Ti and Ti-based alloys

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					            Ti and Ti-based alloys
 Titanium for implant : attempts to use at late 1930s
   - titanium was tolerated in cat femurs as was stainless steel and
     CoCrMo alloy
   - lightness (4.5 g/ cm3 )
     cf. 316 stainless steel ( 7.9 g/cm3)
        cast CoCrMo ( 8.3 g/cm3)
        wrought CoNiCrMo alloys (9.2 g/cm3)
   - good mechanochemical properties : salient features for implant
   - higher degree of corrosion resistance than other metals
      ; because oxide films on surface transformed into passive state metals
   - no stress corrosion fracture
      ; weak point of stainless steel
 Compositions of Ti and Ti-Based Alloys
  Unalloyed titanium for surgical implant applications : four grades
   according to the impurity contents
   - impurities (oxygen, iron, nitrogen) controlled carefully
   - oxygen : great influence on the ductility and strength

  Titanium alloy (Ti6A14V) : widely used to manufacture implants
    - alloying elements : Al (5.5-6.5 wt%), vanadium (3.5-4.5 wt%)
 Structure and Properties of Ti and Ti Alloys
  Titanium : allotropic material
    - up to 882°C : hexagonal close-packed(HCP) structure (-Ti)
    - above 882°C : body-centered cubic structure(BCC) (β-Ti)

  Effect of alloying element
   - aluminum : stabilize the  phase
               ; Increase the transformation temperature from  to β phase
    - vanadium : stabilizes the β phase
               ; Lowering the transformation temperature from  to β phase
  alloys (Fig. a)
   - single-phase microstructure
     ; promotes good weldability
   - stabilizing effect of the high aluminum content
     ; result in : excellent strength characteristics
                 : oxidation resistance at high temperature (300~600°C)
   - cannot be heat-treated for strengthening
     ; because of single-phase

 Addition of a controlled amount of β-stabilizers
   - higher strength β phase to persist below the transformation temp.
     ; results in the two-phase system
   - precipitates of β phase
     ; by heat treatment in the solid solution temperature and subsequent
       quenching, followed by aging at a somewhat lower temperature
   - aging cycle causes the precipitation of some fine  particles from
     the metastable β imparting  structure
      ; stronger than the annealed -β structure (Fig. b)
Fig. a   Fig. b

Fig. c
 higher percentage of β-stabilizing elements (13 wt% V in Ti13V11Cr3Al alloy)
   - result in microstructure that is substantially β
   - β : strengthened by the heat treatment (Fig. c)

 Mechanical properties of the pure titanium and 6A14V alloy
  - modulus of elasticity : 110 Gpa
     ; half the value of Co-based alloys (220-234 Gpa)
   - higher impurity content : higher strength and reduced ductility.

 The strength of titanium
   - varies from a value much lower than that of 316 stainless steel or
     the Co-based alloys to a value about equal to that of cast CoCrMo alloy
   - specific strength (strength per density)
     : excels over any other implant material
   - has poor shear strength
     : less desirable for bone screws, plates, and similar applications
     : gall or seize when in sliding contact with itself or another metal
 Corrosion resistance
  - derived from formation of a solid oxide layer
  - oxide layer forms a thin adherent film and passivates
                        Other Metals
 Tantalum, Ta
   - has good biocompatibility, but poor mechanical properties
   - higher density (16.6 g/cm3)
   - restricted to few applications
      ; wire sutures for surgeons, radioisotope for bladder
 Platinum, Pt
   - good corrosion resistance, but poor mechanical properties
   - electrode of cardiac pacemaker
 Gold
  - useful metals in dentistry
  - good durability, stability and corrosion resistance
  - softer alloys (>83% gold) : used for inlays (not much stress)
  - harder alloys (containing less gold) : used for crowns (heavily stressed)
 Dental amalgam : tooth filling material
  - one of the component metals is mercury
  - The solid alloy : at least 65% Ag
      ; not more than 29% Sn, 6% copper, 2% zinc, and 3% mercury
 Nickel-Titanium Shape Memory Alloy
  - Shape Memory Effect (SME) : first observed by Buehler and Wiley
     ; plastically deformed below the transformation temperature, it reverts
      back to its original shape as the temperature is raised
     ; the SME can be generally related to a diffusionless martensitic phase
  - equiatomic Ni-Ti alloy (Nitinol®)
    : exhibits an exceptional SME near room temperature
  - applications
    : orthodontic dental archwire
    : intracranial aneurysm clips
    : a vena cava filter
    : contractile artificial muscles for an artificial heart
    : orthopedic implants and other medical devices
      생체용 금속재료의 활용
 골절 고정 창치 (Fracture fixation devices)
  - 골절된 뼈를 접합시키기 위한 도구
    ; 철사, 핀, 나사, 골절판, 골내삽입장치(intramedully device),
      척추고정장치(spinal fixation device)

 관절 대체물 (Joint replacemants)
  - hip joint : 대표적인 인공관절
      ; 대퇴골과 implant를 고정시키기
        위한 stem과 femoral head 역할
        의 ball, 그리고 socket으로 구성
  - 인공관절은 높은 하중을 전달해야
    하며, 이로 인한 하중으로 마찰면에
    심각한 마모가 발생하지 않아야 함
치과용 임플란트 (Dental implants)
  - compressive stress (>850N), torque, shear stress에 견뎌야 하고
    pH, 온도 화학적 조성이 수시로 변하는 구강환경에 적합해야 함
  - endosseous implant : 치아의 원래 기능을 회복하기 위하여
                         원래의 치아자리에 삽입하는 임플란트
     ; root form, blade, staple, frame, bone plate, screw
  - subperiosteal implant : 의치를 지탱하기 위한 임플란트
                         : 골막하부의 치조골 표면에 위치
 다공성 코팅 임플란트 (Porous-coated implants)
 implant 고정방식
   - 의학용 시멘트로 뼈조직과 임플란트를 접착
     ; 뼈와 임플란트를 즉시 고정할 수 있는 장점이 있지만, 어린이나
       활동성이 많은 사람은 실패율이 높다
  - 다공성 표면구조를 가진 임플란트를 삽입하여 표면의 Pore속으로
    뼈조직이 성장하면서 접합
Porous coating method
  - 뼈조직의 성장과 강도 및 피로강도 등의 기계적 특성을 부여하기
    위해 주로 Cobalt 합금이나 Titanium 합금의 구형입자 또는 fiber-
     metal 을 implant 표면에 코팅
  - Sintering : melting point의 1/2 이상 온도에서 열처리 함으로서
               입자간에 neck 를 형성시켜 기공을 생성
     ; Cobalt 합금의 경우 크롬탄화물(Cr23C6)를 재분산시키고,
      Titanium 합금에서는 notch를 발생시켜 피로강도가 감소
  - Diffusion bonding : sintering 방법에 비헤 상대적으로 낮은 온도에서
                       titanium의 금속섬유로 다공성 표면을 제조
     ; notch-sensitivity 효과를 유발시키고 피로강도가 감소
  - Plasma spraying : 고온의 플라즈마로 금속분말을 용융시키고
                     이를 운반가스와 함께 임픞란트 표면에 분사
     ; textured surface를 만들지만, 다른 코팅법에 비해 기공(pore)간의
       연결성이 좋지않고, notch-sensitivity 효과때문에 피로강도가 감소

 뼈조직이 성장하기 위한 최적의 pore크기 : 100~400m
                    porosity : 30~40
 Three different porous coated materials

               (a)                                    (b)

                                     (a) : Co-based sintering
                                     (b) : Ti fiber-metal diffusion bonding
                                     (c) : Ti plasma spraying