug-ins should be used as follows:
    1. Process one set of images with the Russ plug-ins to discriminate the grains
    2. Use the Russ plug-ins to determine:
      • Volume fraction of primary alpha
      • Size distribution of primary alpha particles.

2. Prior beta grains in alpha+beta alloys

  1. Manual intercept length for grain size of prior beta grains is to be done separately by each student on the same images as follows:
    1. Discriminate prior beta grains by outlining a printed image
    2. Conduct a linear intercept analysis, using random arrangements of five lines, each measuring about 120 mm
    3. Compare grain sizes determined by your fellow group  members.
  2. Automated analysis using the Russ plug-ins should be done as follows:
    1. Process one set of images with the Russ plug-ins to discriminate the grains
    2. Use the "grain size" plug-in to determine area of each prior beta grain, and express it as average grain size and grain size distribution.


3. Widmanstätten microstructure of transformed beta and the prior beta regions in alpha+beta alloys

Estimate the size of the alpha needles in the Widmanstätten microstructure in both specimens.  Which dimension is more important, length or thickness? Discuss if you expect these dimensions to be the same in both microstructures.   (Hint: do the prior beta grains have the same composition after the alpha+beta anneal?) 

4. Publication-quality micrographs

For the images you analyzed above, use Adobe Photoshop or other similar software products to prepare publication-quality micrographs for your report.  Adjust the contrast and other parameters to enhance the important features in the image obtained from the printer. Note that you may need to modify several parameters before obtaining a print you like.  Annotate the micrograph with a legend and a magnification marker and point out any notable features.

The ability of Digital Darkroom or other similar software products to distinguish the primary alpha from the transformed beta will depend upon the contrast in the transformed beta regions of the image.   Digital Darkroom analyzes the image after it has been converted to a binary black/white image.  If your starting image has dark gray contrast in the transformed beta and nicely white primary alpha, you might be able to simply create a binary image by choosing a suitable threshold.  However, if the alpha plates in transformed beta are of the same contrast as the primary alpha, you might have trouble finding a correct threshold value.  If Digital Darkroom does not adequately distinguish white primary alpha from zebra-striped transformed beta, you may have to artificially enhance the contrast of the alpha grain boundaries and the transformed beta grains before Digital Darkroom can reliably create the binary image for analysis.

For the particular images analyzed in 1. and 2. above, use Adobe Photoshop or other similar software products to prepare publication-quality micrographs for your report. Adjust the contrast and other parameters to enhance the important features in the image from the printer. Expect to try several combinations before you get a print you like. Annotate the micrographs with a legend and a magnification marker. Point out any notable features. 
SOP: Nikon Optiphot Planar MicroscopeFEI (Philips) XL30 Teaching SEM with BSE mode and EDAX analysis


E. X-ray Diffraction
Each group will obtain x-ray powder patterns from approximately 20-80° in 2θ, using the Rigaku Miniflex instrument, with a step size of 0.05°, and an integration time of 1 second.  This scan will capture all lattice spacings from 0.12 to 0.44 nm, covering the major peaks for the phases of the titanium alloys.  

Analyze the diffraction pattern with the JADE software and the JCPDS Powder Diffraction file.  Identify all the peaks in the pattern and index the pattern by labeling each peak in each phase with the d-spacing and the Miller index of the plane it represents. Note that the alloying of titanium and the selective partitioning of certain alloying elements to either the alpha phase or the beta phase may change the d-spacings from those expected for pure titanium.
SOP: Rigaku Miniflex X-ray Diffraction System


III - Theory/Background Information:

III. Background for Physical Metallurgy of Multi-phase Titanium Alloys


Titanium alloys have widespread use. In the aerospace industry, for airframe and engine components, they are prized for their excellent strength-to-weight ratio and relatively high temperature capability.  The high strength-to-weight and stiffness-to-weight are useful in many other applications. Titanium alloys have excellent corrosion resistance, and are useful in the chemical industry and for biomedical implants.  Also, some of the predominant “low-temperature” superconducting materials are titanium-niobium alloys.


The major element in phase chemistry and microstructure development of titanium alloys is the transformation on heating from the low temperature hcp alpha phase to the high temperature bcc beta phase.  In pure Ti, the alpha-beta transformation occurs at 883°C.  Certain alloying elements, e.g., bcc transition metals such as Mo, V, or Nb, stabilize the cubic beta-phase and decrease the alpha-beta transformation temperature.  These transition metals are termed beta stabilizers.   Simple metals, such as Al and interstitial solutes [O, N, C] are alpha stabilizers and increase the temperature of the alpha-beta transformation.  Some elements, such as Zr or Sn, have little effect on the transformation (at low concentrations and can be considered neutral.  Technical alloys are classified as alpha alloys, beta alloys or two-phase alpha+beta alloys, depending on the phases and microstructures obtained.


Two of the more popular alpha+beta alloys are Ti-6Al-4V, a general purpose alloy and Ti-6242 (Ti-6Al-2Sn-4Zr-2Mo) [compositions in wt.%], a forgeable alloy with improved high- temperature resistance.  We will examine the behavior of the Ti-6242 alloy.  On the following page is a pseudo-binary phase diagram for Ti-6Al-2Sn-XMo, showing the temperature of the alpha and beta fields as a function of the Mo content.  Note that for our 2 wt.% Mo composition, the alloy is single-phase beta above 990°C and two-phase alpha+beta below this temperature.  The line marked "Ms" denotes the starting temperature of a martensitic-like transformation for the supercooled beta phase.  In this alloy, the martensitic transformation only occurs with very rapid water quenching to temperatures below MS, so it will not concern us.


With milder air-quenching, alloys annealed above 990°C in the beta field transform to an alpha+beta mixture with a characteristic Widmanstätten microstructure, consisting of needles or platelets of alpha in a beta matrix.  The beta phase contains almost all of the Mo solute, but very little of the Al, while the alpha phase contains most of the Al and virtually no Mo.  The Widmanstätten microstructure, also called transformed beta, is illustrated on the following page.


Alloys annealed in the alpha+beta field consist, at annealing temperature, of two phases:  globular alpha (also called primary alpha) and beta.  The relative amount of alpha and beta phases at temperature can be roughly indicated from the pseudo-binary phase diagram .  After an air quench, the original beta phase transforms to the Widmanstätten alpha+beta. This is illustrated in the enclosed micrograph.  Thus, the quenched alpha+beta alloy contains two microconstituents (primary alpha and Widmanstätten alpha+beta [transformed beta]) and two phases (alpha+beta).

 

Note the alpha phase is present in two morphologies:  primary alpha and alpha needles in the Widmanstätten morphology.  The primary alpha develops during the anneal, and has no orientation relationship with the beta phase.  The Widmanstätten alpha forms during the quench by precipitation in the beta, forming as platelets with the {0001} basal planes of the hexagonal alpha phase parallel to the {110} planes of the parent retained beta. Under optical magnification, the alpha platelets have a "basket weave" structure from the different variants of {110}.


We should expect that our as-received alloy rods were extruded hot, but this should be verified by inspection of the rods and their microstructures.  If the rods were extruded, it is likely that they were extruded above the beta transition temperature. Thus, the deformation would have taken place in the beta field, i.e., they were “beta forged”.  Such hot-forged alloys typically have a recognizable microstructure, often with good creep resistance.


IV - Theory/Background References:

  1. Titanium: A Technical Guide, M.J. Donachie, ed., ASM International, 1988. [ISBN: 0-87170-309-2].

  2. Alloying, J.L. Walter, M.R. Jackson, C.T. Sims, eds., ASM International, 1988, pp. 257-370 (article by E.W. Collins). [ISBN: 0-87170-326-2].

  3. Metals Handbook, Volume 2, (Formerly) Tenth Edition, Properties and Selection: Nonferrous Alloys and Special Purpose Materials, ASM International, 1990, pp. 586-633 (also pp. 634-660). [TA  459.M43; ISBN: 0-87170-378-5(v.2)].

  4. Metals Handbook, Desk Edition, Second Edition, J.R. Davis, ed., ASM International, 1998, pp. 575-588. [TA 459.M288; ISBN: 0-87170-654-7].

  5. R.I. Jaffe, ThePhysical Metallurgy of Titanium Alloys, Progress in Metal Physics, Volume 7, B. Chalmers and R. King, eds., Pergamon Press, New York, 1958, pp. 65-163.

  6. Several physical metallurgy texts also have useful information on titanium alloys. See for example texts by A.G. Guy and J. J. Hren, Elements of Physical Metallurgy, 1974.


V- Activity Schedule:

Date/Time Group 1 Group 2 Group 3 3/15 – 3/17 1:30 - 3:30 Heat Treatment Heat Treatment Heat Treatment 3/15 – 3/17 3:30 - 5:30 X-ray Diffraction Metallography Hardness Testing 3/22 - 3/24 1:30 - 3:30 Metallography X-ray Diffraction Metallography 3/22 - 3/24 3:30 - 5:30 Hardness Testing Hardness Testing X-ray Diffraction


VI -Format and Important Questions for Lab Report:

The report should include the publication-quality micrographs. In your report, include the quantitative analysis of microstructural features, comparing your group’s results with the other groups from the class. The setting of threshold levels and contrast during image analysis introduces some variation between analyses, so compare your group’s transformed beta data with the others.  Interpret the origin of Widmanstätten structure. In addition, compare the hardness of the transformed beta, the alpha + transformed beta, and the alpha case region.  Finally, discuss the relationship between the structure, microstructure, and hardness of the titanium alloys.

  1. There are four major tasks in this experiment. What are they? Describe each one briefly.

  2. In the solid state, titanium exists in two allotropic/polymorphic forms. What are they? What is the crystal structure of each form and at what temperatures is each the stable phase?

  3. The write-up for this experiment talks about a “pseudo-binary” phase diagram for titanium-base alloys. What is this type of diagram? Give an example (either general or specific).

  4. The titanium alloy used in this experiment will be examined in three different conditions/treatments. What are they? Describe each one briefly.

  5. The write-up for this experiment describes one of the microstructures to be examined as “alpha + beta”, including primary alpha. What is this microstructure? Draw a schematic sketch of what this microstructure might look like when observed by optical microscopy.

  6. We will use T-T-T diagrams as a basis for heat treating titanium-base alloys. What is a T-T-T diagram? Give an example relevant to titanium alloys.

  7. What alloy of titanium is to be used in this experiment? What is the purpose of each of the solute element additions in this alloy?

  8. Oxygen as a solute in titanium plays a significant role in general on the properties of the alloys and has specific effects in the conduct of this experiment. Describe these effects briefly.

  9. The write-up for this experiment describes one of the microstructures to be examined as “Widmanstätten”. What is this microstructure? Draw a schematic sketch of what this microstructure might look like when observed by optical microscopy.

Phase Information and Microstructure of Ti-6242 Alloy